LOCC distinguishability of unilaterally transformable quantum states

We consider the question of perfect local distinguishability of mutually orthogonal bipartite quantum states, with the property that every state can be specified by a unitary operator acting on the local Hilbert space of Bob. We show that if the states can be exactly discriminated by one-way LOCC where Alice goes first, then the unitary operators can also be perfectly distinguished by an orthogonal measurement on Bob's Hilbert space. We give examples of sets of N<=d maximally entangled states in $d \otimes d$ for d=4,5,6 that are not perfectly distinguishable by one-way LOCC. Interestingly for d=5,6 our examples consist of four and five states respectively. We conjecture that these states cannot be perfectly discriminated by two-way LOCC.Comment: Revised version, new proofs added; to appear in New Journal of Physic

Twist-teleportation based local discrimination of maximally entangled states

In this work, we study the local distinguishability of maximally entangled states (MESs). In particular, we are concerned with whether any fixed number of MESs can be locally distinguishable for sufficiently large dimensions. Fan and Tian \emph{et al.} have already obtained two satisfactory results for the generalized Bell states (GBSs) and the qudit lattice states when applied to prime or prime power dimensions. We construct a general twist-teleportation scheme for any orthonormal basis with MESs that is inspired by the method used in [Phys. Rev. A \textbf{70}, 022304 (2004)]. Using this teleportation scheme, we obtain a sufficient and necessary condition for one-way distinguishable sets of MESs, which include the GBSs and the qudit lattice states as special cases. Moreover, we present a generalized version of the results in [Phys. Rev. A \textbf{92}, 042320 (2015)] for the arbitrary dimensional case.Comment: 7 pages, 2 figure

Tight bounds on the distinguishability of quantum states under separable measurements

One of the many interesting features of quantum nonlocality is that the states of a multipartite quantum system cannot always be distinguished as well by local measurements as they can when all quantum measurements are allowed. In this work, we characterize the distinguishability of sets of multipartite quantum states when restricted to separable measurements -- those which contain the class of local measurements but nevertheless are free of entanglement between the component systems. We consider two quantities: The separable fidelity -- a truly quantum quantity -- which measures how well we can "clone" the input state, and the classical probability of success, which simply gives the optimal probability of identifying the state correctly. We obtain lower and upper bounds on the separable fidelity and give several examples in the bipartite and multipartite settings where these bounds are optimal. Moreover the optimal values in these cases can be attained by local measurements. We further show that for distinguishing orthogonal states under separable measurements, a strategy that maximizes the probability of success is also optimal for separable fidelity. We point out that the equality of fidelity and success probability does not depend on an using optimal strategy, only on the orthogonality of the states. To illustrate this, we present an example where two sets (one consisting of orthogonal states, and the other non-orthogonal states) are shown to have the same separable fidelity even though the success probabilities are different.Comment: 19 pages; published versio

Relativistic quantum coin tossing

A relativistic quantum information exchange protocol is proposed allowing two distant users to realize ``coin tossing'' procedure. The protocol is based on the point that in relativistic quantum theory reliable distinguishing between the two orthogonal states generally requires a finite time depending on the structure of these states.Comment: 6 pages, no figure

Distinguishability-based genuine nonlocality with genuine multipartite entanglement

A set of orthogonal multipartite quantum states is said to be distinguishability-based genuinely nonlocal (also genuinely nonlocal, for abbreviation) if the states are locally indistinguishable across any bipartition of the subsystems. This form of multipartite nonlocality, although more naturally arising than the recently popular "strong nonlocality" in the context of local distinguishability, receives much less attention. In this work, we study the distinguishability-based genuine nonlocality of a special type of genuinely multipartite entangled states -- the Greenberger-Horne-Zeilinger (GHZ)-like states. We first show that any 5 states of the three-qubit GHZ basis are genuinely nonlocal, while any 4 states of them are not. Then for more general tripartite systems, we present a universal bound about the cardinality for an arbitrary set of GHZ-like states to be genuinely nonlocal. Although not necessary, entanglement is believed to raise difficulty in state discrimination in many situations. In the literature, there has been lots of studies in favor of this perspective, including the efforts seeking for small nonlocal sets consisting of maximally entangled states in bipartite systems. Here in the tripartite case, where GHZ-like states are studied, we also find the existence of some small genuinely nonlocal sets: we show that the cardinality can scale down to linear in the local dimension d. This result not only substantiates the aforemention perspective in multipartite scenario, but also suggests that there might exist substantial difference between strong nonlocality and the normal distinguishability-based multipartite nonlocality.Comment: 13 pages, 1 figure, submitted to "New journal of physics" in Sep, 202