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arXiv:quant-ph/0506149 · International Journal of Quantum Information

DISTINGUISHING UNENTANGLED STATES WITH AN UNENTANGLED MEASUREMENT

William K. Wootters

Published Feb 1, 2006• 29 citations
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Abstract

In a 1991 paper, Asher Peres and the author theoretically analyzed a set of unentangled bipartite quantum states that could apparently be distinguished better by a global measurement than by any sequence of local measurements on the individual subsystems. The present paper returns to the same example, and shows that the best result so far achieved can alternatively be attained by a measurement that, while still global, is "unentangled" in the sense that the operator associated with each measurement outcome is a tensor product.

DOI
10.1142/s0219749906001724

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In a 1991 paper, Asher Peres and the author theoretically analyzed a set of unentangled bipartite quantum states that could apparently be distinguished better by a global measurement than by any sequence of local measurements on the individual subsystems. The present paper returns to the same example, and shows that the best result so far achieved can alternatively be attained by a measurement that, while still global, is "unentangled" in the sense that the operator associated with each measurement outcome is a tensor product.
Nonlocality without entanglement is a rather counterintuitive phenomenon in which information may be encoded entirely in product (unentangled) states of composite quantum systems in such a way that local measurement of the subsystems is not enough for optimal decoding. For simple examples of pure product states, the gap in performance is known to be rather small when arbitrary local strategies are allowed. Here we restrict to local strategies readily achievable with current technology: those requiring neither a quantum memory nor joint operations. We show that even for measurements on pure product states, there can be a large gap between such strategies and theoretically optimal performance. Thus, even in the absence of entanglement, physically realizable local strategies can be far from optimal for extracting quantum information.
Entanglement is a useful resource because some global operations cannot be locally implemented using classical communication. We prove a number of results about what is and what is not locally possible. We focus on orthogonal states, which can always be globally distinguished. We establish the necessary and sufficient conditions for a general set of 2 x 2 quantum states to be locally distinguishable, and for a general set of 2 x n quantum states to be distinguished given an initial measurement of the qubit. These results reveal a fundamental asymmetry to nonlocality, which is the origin of "nonlocality without entanglement," and we present a very simple proof of this phenomenon.
The entangled states of composite quantum systems are well studied. The particle-like nature of these systems also means that, while entangled, they can be physically separated and measurements performed on the separated subsystems. Measurements on the separated subsystems A and B should pertain to vectors in the local Hilbert space of each subsystem, but to date it has not been clear how to elucidate the relevant states of the separated subsystems because it is not obvious how to resolve them from states given in the tensor product basis, except for the separable states. Here it is shown that the projections of any general entangled state that are detected by measurements on the separated subsystems can be obtained considering the corresponding (cosets of) states in the free vector space from which the tensor product space is defined. The result eliminates the need to invoke random collapse, and from this perspective nonlocality arises because of the way measurements on each separated subsystem projects possible measurement outcomes.
We prove a tight and close-to-optimal lower bound on the effectiveness of local quantum measurements (without classical communication) at discriminating any two bipartite quantum states. Our result implies, for example, that any two orthogonal quantum states of a nA×nBn_{A}\times n_{B}nA​×nB​ bipartite quantum system can be discriminated via local measurements with an error probability no larger than 12(1−1cmin⁡{nA,nB})\frac {1}2 \left ({1 - \frac {1}{c \min \{n_{A}, n_{B}\}} }\right)21​(1−cmin{nA​,nB​}1​) , where 1≤c≤221\leq c\leq 2\sqrt {2}1≤c≤22​ is a universal constant, and our bound scales provably optimally with the local dimensions nA,nBn_{A},n_{B}nA​,nB​ . Mathematically, this is achieved by showing that the distinguishability norm ∥⋅∥LO\|\cdot \|_{ \mathrm {LO}}∥⋅∥LO​ associated with local measurements satisfies that ∥⋅∥1≤22min⁡{nA,nB}∥⋅∥LO\|\cdot \|_{1}\leq 2\sqrt {2} \min \{n_{A},n_{B}\} \|\cdot \|_{ \mathrm {LO}}∥⋅∥1​≤22​min{nA​,nB​}∥⋅∥LO​ , where ∥⋅∥1\|\cdot \|_{1}∥⋅∥1​ is the trace norm.
Although the solution, within standard quantum physics, of the problem of outcomes has been published several times, many authors continue to treat measurement as an unsolved fundamental dilemma. The solution lies in the formation of entangled subsystems, the non-local nature of the measurement state, and the resulting distinction between mixed-state local outcomes and the pure-state global outcome. Upon "measurement" (i.e. entanglement), the quantum system and its measurement apparatus both decohere and collapse into local mixed states while the unitarily-evolving global state remains coherent and un-collapsed. The states we observe are the local, collapsed states. Considerable experimental evidence supports this conclusion. Theoretical objections to this conclusion are rebutted, and a new perspective on measurement and entanglement is noted.
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 introduce a way to locally manipulate the original quantum system: either identify the state successfully or distill some pure entanglement. Remarkably, if many copies are available, we show that any finite set of entangled pure states, whether orthogonal or not, can be locally distinguished in this way, which further implies that pure entanglement can be deterministically extracted from unknown entanglement. These results make it clear why a large class of entangled bipartite quantum operations including unitary operations and measurements that are globally distinguishable can also be locally distinguishable: They can generate pure entanglement consistently.