9 research outputs found
Nonlocality, Asymmetry, and Distinguishing Bipartite States
Entanglement is an useful resource because some global operations cannot be
locally implemented using classical communication. We prove a number of results
about what is and 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 2x2 quantum states to be locally
distinguishable, and for a general set of 2xn 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.Comment: 5 pages, 1 figure. Improved in line with referees comments,
references added, typo corrected. To appear in Phys. Rev. Let
Local Distinguishability of Multipartite Orthogonal Quantum States
We consider one copy of a quantum system prepared in one of two orthogonal
pure states, entangled or otherwise, and distributed between any number of
parties. We demonstrate that it is possible to identify which of these two
states the system is in by means of local operations and classical
communication alone. The protocol we outline is both completely reliable and
completely general - it will correctly distinguish any two orthogonal states
100% of the time.Comment: 5 pages, revte
Classical and quantum fingerprinting with shared randomness and one-sided error
Within the simultaneous message passing model of communication complexity,
under a public-coin assumption, we derive the minimum achievable worst-case
error probability of a classical fingerprinting protocol with one-sided error.
We then present entanglement-assisted quantum fingerprinting protocols
attaining worst-case error probabilities that breach this bound.Comment: 10 pages, 1 figur
Generic local distinguishability and completely entangled subspaces
A subspace of a multipartite Hilbert space is completely entangled if it
contains no product states. Such subspaces can be large with a known maximum
size, S, approaching the full dimension of the system, D. We show that almost
all subspaces with dimension less than or equal to S are completely entangled,
and then use this fact to prove that n random pure quantum states are
unambiguously locally distinguishable if and only if n does not exceed D-S.
This condition holds for almost all sets of states of all multipartite systems,
and reveals something surprising. The criterion is identical for separable and
for nonseparable states: entanglement makes no difference.Comment: 12 page
On the logical structure of Bell theorems without inequalities
Bell theorems show how to experimentally falsify local realism. Conclusive
falsification is highly desirable as it would provide support for the most
profoundly counterintuitive feature of quantum theory - nonlocality. Despite
the preponderance of evidence for quantum mechanics, practical limits on
detector efficiency and the difficulty of coordinating space-like separated
measurements have provided loopholes for a classical worldview; these loopholes
have never been simultaneously closed. A number of new experiments have
recently been proposed to close both loopholes at once. We show some of these
novel designs fail in the most basic way, by not ruling out local hidden
variable models, and we provide an explicit classical model to demonstrate
this. They share a common flaw, which reveals a basic misunderstanding of how
nonlocality proofs work. Given the time and resources now being devoted to such
experiments, theoretical clarity is essential. Our explanation is presented in
terms of simple logic and should serve to correct misconceptions and avoid
future mistakes. We also show a nonlocality proof involving four participants
which has interesting theoretical properties.Comment: 8 pages, text clarified, explicit LHV model provided for flawed
nonlocality tes
Local distinguishability of quantum states
EThOS - Electronic Theses Online ServiceGBUnited Kingdo