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Professor Runyao Duan

Future Fellow and Professor, A/DRsch Ctr Quantum Computat'n & Intelligent Systs
Core Member, QCIS - Quantum Computation and Intelligent Systems
NA
 
Phone
+61 2 9514 4619
Can supervise: Yes

Chapters

Ying, M., Duan, R., Feng, Y. & Ji, Z. 2010, 'Predicate Transformer Semantics of Quantum Programs' in Gay, S. & Mackie, I. (eds), Semantic Techniques in Quantum Computation, Cambridge University Press, Cambridge, pp. 311-360.
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This chapter presents a systematic exposition of predicate transformer semantics for quantum programs. It is divided into two parts: The first part reviews the state transformer (forward) semantics of quantum programs according to Selingerâs suggestion of representing quantum programs by superoperators and elucidates DâHondt-Panangadenâs theory of quantum weakest preconditions in detail. In the second part, we develop a quite complete predicate transformer semantics of quantum programs based on Birkhoffâvon Neumann quantum logic by considering only quantum predicates expressed by projection operators. In particular, the universal coujunctivity and termination law of quantum programs are proved, and Hoareâs induction rule is established in the quantum setting.

Conferences

Feng, Y., Duan, R. & Ying, M. 2011, 'Bisimulation for Quantum Processes', Proceedings of the 38th annual ACM SIGPLAN-SIGACT symposium on Principles of programming language, annual ACM SIGPLAN-SIGACT symposium on Principles of programming languages, ACM, Austin, Texas, USA, pp. 523-534.
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Quantum cryptographic systems have been commercially available, with a striking advantage over classical systems that their security and ability to detect the presence of eavesdropping are provable based on the principles of quantum mechanics. On the other hand, quantum protocol designers may commit much more faults than classical protocol designers since human intuition is much better adapted to the classical world than the quantum world. To offer formal techniques for modeling and verification of quantum protocols, several quantum extensions of process algebra have been proposed. One of the most serious issues in quantum process algebra is to discover a quantum generalization of the notion of bisimulation, which lies in a central position in process algebra, preserved by parallel composition in the presence of quantum entanglement, which has no counterpart in classical computation. Quite a few versions of bisimulation have been defined for quantum processes in the literature, but in the best case they are only proved to be preserved by parallel composition of purely quantum processes where no classical communications are involved. Many quantum cryptographic protocols, however, employ the LOCC (Local Operations and Classical Communications) scheme, where classical communications must be explicitly specified. So, a notion of bisimulation preserved by parallel composition in the circumstance of both classical and quantum communications is crucial for process algebra approach to verification of quantum cryptographic protocols. In this paper we introduce a novel notion of bisimulation for quantum processes and prove that it is congruent with respect to various process algebra combinators including parallel composition even when both classical and quantum communications are present.We also establish some basic algebraic laws for this bisimulation.
Duan, R., Severini, S. & Winter, A. 2011, 'Zero-error communication via quantum channels and a quantum Lovász -function', 2011 IEEE International Symposium on Information Theory Proceedings (ISIT), IEEE International Symposium on Information Theory Proceedings (ISIT), IEEE, St Petersburg, Russia, pp. 64-68.
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We study the quantum channel version of Shannon's zero-error capacity problem. Motivated by recent progress on this question, we propose to consider a certain linear space operators as the quantum generalisation of the adjacency matrix, in terms of which the plain, quantum and entanglement-assisted capacity can be formulated, and for which we show some new basic properties. Most importantly, we define a quantum version of Lova´sz' famous ? function, as the norm-completion (or stabilisation) of a naive generalisation of ?. We go on to show that this function upper bounds the number of entanglement-assisted zero-error messages, that it is given by a semidefinite programme, whose dual we write down explicitly, and that it is multiplicative with respect to the natural (strong) graph product. We explore various other properties of the new quantity, which reduces to Lova´sz' original ? in the classical case, give several applications, and propose to study the linear spaces of operators associated to channels as non-commutative graphs, using the language of operator systems and Hilbert modules.
Duan, R., Grassl, M., Ji, Z. & Zeng, B. 2010, 'Multi-error-correcting amplitude damping codes', IEEE International Symposium on Information Theory - Proceedings, International Symposium on Information Theory, IEEE, Austin, USA, pp. 2672-2676.
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We construct new families of multi-error-correcting quantum codes for the amplitude damping channel. Our key observation is that, with proper encoding, two uses of the amplitude damping channel simulate a quantum erasure channel. This allows us to use co

Journal articles

Duan, R. & Winter, A. 2016, 'No-Signalling Assisted Zero-Error Capacity of Quantum Channels and an Information Theoretic Interpretation of the Lovasz Number', IEEE Transactions on Information Theory, vol. 62, no. 2, pp. 891-914.
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We study the one-shot zero-error classical capacity of a quantum channel assisted by quantum no-signalling correlations, and the reverse problem of exact simulation of a prescribed channel by a noiseless classical one. Quantum no-signalling correlations are viewed as two-input and two-output completely positive and trace preserving maps with linear constraints enforcing that the device cannot signal. Both problems lead to simple semidefinite programmes (SDPs) that depend only on the Kraus operator space of the channel. In particular, we show that the zero-error classical simulation cost is precisely the conditional min-entropy of the Choi-Jamiolkowski matrix of the given channel. The zero-error classical capacity is given by a similar-looking but different SDP; the asymptotic zero-error classical capacity is the regularization of this SDP, and in general we do not know of any simple form. Interestingly however, for the class of classical-quantum channels, we show that the asymptotic capacity is given by a much simpler SDP, which coincides with a semidefinite generalization of the fractional packing number suggested earlier by Aram Harrow. This finally results in an operational interpretation of the celebrated Lovasz $\vartheta$ function of a graph as the zero-error classical capacity of the graph assisted by quantum no-signalling correlations, the first information theoretic interpretation of the Lovasz number.
Duan, R., Severini, S. & Winter, A. 2016, 'On zero-error communication via quantum channels in the presence of noiseless feedback'.
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We initiate the study of zero-error communication via quantum channels when the receiver and sender have at their disposal a noiseless feedback channel of unlimited quantum capacity, generalizing Shannon's zero-error communication theory with instantaneous feedback. We first show that this capacity is a function only of the linear span of Choi-Kraus operators of the channel, which generalizes the bipartite equivocation graph of a classical channel, and which we dub "non-commutative bipartite graph". Then we go on to show that the feedback-assisted capacity is non-zero (with constant activating noiseless communication) if and only if the non-commutative bipartite graph is non-trivial, and give a number of equivalent characterizations. This result involves a far-reaching extension of the "conclusive exclusion" of quantum states [Pusey/Barrett/Rudolph, Nature Phys. 8:475-478]. We then present an upper bound on the feedback-assisted zero-error capacity, motivated by a conjecture originally made by Shannon and proved later by Ahlswede. We demonstrate this bound to have many good properties, including being additive and given by a minimax formula. We also prove that this quantity is the entanglement-assisted capacity against an adversarially chosen channel from the set of all channels with the same Choi-Kraus span, which can also be interpreted as the feedback-assisted unambiguous capacity. The proof relies on a generalization of the "Postselection Lemma" [Christandl/Koenig/Renner, PRL 102:020504] that allows to reflect additional constraints, and which we believe to be of independent interest. We illustrate our ideas with a number of examples, including classical-quantum channels and Weyl diagonal channels, and close with an extensive discussion of open questions.
Lai, C.-.Y. & Duan, R. 2016, 'On the One-Shot Zero-Error Classical Capacity of Classical-Quantum Channels Assisted by Quantum Non-signalling Correlations'.
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Duan and Winter studied the one-shot zero-error classical capacity of a quantum channel assisted by quantum non-signalling correlations, and formulated this problem as a semidefinite program depending only on the Kraus operator space of the channel. For the class of classical-quantum channels, they showed that the asymptotic zero-error classical capacity assisted by quantum non-signalling correlations, minimized over all classical-quantum channels with a confusability graph $G$, is exactly $\log \vartheta(G)$, where $\vartheta(G)$ is the celebrated Lov\'{a}sz theta function. In this paper, we show that the same result holds in the one-shot setting for a class of circulant graphs defined by equal-sized cyclotomic cosets, which include the cycle graphs of odd length, the Paley graphs of prime vertices, and the cubit residue graphs of prime vertices. Examples of other graphs are also discussed. This endows the Lov\'{a}sz $\theta$ function with a more straightforward operational meaning.
Chitambar, E.A., Duan, R. & Hsieh, M. 2014, 'When Do Local Operations and Classical Communication Suffice for Two-Qubit State Discrimination?', IEEE Transactions On Information Theory, vol. 60, no. 3, pp. 1549-1561.
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In this paper, we consider the conditions under which a given ensemble of two-qubit states can be optimally distinguished by local operations and classical communication (LOCC). We begin by completing the perfect distinguishability problem of two-qubit ensembles-both for separable operations and LOCC-by providing necessary and sufficient conditions for the perfect discrimination of one pure and one mixed state. Then, for the well-known task of minimum error discrimination, it is shown that almost all two-qubit ensembles consisting of three pure states cannot be optimally discriminated using LOCC. This is surprising considering that any two pure states can be distinguished optimally by LOCC. Special attention is given to ensembles that lack entanglement, and we prove an easy sufficient condition for when a set of three product states cannot be optimally distinguished by LOCC, thus providing new examples of the phenomenon known as non-locality without entanglement. We next consider an example of N parties who each share the same state but who are ignorant of its identity. The state is drawn from the rotationally invariant trine ensemble, and we establish a tight connection between the N-copy ensemble and Shor's lifted single-copy ensemble. For any finite N, we prove that optimal identification of the states cannot be achieved by LOCC; however, as N?8, LOCC can indeed discriminate the states optimally. This is the first result of its kind. Finally, we turn to the task of unambiguous discrimination and derive new lower bounds on the LOCC inconclusive probability for symmetric states. When applied to the double trine ensemble, this leads to a rather different distinguishability character than when the minimum error probability is considered.
Yu, N., Duan, R. & Ying, M. 2014, 'Distinguishability of Quantum States by Positive Operator-Valued Measures with Positive Partial Transpose', IEEE Transactions on Information Theory, vol. 60, no. 4, pp. 2069-2079.
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We study the distinguishability of bipartite quantum states by Positive Operator-Valued Measures with positive partial transpose (PPT POVMs). The contributions of this paper include: (1). We give a negative answer to an open problem of [M. Horodecki $et. al$, Phys. Rev. Lett. 90, 047902(2003)] showing a limitation of their method for detecting nondistinguishability. (2). We show that a maximally entangled state and its orthogonal complement, no matter how many copies are supplied, can not be distinguished by PPT POVMs, even unambiguously. This result is much stronger than the previous known ones \cite{DUAN06,BAN11}. (3). We study the entanglement cost of distinguishing quantum states. It is proved that $\sqrt{2/3}\ket{00}+\sqrt{1/3}\ket{11}$ is sufficient and necessary for distinguishing three Bell states by PPT POVMs. An upper bound of entanglement cost of distinguishing a $d\otimes d$ pure state and its orthogonal complement is obtained for separable operations. Based on this bound, we are able to construct two orthogonal quantum states which cannot be distinguished unambiguously by separable POVMs, but finite copies would make them perfectly distinguishable by LOCC. We further observe that a two-qubit maximally entangled state is always enough for distinguishing a $d\otimes d$ pure state and its orthogonal complement by PPT POVMs, no matter the value of $d$. In sharp contrast, an entangled state with Schmidt number at least $d$ is always needed for distinguishing such two states by separable POVMs. As an application, we show that the entanglement cost of distinguishing a $d\otimes d$ maximally entangled state and its orthogonal complement must be a maximally entangled state for $d=2$,which implies that teleportation is optimal; and in general, it could be chosen as $\mathcal{O}(\frac{\log d}{d})$.
Yu, N., Guo, C. & Duan, R. 2014, 'Obtaining a W state from a Greenberger-Horne-Zeilinger state via stochastic local operations and classical communication with a rate approaching unity', Physical Review Letters, vol. 112, no. 16.
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We introduce a notion of the entanglement transformation rate to characterize the asymptotic comparability of two multipartite pure entangled states under stochastic local operations and classical communication (SLOCC). For two well known SLOCC inequivalent three-qubit states |GHZ=(1/2)(|000+|111) and |W=(1/3)(|100+|010+|001), we show that the entanglement transformation rate from |GHZ to |W is exactly 1. That means that we can obtain one copy of the W state from one copy of the Greenberg-Horne-Zeilinger (GHZ) state by SLOCC, asymptotically. We then apply similar techniques to obtain a lower bound on the entanglement transformation rates from an N-partite GHZ state to a class of Dicke states, and prove the tightness of this bound for some special cases which naturally generalize the |W state. A new lower bound on the tensor rank of the matrix permanent is also obtained by evaluating the tensor rank of Dicke states. © 2014 American Physical Society.
Ban, Y. & Chen, X. 2014, 'Counter-diabatic driving for fast spin control in a two-electron double quantum dot', Scientific Reports, vol. 4, pp. 6258-6258.
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Ying, M., Yu, N., Feng, Y. & Duan, R. 2013, 'Verification of quantum programs', Science Of Computer Programming, vol. 78, no. 9, pp. 1679-1700.
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This paper develops verification methodology for quantum programs, and the contribution of the paper is two-fold. Sharir, Pnueli and Hart [M. Sharir, A. Pnueli, S. Hart, Verification of probabilistic programs, SIAM Journal of Computing 13 (1984) 292314] presented a general method for proving properties of probabilistic programs, in which a probabilistic program is modeled by a Markov chain and an assertion on the output distribution is extended to an invariant assertion on all intermediate distributions. Their method is essentially a probabilistic generalization of the classical Floyd inductive assertion method. In this paper, we consider quantum programs modeled by quantum Markov chains which are defined by super-operators. It is shown that the SharirPnueliHart method can be elegantly generalized to quantum programs by exploiting the SchrödingerHeisenberg duality between quantum states and observables. In particular, a completeness theorem for the SharirPnueliHart verification method of quantum programs is established.
Duan, R., Severini, S. & Winter, A. 2013, 'Zero-Error Communication via Quantum Channels, Noncommutative Graphs, and a Quantum Lovász Number', IEEE Transactions On Information Theory, vol. 59, no. 2, pp. 1164-1174.
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We study the quantum channel version of Shannon's zero-error capacity problem. Motivated by recent progress on this question, we propose to consider a certain subspace of operators (so-called operator systems) as the quantum generalization of the adjacency matrix, in terms of which the zero-error capacity of a quantum channel, as well as the quantum and entanglement-assisted zero-error capacities can be formulated, and for which we show some new basic properties. Most importantly, we define a quantum version of Lova´sz' famous ? function on general operator systems, as the norm-completion (or stabilization) of a naive generalization of ?. We go on to show that this function upper bounds the number of entanglement-assisted zero-error messages, that it is given by a semidefinite program, whose dual we write down explicitly, and that it is multiplicative with respect to the tensor product of operator systems (corresponding to the tensor product of channels). We explore various other properties of the new quantity, which reduces to Lova´sz' original ? in the classical case, give several applications, and propose to study the operator systems associated with channels as noncommutative graphs, using the language of Hilbert modules.
Yu, N., Duan, R. & Ying, M. 2013, 'Five two-qubit gates are necessary for implementing the Toffoli gate', PHYSICAL REVIEW A, vol. 88, no. 1.
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Lu, C., Chen, J.F. & Duan, R. 2012, 'Some Bounds On The Minimum Number Of Queries Required For Quantum Channel Perfect Discrimination', Quantum Information and Computation, vol. 12, no. 1-2, pp. 138-148.
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We prove a lower bound on the q-maximal fidelities between two quantum channels epsilon(0) and epsilon(1) and an upper bound on the q-maximal fidelities between a quantum channel epsilon and an identity tau. Then we apply these two bounds to provide a si
Ying, M., Feng, Y., Duan, R., Li, Y. & Yu, N. 2012, 'Quantum Programming: From Theories To Implementations', Chinese Science Bulletin, vol. 57, no. 16, pp. 1903-1909.
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This paper surveys the new field of programming methodology and techniques for future quantum computers, including design of sequential and concurrent quantum programming languages, their semantics and implementations. Several verification methods for qu
Feng, Y., Duan, R. & Ying, M. 2012, 'Bisimulation For Quantum Processes', ACM Transactions pn Programming Language and Systems (TOPLAS), vol. 34, no. 4, pp. 1-43.
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Quantum cryptographic systems have been commercially available, with a striking advantage over classical systems that their security and ability to detect the presence of eavesdropping are provable based on the principles of quantum mechanics. On the oth
Yu, N., Duan, R. & Ying, M. 2012, 'Four Locally Indistinguishable Ququad-Ququad Orthogonal Maximally Entangled States', PHYSICAL REVIEW LETTERS, vol. 109, no. 2.
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Chen, J., Chen, X., Duan, R., Ji, Z. & Zeng, B. 2011, 'No-go Theorem For One-way Quantum Computing On Naturally Occurring Two-level Systems', Physical Review A, vol. 83, no. 5, pp. 0-0.
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The ground states of some many-body quantum systems can serve as resource states for the one-way quantum computing model, achieving the full power of quantum computation. Such resource states are found, for example, in spin- 5 2 and spin- 3 2 systems. It is, of course, desirable to have a natural resource state in a spin- 1 2 , that is, qubit system. Here, we give a negative answer to this question for frustration-free systems with two-body interactions. In fact, it is shown to be impossible for any genuinely entangled qubit state to be a nondegenerate ground state of any two-body frustration-free Hamiltonian. What is more, we also prove that every spin- 1 2 frustration-free Hamiltonian with two-body interaction always has a ground state that is a product of single- or two-qubit states. In other words, there cannot be any interesting entanglement features in the ground state of such a qubit Hamiltonian.
Yu, N., Duan, R. & Ying, M. 2011, 'Any 2 Circle Times N Subspace Is Locally Distinguishable', Physical Review A, vol. 84, no. 1, pp. 1-3.
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A subspace of a multipartite Hilbert space is said to be locally indistinguishable if any orthonormal basis of this subspace cannot be perfectly distinguished by local operations and classical communication. Previously it was shown that any m . n bipartite system with m > 2 and n > 2 has a locally indistinguishable subspace. However, it has been an open problem since 2005 whether there is a locally indistinguishable bipartite subspace with a qubit subsystem.We settle this problem in negative by showing that any 2 . n bipartite subspace contains a basis that is locally distinguishable. As an interesting application, we show that any quantum channel with two Kraus operators has optimal environment-assisted classical capacity.
Duan, R. & Shi, Y. 2010, 'When Is There A Multipartite Maximum Entangled State?', Quantum Information and Computation, vol. 10, no. 11-12, pp. 925-935.
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For a multipartite quantum system of the dimension d(1) circle times d(2) circle times ... circle times d(n), where d(1) >= d(2) >= ... >= d(n) >= 2, is there an entangled state maximum in the sense that all other states in the system can be obtained from the state through local quantum operations and classical communications (LOCC)? When d(1) >= n i=2d(i), such state exists. We show that this condition is also necessary. Our proof, somewhat surprisingly, uses results from algebraic complexity theory.
Li, Y., Duan, R. & Ying, M. 2010, 'Local Unambiguous Discrimination With Remaining Entanglement', Physical Review A, vol. 82, no. 3, pp. 1-6.
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A bipartite state, which is secretly chosen from a finite set of known entangled pure states, cannot immediately be useful in standard quantum information processing tasks. To effectively make use of the entanglement contained in this unknown state, we i
Chen, X., Duan, R., Ji, Z. & Zeng, B. 2010, 'Quantum State Reduction For Universal Measurement Based Computation', Physical Review Letters, vol. 105, no. 2, pp. 1-4.
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Measurement based quantum computation, which requires only single particle measurements on a universal resource state to achieve the full power of quantum computing, has been recognized as one of the most promising models for the physical realization of
Yu, N., Duan, R. & Ying, M. 2010, 'Optimal Simulation Of A Perfect Entangler', Physical Review A, vol. 81, no. 3, pp. 1-4.
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A2 circle times 2 unitary operation is called a perfect entangler if it can generate a maximally entangled state from some unentangled input. We study the following question: How many runs of a given two-qubit entangling unitary operation are required to
Duan, R., Xin, Y. & Ying, M. 2010, 'Locally Indistinguishable Subspaces Spanned By Three-Qubit Unextendible Product Bases', Physical Review A, vol. 81, no. 3, pp. 1-10.
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We study the local distinguishability of general multiqubit states and show that local projective measurements and classical communication are as powerful as the most general local measurements and classical communication. Remarkably, this indicates that
Yu, N., Chitambar, E.A., Guo, C. & Duan, R. 2010, 'Tensor Rank Of The Tripartite State Vertical Bar W >(Circle Times N)', Physical Review A, vol. 81, no. 1, pp. 1-3.
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Tensor rank refers to the number of product states needed to express a given multipartite quantum state. Its nonadditivity as an entanglement measure has recently been observed. In this Brief Report, we estimate the tensor rank of multiple copies of the
Chen, L., Chitambar, E.A., Duan, R., Ji, Z. & Winter, A. 2010, 'Tensor Rank And Stochastic Entanglement Catalysis For Multipartite Pure States', Physical Review Letters, vol. 105, no. 20, pp. 1-4.
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The tensor rank (also known as generalized Schmidt rank) of multipartite pure states plays an important role in the study of entanglement classifications and transformations. We employ powerful tools from the theory of homogeneous polynomials to investig
Chitambar, E.A., Duan, R. & Shi, Y. 2010, 'Multipartite-To-Bipartite Entanglement Transformations And Polynomial Identity Testing', Physical Review A, vol. 81, no. 5, pp. 1-4.
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We consider the problem of deciding if some multiparty entangled pure state can be converted, with a nonzero success probability, into a given bipartite pure state shared between two specified parties through local quantum operations and classical commun
Chen, L., Chitambar, E., Duan, R., Ji, Z. & Winter, A. 2010, 'Tensor rank and stochastic entanglement catalysis for multipartite pure states.', Physical review letters, vol. 105, no. 20, p. 200501.
The tensor rank (also known as generalized Schmidt rank) of multipartite pure states plays an important role in the study of entanglement classifications and transformations. We employ powerful tools from the theory of homogeneous polynomials to investigate the tensor rank of symmetric states such as the tripartite state |W3>=1/3(|100> + |010> + |001>) and its N-partite generalization |W(N)>. Previous tensor rank estimates are dramatically improved and we show that (i) three copies of |W3> have a rank of either 15 or 16, (ii) two copies of |W(N)> have a rank of 3N - 2, and (iii) n copies of |W(N)> have a rank of O(N). A remarkable consequence of these results is that certain multipartite transformations, impossible even probabilistically, can become possible when performed in multiple-copy bunches or when assisted by some catalyzing state. This effect is impossible for bipartite pure states.
Duan, R., Feng, Y., Xin, Y. & Ying, M. 2009, 'Distinguishability of Quantum States by Separable Operations', IEEE Transactions On Information Theory, vol. 55, no. 3, pp. 1320-1330.
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In this paper, we study the distinguishability of multipartite quantum states by separable operations. We first present a necessary and sufficient condition for a finite set of orthogonal quantum states to be distinguishable by separable operations. An analytical version of this condition is derived for the case of (D - 1) pure states, where D is the total dimension of the state space under consideration. A number of interesting consequences of this result are then carefully investigated. Remarkably, we show there exists a large class of 2 circle times 2 separable operations not being realizable by local operations and classical communication. Before our work, only a class of 3 circle times 3 nonlocal separable operations was known [Bennett et al, Phys. Rev. A 59, 1070 (1999)]. We also show that any basis of the orthogonal complement of a multipartite pure state is indistinguishable by separable operations if and only if this state cannot be a superposition of one or two orthogonal product states, i.e., has an orthogonal Schmidt number not less than three, thus generalize the recent work about indistinguishable bipartite subspaces [Watrous, Phys. Rev. Lett. 95, 080505 (2005)]. Notably, we obtain an explicit construction of indistinguishable subspaces of dimension 7 (or 6) by considering a composite quantum system consisting of two qutrits (resp., three qubits), which is slightly better than the previously known indistinguishable bipartite subspace with dimension 8.
Ying, M., Feng, Y., Duan, R. & Ji, Z. 2009, 'An Algebra Of Quantum Processes', Acm Transactions On Computational Logic, vol. 10, no. 3, pp. 1-36.
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We introduce an algebra qCCS of pure quantum processes in which communications by moving quantum states physically are allowed and computations are modeled by super-operators, but no classical data is explicitly involved. An operational semantics of qCCS is presented in terms of (nonprobabilistic) labeled transition systems. Strong bisimulation between processes modeled in qCCS is defined, and its fundamental algebraic properties are established, including uniqueness of the solutions of recursive equations. To model sequential computation in qCCS, a reduction relation between processes is defined. By combining reduction relation and strong bisimulation we introduce the notion of strong reduction-bisimulation, which is a device for observing interaction of computation and communication in quantum systems. Finally, a notion of strong approximate bisimulation (equivalently, strong bisimulation distance) and its reduction counterpart are introduced. It is proved that both approximate bisimilarity and approximate reduction-bisimilarity are preserved by various constructors of quantum processes. This provides us with a formal tool for observing robustness of quantum processes against inaccuracy in the implementation of its elementary gates.
Chitambar, E.A. & Duan, R. 2009, 'Nonlocal Entanglement Transformations Achievable by Separable Operations', Physical Review Letters, vol. 103, no. 11, pp. 1-4.
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The weird phenomenon of "quantum nonlocality without entanglement" means that local quantum operations assisted by classical communication constitute a proper subset of the class of separable quantum operations. Despite considerable recent advances, little is known to what extent the class of separable operations differs from local quantum operations and classical communication. In this Letter we show that separable operations are generally stronger than local quantum operations and classical communication when distilling a mixed state into a pure entangled state and thus confirm the existence of entanglement monotones that can increase under separable operations. Our finding can also be interpreted as confirming the ability of separable operations to enhance the entanglement of mixed states relative to certain measures, a sensible but important fact that has never been rigorously proven before.
Feng, Y., Duan, R. & Ying, M. 2009, 'Locally undetermined states, generalized Schmidt decomposition, and application in distributed computing', Quantum Information and Computation, vol. 9, no. 11, pp. 997-1012.
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Multipartite quantum states that cannot be uniquely determined by their reduced states of all proper subsets of the parties exhibit some inherit `high-order' correlation. This paper elaborates this issue by giving necessary and sufficient conditions for a pure multipartite state to be locally undetermined, and moreover, characterizing precisely all the pure states sharing the same set of reduced states with it. Interestingly, local determinability of pure states is closely related to a generalized notion of Schmidt decomposition. Furthermore, we find that locally undetermined states have some applications to the well-known consensus problem in distributed computation. To be specific, given some physically separated agents, when communication between them, either classical or quantum, is unreliable and they are not allowed to use local ancillary quantum systems, then there exists a totally correct and completely fault-tolerant protocol for them to reach a consensus if and only if they share a priori a locally undetermined quantum state
Chitambar, E. & Duan, R. 2009, 'Nonlocal entanglement transformations achievable by separable operations.', Physical review letters, vol. 103, no. 11, p. 110502.
The weird phenomenon of "quantum nonlocality without entanglement" means that local quantum operations assisted by classical communication constitute a proper subset of the class of separable quantum operations. Despite considerable recent advances, little is known to what extent the class of separable operations differs from local quantum operations and classical communication. In this Letter we show that separable operations are generally stronger than local quantum operations and classical communication when distilling a mixed state into a pure entangled state and thus confirm the existence of entanglement monotones that can increase under separable operations. Our finding can also be interpreted as confirming the ability of separable operations to enhance the entanglement of mixed states relative to certain measures, a sensible but important fact that has never been rigorously proven before.
Duan, R., Feng, Y. & Ying, M. 2009, 'Perfect Distinguishability of Quantum Operations', PHYSICAL REVIEW LETTERS, vol. 103, no. 21.
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Ji, Z., Wang, G., Duan, R., Feng, Y. & Ying, M. 2008, 'Parameter Estimation of Quantum Channels', IEEE Transactions On Information Theory, vol. 54, no. 11, pp. 5172-5185.
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The efficiency of parameter estimation of quantum channels is studied in this paper. We introduce the concept of programmable parameters to the theory of estimation. It is found that programmable parameters obey the standard quantum limit strictly; hence, no speedup is possible in its estimation. We also construct a class of nonunitary quantum channels whose parameter can be estimated in a way that the standard quantum limit is broken. The study of estimation of general quantum channels also enables an investigation of the effect of noises on quantum estimation.
Duan, R. & Shi, Y. 2008, 'Entanglement Between Two Uses Of A Noisy Multipartite Quantum Channel Enables Perfect Transmission Of Classical Information', Physical Review Letters, vol. 101, no. 2, pp. 1-4.
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Suppose that m senders want to transmit classical information to n receivers with zero probability of error using a noisy multipartite communication channel. The senders are allowed to exchange classical, but not quantum, messages among themselves, and t
Chen, J.F., Duan, R., Ji, Z., Ying, M. & Yu, J.X. 2008, 'Existence Of Universal Entangler', Journal of Mathematical Physics, vol. 49, no. 1, pp. 1-7.
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A gate is called an entangler if it transforms some (pure) product states to entangled states. A universal entangler is a gate which transforms all product states to entangled states. In practice, a universal entangler is a very powerful device for gener
Duan, R., Feng, Y. & Ying, M. 2008, 'Local Distinguishability Of Multipartite Unitary Operations', Physical Review Letters, vol. 100, no. 2, pp. 1-4.
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We show that any two different unitary operations acting on an arbitrary multipartite quantum system can be perfectly distinguished by local operations and classical communication when a finite number of runs is allowed. Intuitively, this result indicate
Chitambar, E.A., Duan, R. & Shi, Y. 2008, 'Tripartite Entanglement Transformations And Tensor Rank', Physical Review Letters, vol. 101, no. 14, pp. 1-4.
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A basic question regarding quantum entangled states is whether one can be probabilistically converted to another through local operations and classical communication exclusively. While the answer for bipartite systems is known, we show that for tripartit
Wu, X. & Duan, R. 2008, 'Exact Quantum Search By Parallel Unitary Discrimination Schemes', Physical Review A, vol. 78, no. 1, pp. 1-8.
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We study the unsorted database search problem with items N from the viewpoint of unitary discrimination. Instead of considering the famous O(root N) Grover bounded-error algorithm for the original problem, we seek the results for the exact algorithms, i.
Xin, Y. & Duan, R. 2008, 'Local Distinguishability Of Orthogonal 2 Circle Times 3 Pure States', Physical Review A, vol. 77, no. 1, pp. 1-10.
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We present a complete characterization for the local distinguishability of orthogonal 2 circle times 3 pure states except for some special cases of three states. Interestingly, we find there is a large class of four or three states that are indistinguish
Duan, R., Feng, Y. & Ying, M. 2008, 'Local distinguishability of multipartite unitary operations.', Physical review letters, vol. 100, no. 2, p. 020503.
We show that any two different unitary operations acting on an arbitrary multipartite quantum system can be perfectly distinguished by local operations and classical communication when a finite number of runs is allowed. Intuitively, this result indicates that the lost identity of a nonlocal unitary operation can be recovered locally. No entanglement between distant parties is required.
Chitambar, E., Duan, R. & Shi, Y. 2008, 'Tripartite entanglement transformations and tensor rank.', Physical review letters, vol. 101, no. 14, p. 140502.
A basic question regarding quantum entangled states is whether one can be probabilistically converted to another through local operations and classical communication exclusively. While the answer for bipartite systems is known, we show that for tripartite systems, this question encodes some of the most challenging open problems in mathematics and computer science. In particular, we show that there is no easy general criterion to determine the feasibility, and in fact, the problem is NP hard. In addition, we find obtaining the most efficient algorithm for matrix multiplication to be precisely equivalent to determining the maximum rate to convert the Greenberger-Horne-Zeilinger state to a triangular distribution of three EPR states. Our results are based on connections between multipartite entanglement and tensor rank (also called Schmidt rank), a key concept in algebraic complexity theory.
Ying, M., Chen, J.F., Feng, Y. & Duan, R. 2007, 'Commutativity Of Quantum Weakest Preconditions', Information Processing Letters, vol. 104, no. 4, pp. 152-158.
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The notion of quantum weakest precondition was introduced by D'Hondt and P. Panangaden [E. D'Hondt, P. Panangaden, Quantum weakest preconditions, Mathematical Structures in Computer Science 16 (2006) 429-451], and they presented a representation of weake
Xin, Y. & Duan, R. 2007, 'Conditions For Entanglement Transformation Between A Class Of Multipartite Pure States With Generalized Schmidt Decompositions', Physical Review A, vol. 76, no. 4, pp. 1-3.
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We generalize Nielsen's majorization criterion for the convertibility of bipartite pure states [Phys. Rev. Lett. 83, 436 (1999)] to a special class of multipartite pure states with generalized Schmidt decompositions.
Duan, R., Feng, Y., Ji, Z. & Ying, M. 2007, 'Distinguishing Arbitrary Multipartite Basis Unambiguously Using Local Operations And Classical Communication', Physical Review Letters, vol. 98, no. 23, pp. 1-4.
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We show that an arbitrary basis of a multipartite quantum state space consisting of K distant parties such that the kth party has local dimension d(k) always contains at least N=Sigma(K)(k=1)(d(k)-1)+1 members that are unambiguously distinguishable using
Duan, R., Feng, Y. & Ying, M. 2007, 'Entanglement Is Not Necessary For Perfect Discrimination Between Unitary Operations', Physical Review Letters, vol. 98, no. 10, pp. 1-4.
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We show that a unitary operation (quantum circuit) secretly chosen from a finite set of unitary operations can be determined with certainty by sequentially applying only a finite amount of runs of the unknown circuit. No entanglement or joint quantum ope
Feng, Y., Duan, R., Ji, Z. & Ying, M. 2007, 'Probabilistic Bisimulations For Quantum Processes', Information And Computation, vol. 205, no. 11, pp. 1608-1639.
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Modeling and reasoning about concurrent quantum systems is very important for both distributed quantum computing and quantum protocol verification. As a consequence, a general framework formally describing communication and concurrency in complex quantum
Feng, Y., Duan, R., Ji, Z. & Ying, M. 2007, 'Proof Rules For The Correctness Of Quantum Programs', Theoretical Computer Science, vol. 386, no. 1-2, pp. 151-166.
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We apply the notion of quantum predicate proposed by D'Hondt and Panangaden to analyze a simple language fragment which may describe the quantum part of a future quantum computer in Knill's architecture. The notion of weakest liberal precondition semanti
Duan, R., Feng, Y. & Ying, M. 2007, 'Entanglement is Not Necessary for Perfect Discrimination between Unitary Operations', Physical Review Letters, vol. 98, no. 10.
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Feng, Y., Duan, R., Ji, Z. & Ying, M. 2007, 'Probabilistic bisimulations for quantum processes', Information and Computation.
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Duan, R., Feng, Y., Ji, Z. & Ying, M. 2007, 'Distinguishing arbitrary multipartite basis unambiguously using local operations and classical communication.', Physical review letters, vol. 98, no. 23, p. 230502.
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We show that an arbitrary basis of a multipartite quantum state space consisting of K distant parties such that the kth party has local dimension dk always contains at least N= Sigma(k=1)(K) (dk-1)+1 members that are unambiguously distinguishable using local operations and classical communication (LOCC). We further show that this lower bound is optimal by analytically constructing a special product basis having only N members unambiguously distinguishable by LOCC. Interestingly, such a special product basis not only gives a stronger form of the weird phenomenon "nonlocality without entanglement," but also implies the existence of a locally distinguishable entangled basis.
Duan, R., Feng, Y. & Ying, M. 2007, 'Entanglement is not necessary for perfect discrimination between unitary operations.', Physical review letters, vol. 98, no. 10, p. 100503.
We show that a unitary operation (quantum circuit) secretly chosen from a finite set of unitary operations can be determined with certainty by sequentially applying only a finite amount of runs of the unknown circuit. No entanglement or joint quantum operations are required in our scheme. We further show that our scheme is optimal in the sense that the number of the runs is minimal when discriminating only two unitary operations.
Ji, Z., Feng, Y., Duan, R. & Ying, M. 2006, 'Boundary Effect Of Deterministic Dense Coding', Physical Review A, vol. 73, no. 3, pp. 1-3.
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We present a rigorous proof of an interesting boundary effect of deterministic dense coding first observed by S. Mozes, J. Oppenheim, and B. Reznik [Phys. Rev. A 71, 012311 (2005)]. Namely, it is shown that d(2)-1 cannot be the maximal alphabet size of a
Ji, Z., Feng, Y., Duan, R. & Ying, M. 2006, 'Identification And Distance Measures Of Measurement Apparatus', Physical Review Letters, vol. 96, no. 20, pp. 1-4.
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We propose simple schemes that can perfectly identify projective measurement apparatuses secretly chosen from a finite set. Entanglement is used in these schemes both to make possible the perfect identification and to improve the efficiency significantly
Feng, Y., Duan, R. & Ji, Z. 2006, 'Optimal Dense Coding With Arbitrary Pure Entangled States', Physical Review A, vol. 74, no. 1, pp. 1-5.
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We examine dense coding with an arbitrary pure entangled state sharing between the sender and the receiver. Upper bounds on the average success probability in approximate dense coding and on the probability of conclusive results in unambiguous dense codi
Duan, R., Feng, Y. & Ying, M. 2006, 'Partial Recovery Of Quantum Entanglement', IEEE Transactions On Information Theory, vol. 52, no. 7, pp. 3080-3104.
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Suppose Alice and Bob try to transform an entangled state shared between them into another one by local operations and classical communications. Then in general a certain amount of entanglement contained in the initial state will decrease in the process
Feng, Y., Duan, R. & Ying, M. 2006, 'Relation Between Catalyst-Assisted Transformation And Multiple-Copy Transformation For Bipartite Pure States', Physical Review A, vol. 74, no. 4, pp. 1-7.
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We show that in some cases, catalyst-assisted entanglement transformation cannot be implemented by multiple-copy transformation for pure states. This fact, together with the result we obtained in R. Y. Duan, Y. Feng, X. Li, and M. S. Ying, Phys. Rev. A 7
Duan, R., Ji, Z., Feng, Y. & Ying, M. 2006, 'Some Issues In Quantum Information Theory', Journal Of Computer Science And Technology, vol. 21, no. 5, pp. 776-789.
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Quantum information theory is a new interdisciplinary research field related to quantum mechanics, computer science, information theory, and applied mathematics. It provides completely new paradigms to do information processing tasks by employing the pri
Feng, Y., Duan, R. & Ying, M. 2006, 'Relation between catalyst-assisted transformation and multiple-copy transformation for bipartite pure states', Physical Review A, vol. 74, no. 4.
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Ji, Z., Feng, Y., Duan, R. & Ying, M. 2006, 'Identification and distance measures of measurement apparatus.', Physical review letters, vol. 96, no. 20, p. 200401.
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We propose simple schemes that can perfectly identify projective measurement apparatuses secretly chosen from a finite set. Entanglement is used in these schemes both to make possible the perfect identification and to improve the efficiency significantly. Based on these results, a brief discussion on the problem of how to appropriately define distance measures of measurements is also provided.
Ji, Z., Feng, Y., Duan, R. & Ying, M. 2006, 'Boundary effect of deterministic dense coding', Physical Review A, vol. 73, no. 3.
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Feng, Y., Duan, R. & Ying, M. 2005, 'Catalyst-Assisted Probabilistic Entanglement Transformation', IEEE Transactions On Information Theory, vol. 51, no. 3, pp. 1090-1101.
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We are concerned with catalyst-assisted probabilistic entanglement transformations. A necessary and sufficient condition is presented under which there exist partial catalysts that can increase the maximal transforming probability of a given entanglement
Feng, Y., Duan, R. & Ji, Z. 2005, 'Condition And Capability Of Quantum State Separation', Physical Review A, vol. 72, no. 1, pp. 1-6.
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The linearity of quantum operations puts many fundamental constraints on the information processing tasks we can achieve on a quantum system whose state is not exactly known, just as we observe in quantum cloning and quantum discrimination. In this paper
Duan, R., Feng, Y., Ji, Z. & Ying, M. 2005, 'Efficiency Of Deterministic Entanglement Transformation', Physical Review A, vol. 71, no. 2, pp. 1-7.
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We prove that sufficiently many copies of a bipartite entangled pure state can always be transformed into some copies of another one with certainty by local quantum operations and classical communication. The efficiency of such a transformation is charac
Duan, R., Feng, Y., Li, X. & Ying, M. 2005, 'Multiple-Copy Entanglement Transformation And Entanglement Catalysis', Physical Review A, vol. 71, no. 4, pp. 1-11.
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We prove that any multiple-copy entanglement transformation [S. Bandyopadhyay, V. Roychowdhury, and U. Sen, Phys. Rev. A 65, 052315 (2002)] can be implemented by a suitable entanglement-assisted local transformation [D. Jonathan and M. B. Plenio, Phys. R
Sun, X., Duan, R. & Ying, M. 2005, 'The Existence Of Quantum Entanglement Catalysts', IEEE Transactions On Information Theory, vol. 51, no. 1, pp. 75-80.
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Without additional resources, it is often impossible to transform one entangled quantum state into another with local quantum operations and classical communication. Jonathan and Plenio (Phys. Rev. Lett., vol. 83, p. 3566, 1999) presented an interesting
Duan, R., Feng, Y., Li, X. & Ying, M. 2005, 'Trade-Off Between Multiple-Copy Transformation And Entanglement Catalysis', Physical Review A, vol. 71, no. 6, pp. 1-7.
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We demonstrate that multiple copies of a bipartite entangled pure state may serve as a catalyst for certain entanglement transformations while a single copy cannot. Such a state is termed a
Duan, R., Feng, Y. & Ying, M. 2005, 'Entanglement-Assisted Transformation Is Asymptotically Equivalent To Multiple-Copy Transformation', Physical Review A, vol. 72, no. 2, pp. 1-5.
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We show that two ways of manipulating quantum entanglement-namely, entanglement-assisted local transformation [D. Jonathan and M. B. Plenio, Phys. Rev. Lett. 83, 3566 (1999)] and multiple-copy transformation [S. Bandyopadhyay, V. Roychowdhury, and U. Sen
Duan, R., Feng, Y., Li, X. & Ying, M. 2005, 'Trade-off between multiple-copy transformation and entanglement catalysis', Physical Review A, vol. 71, no. 6.
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Duan, R., Feng, Y., Li, X. & Ying, M. 2005, 'Multiple-copy entanglement transformation and entanglement catalysis', Physical Review A, vol. 71, no. 4.
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Duan, R., Feng, Y., Ji, Z. & Ying, M. 2005, 'Efficiency of deterministic entanglement transformation', Physical Review A, vol. 71, no. 2.
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Duan, R., Feng, Y. & Ying, M. 2005, 'Entanglement-assisted transformation is asymptotically equivalent to multiple-copy transformation', Physical Review A, vol. 72, no. 2.
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Feng, Y., Duan, R., Ji, Z.-.F. & Ying, M. 2005, 'Proof rules for purely quantum programs', CoRR, vol. abs/cs/0507043.
Ji, Z., Duan, R. & Ying, M. 2004, 'Comparability Of Multipartite Entanglement', Physics Letters A, vol. 330, no. 6, pp. 418-423.
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We prove, in a multipartite setting, that it is always feasible to exactly transform a genuinely m-partite entangled pure state with sufficient many copies to any other m-partite state via local quantum operation and classical communication. This result
Duan, R., Ji, Z., Feng, Y. & Ying, M. 2004, 'Quantum Operation Quantum Fourier Transform And Semi-Definite Programming', Physics Letters A, vol. 323, no. 1-2, pp. 48-56.
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We analyze a class of quantum operations based on a geometrical representation of d-level quantum system (or qudit for short). A sufficient and necessary condition of complete positivity, expressed in terms of the quantum Fourier transform, is found for
Feng, Y., Duan, R. & Ying, M. 2004, 'When Catalysis Is Useful For Probabilistic Entanglement Transformation', Physical Review A, vol. 69, no. 6, pp. 1-5.
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We determine all 2x2 quantum states that can serve as useful catalysts for a given probabilistic entanglement transformation, in the sense that they can increase the maximal transformation probability. When higher-dimensional catalysts are considered, a
Feng, Y.A., Duan, R.Y. & Ying, M.S. 2004, 'Unambiguous discrimination between mixed quantum states', PHYSICAL REVIEW A, vol. 70, no. 1.
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Feng, Y., Duan, R. & Ying, M. 2004, 'When catalysis is useful for probabilistic entanglement transformation', Physical Review A - Atomic, Molecular, and Optical Physics, vol. 69, no. 6, pp. 062310-062311.
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The quantum states that served as useful catalysts for a probabilistic entanglement transformation were discussed. It was stated that the quantum states provided maximum transformation probability. A sufficiently necessary condition was derived when higher-dimensional catalysts were considered. It was found that among these conditions certain probabilistic transformation had useful catalysts.
Feng, Y., Zhang, S., Duan, R. & Ying, M. 2002, 'Lower Bound On Inconclusive Probability Of Unambiguous Discrimination', Physical Review A, vol. 66, no. 6, pp. 1-4.
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We derive a lower bound on the inconclusive probability of unambiguous discrimination among n linearly independent quantum states by using the constraint of no signaling. It improves the bound presented in the. paper of Zhang, Feng, Sun, and Ying [Phys.
Duan, R., 'Super-Activation of Zero-Error Capacity of Noisy Quantum Channels'.
We study various super-activation effects in the following zero-error communication scenario: One sender wants to send classical or quantum information through a noisy quantum channel to one receiver with zero probability of error. First we show that there are quantum channels of which a single use is not able to transmit classical information perfectly yet two uses can. This is achieved by employing entangled input states between different uses of the given channel and thus cannot happen for classical channels. Second we exhibit a class of quantum channel with vanishing zero-error classical capacity such that when a noiseless qubit channel or one ebit shared entanglement are available, it can be used to transmit $\log_2 d$ noiseless qubits, where 2d is the dimension of input state space. Third we further construct quantum channels with vanishing zero-error classical capacity when assisted with classical feedback can be used to transmit both classical and quantum information perfectly. These striking findings not only indicate both the zero-error quantum and classical capacities of quantum channels satisfy a strong super-additivity beyond any classical channels, but also highlight the activation power of auxiliary physical resources in zero-error communication.
Xin, Y. & Duan, R., 'Local distinguishability of orthogonal 2\otimes3 pure states', Phys.Rev.A, vol. 77, p. 012315.
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We present a complete characterization for the local distinguishability of orthogonal $2\otimes 3$ pure states except for some special cases of three states. Interestingly, we find there is a large class of four or three states that are indistinguishable by local projective measurements and classical communication (LPCC) can be perfectly distinguishable by LOCC. That indicates the ability of LOCC for discriminating $2\otimes 3$ states is strictly more powerful than that of LPCC, which is strikingly different from the case of multi-qubit states. We also show that classical communication plays a crucial role for local distinguishability by constructing a class of $m\otimes n$ states which require at least $2\min\{m,n\}-2$ rounds of classical communication in order to achieve a perfect local discrimination.
Xin, Y. & Duan, R., 'Conditions for entanglement transformation between a class of multipartite pure states with generalized Schmidt decompositions', Phys.Rev.A, vol. 76, p. 044301.
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In this note we generalize Nielsen's marjoization criterion for the convertibility of bipartite pure states [Phys. Rev. Lett \textbf{83}, 436(1999)] to a special class of multipartite pure states which have generalized Schmidt decompositions.
Duan, R. & Shi, Y., 'Entanglement between Two Uses of a Noisy Multipartite Quantum Channel Enables Perfect Transmission of Classical Information', Phys. Rev. Lett., vol. 101, p. 020501.
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Suppose that $m$ senders want to transmit classical information to $n$ receivers with zero probability of error using a noisy multipartite communication channel. The senders are allowed to exchange classical, but not quantum, messages among themselves, and the same holds for the receivers. If the channel is classical, a single use can transmit information if and only if multiple uses can. In sharp contrast, we exhibit, for each $m$ and $n$ with $m\ge 2$ or $n\ge 2$, a quantum channel of which a single use is not able to transmit information yet two uses can. This latter property requires and is enabled by quantum entanglement.
Duan, R., Feng, Y. & Ying, M., 'An Equivalence of Entanglement-Assisted Transformation and Multiple-Copy Entanglement Transformation'.
We examine the powers of entanglement-assisted transformation and multiple-copy entanglement transformation. First, we find a sufficient condition of when a given catalyst is useful in producing another specific target state. As an application of this condition, for any non-maximally entangled bipartite pure state and any integer $n$ not less than 4, we are able to explicitly construct a set of $n\times n$ quantum states which can be produced by using the given state as a catalyst. Second, we prove that for any positive integer $k$, entanglement-assisted transformation with $k\times k$-dimensional catalysts is useful in producing a target state if and only if multiple-copy entanglement transformation with $k$ copies of state is useful in producing the same target. Moreover, a necessary and sufficient condition for both of them is obtained in terms of the Schmidt coefficients of the target. This equivalence of entanglement-assisted transformation and multiple-copy entanglement transformation implies many interesting properties of entanglement transformation. Furthermore, these results are generalized to the case of probabilistic entanglement transformations.
Duan, R., Feng, Y. & Ying, M., 'Partial Recovery of Quantum Entanglement', IEEE Trans. Inform. Theory, vol. 52, p. 7.
Suppose Alice and Bob try to transform an entangled state shared between them into another one by local operations and classical communications. Then in general a certain amount of entanglement contained in the initial state will decrease in the process of transformation. However, an interesting phenomenon called partial entanglement recovery shows that it is possible to recover some amount of entanglement by adding another entangled state and transforming the two entangled states collectively. In this paper we are mainly concerned with the feasibility of partial entanglement recovery. The basic problem we address is whether a given state is useful in recovering entanglement lost in a specified transformation. In the case where the source and target states of the original transformation satisfy the strict majorization relation, a necessary and sufficient condition for partial entanglement recovery is obtained. For the general case we give two sufficient conditions. We also give an efficient algorithm for the feasibility of partial entanglement recovery in polynomial time. As applications, we establish some interesting connections between partial entanglement recovery and the generation of maximally entangled states, quantum catalysis, mutual catalysis, and multiple-copy entanglement transformation.
Feng, Y., Duan, R. & Ying, M., 'Catalyst-assisted Probabilistic Entanglement Transformation', IEEE Trans. Inform. Theory, vol. 51, p. 3.
We are concerned with catalyst-assisted probabilistic entanglement transformations. A necessary and sufficient condition is presented under which there exist partial catalysts that can increase the maximal transforming probability of a given entanglement transformation. We also design an algorithm which leads to an efficient method for finding the most economical partial catalysts with minimal dimension. The mathematical structure of catalyst-assisted probabilistic transformation is carefully investigated.
Feng, Y., Duan, R. & Ji, Z., 'Condition and capability of quantum state separation', Phys. Rev. A, vol. 72, p. 012313.
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The linearity of quantum operations puts many fundamental constraints on the information processing tasks we can achieve on a quantum system whose state is not exactly known, just as we observe in quantum cloning and quantum discrimination. In this paper we show that in probabilistic manner, linearity is in fact the only one that restricts the physically realizable tasks. To be specific, if a system is prepared in a state secretly chosen from a linearly independent pure state set, then any quantum state separation can be physically realized with a positive probability. Furthermore, we derive a lower bound on the average failure probability of any quantum state separation.
Feng, Y., Duan, R. & Ji, Z., 'Optimal dense coding with arbitrary pure entangled states', Phys. Rev. A, vol. 74, p. 012310.
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We examine dense coding with an arbitrary pure entangled state sharing between the sender and the receiver. Upper bounds on the average success probability in approximate dense coding and on the probability of conclusive results in unambiguous dense coding are derived. We also construct the optimal protocol which saturates the upper bound in each case.
Lu, C., Chen, J. & Duan, R., 'Optimal Perfect Distinguishability between Unitaries and Quantum Operations'.
We study optimal perfect distinguishability between a unitary and a general quantum operation. In 2-dimensional case we provide a simple sufficient and necessary condition for sequential perfect distinguishability and an analytical formula of optimal query time. We extend the sequential condition to general d-dimensional case. Meanwhile, we provide an upper bound and a lower bound for optimal sequential query time. In the process a new iterative method is given, the most notable innovation of which is its independence to auxiliary systems or entanglement. Following the idea, we further obtain an upper bound and a lower bound of (entanglement-assisted) q-maximal fidelities between a unitary and a quantum operation. Thus by the recursion in [1] an upper bound and a lower bound for optimal general perfect discrimination are achieved. Finally our lower bound result can be extended to the case of arbitrary two quantum operations.
Yu, N., Duan, R. & Ying, M., 'Optimal Simulation of a Perfect Entangler', Phys. Rev. A, vol. 81, p. 032328.
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A $2\otimes 2$ unitary operation is called a perfect entangler if it can generate a maximally entangled state from some unentangled input. We study the following question: How many runs of a given two-qubit entangling unitary operation is required to simulate some perfect entangler with one-qubit unitary operations as free resources? We completely solve this problem by presenting an analytical formula for the optimal number of runs of the entangling operation. Our result reveals an entanglement strength of two-qubit unitary operations.
Chitambar, E., Duan, R. & Shi, Y., 'Tripartite to Bipartite Entanglement Transformations and Polynomial Identity Testing', Phys. Rev. A, vol. 81, p. 052310.
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We consider the problem of deciding if a given three-party entangled pure state can be converted, with a non-zero success probability, into a given two-party pure state through local quantum operations and classical communication. We show that this question is equivalent to the well-known computational problem of deciding if a multivariate polynomial is identically zero. Efficient randomized algorithms developed to study the latter can thus be applied to the question of tripartite to bipartite entanglement transformations.
Yu, N., Chitambar, E., Guo, C. & Duan, R., 'The Tensor Rank of the Tripartite State $\ket{W}^{\otimes n}$}', Phys. Rev. A, vol. 81, p. 014301.
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Tensor rank refers to the number of product states needed to express a given multipartite quantum state. Its non-additivity as an entanglement measure has recently been observed. In this note, we estimate the tensor rank of multiple copies of the tripartite state $\ket{W}=\tfrac{1}{\sqrt{3}}(\ket{100}+\ket{010}+\ket{001})$. Both an upper bound and a lower bound of this rank are derived. In particular, it is proven that the tensor rank of $\ket{W}^{\otimes 2}$ is seven, thus resolving a previously open problem. Some implications of this result are discussed in terms of transformation rates between $\ket{W}^{\otimes n}$ and multiple copies of the state $\ket{GHZ}=\tfrac{1}{\sqrt{2}}(\ket{000}+\ket{111})$.
Yu, N., Duan, R. & Xu, Q., 'Bounds on the distance between a unital quantum channel and the convex hull of unitary channels, with applications to the asymptotic quantum Birkhoff conjecture'.
Motivated by the recent resolution of Asymptotic Quantum Birkhoff Conjecture (AQBC), we attempt to estimate the distance between a given unital quantum channel and the convex hull of unitary channels. We provide two lower bounds on this distance by employing techniques from quantum information and operator algebras, respectively. We then show how to apply these results to construct some explicit counterexamples to AQBC. We also point out an interesting connection between the Grothendieck's inequality and AQBC.
Chen, J., Chen, X., Duan, R., Ji, Z. & Zeng, B., 'No-go Theorem for One-way Quantum Computing on Naturally Occurring Two-level Systems', Phys. Rev. A, vol. 83, p. 050301.
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One-way quantum computing achieves the full power of quantum computation by performing single particle measurements on some many-body entangled state, known as the resource state. As single particle measurements are relatively easy to implement, the preparation of the resource state becomes a crucial task. An appealing approach is simply to cool a strongly correlated quantum many-body system to its ground state. In addition to requiring the ground state of the system to be universal for one-way quantum computing, we also want the Hamiltonian to have non-degenerate ground state protected by a fixed energy gap, to involve only two-body interactions, and to be frustration-free so that measurements in the course of the computation leave the remaining particles in the ground space. Recently, significant efforts have been made to the search of resource states that appear naturally as ground states in spin lattice systems. The approach is proved to be successful in spin-5/2 and spin-3/2 systems. Yet, it remains an open question whether there could be such a natural resource state in a spin-1/2, i.e., qubit system. Here, we give a negative answer to this question by proving that it is impossible for a genuinely entangled qubit states to be a non-degenerate ground state of any two-body frustration-free Hamiltonian. What is more, we prove that every spin-1/2 frustration-free Hamiltonian with two-body interaction always has a ground state that is a product of single- or two-qubit states, a stronger result that is interesting independent of the context of one-way quantum computing.