1,539 research outputs found
Quantum de Finetti Theorems under Local Measurements with Applications
Quantum de Finetti theorems are a useful tool in the study of correlations in
quantum multipartite states. In this paper we prove two new quantum de Finetti
theorems, both showing that under tests formed by local measurements one can
get a much improved error dependence on the dimension of the subsystems. We
also obtain similar results for non-signaling probability distributions. We
give the following applications of the results:
We prove the optimality of the Chen-Drucker protocol for 3-SAT, under the
exponential time hypothesis.
We show that the maximum winning probability of free games can be estimated
in polynomial time by linear programming. We also show that 3-SAT with m
variables can be reduced to obtaining a constant error approximation of the
maximum winning probability under entangled strategies of O(m^{1/2})-player
one-round non-local games, in which the players communicate O(m^{1/2}) bits all
together.
We show that the optimization of certain polynomials over the hypersphere can
be performed in quasipolynomial time in the number of variables n by
considering O(log(n)) rounds of the Sum-of-Squares (Parrilo/Lasserre) hierarchy
of semidefinite programs. As an application to entanglement theory, we find a
quasipolynomial-time algorithm for deciding multipartite separability.
We consider a result due to Aaronson -- showing that given an unknown n qubit
state one can perform tomography that works well for most observables by
measuring only O(n) independent and identically distributed (i.i.d.) copies of
the state -- and relax the assumption of having i.i.d copies of the state to
merely the ability to select subsystems at random from a quantum multipartite
state.
The proofs of the new quantum de Finetti theorems are based on information
theory, in particular on the chain rule of mutual information.Comment: 39 pages, no figure. v2: changes to references and other minor
improvements. v3: added some explanations, mostly about Theorem 1 and
Conjecture 5. STOC version. v4, v5. small improvements and fixe
The Quantum Frontier
The success of the abstract model of computation, in terms of bits, logical
operations, programming language constructs, and the like, makes it easy to
forget that computation is a physical process. Our cherished notions of
computation and information are grounded in classical mechanics, but the
physics underlying our world is quantum. In the early 80s researchers began to
ask how computation would change if we adopted a quantum mechanical, instead of
a classical mechanical, view of computation. Slowly, a new picture of
computation arose, one that gave rise to a variety of faster algorithms, novel
cryptographic mechanisms, and alternative methods of communication. Small
quantum information processing devices have been built, and efforts are
underway to build larger ones. Even apart from the existence of these devices,
the quantum view on information processing has provided significant insight
into the nature of computation and information, and a deeper understanding of
the physics of our universe and its connections with computation.
We start by describing aspects of quantum mechanics that are at the heart of
a quantum view of information processing. We give our own idiosyncratic view of
a number of these topics in the hopes of correcting common misconceptions and
highlighting aspects that are often overlooked. A number of the phenomena
described were initially viewed as oddities of quantum mechanics. It was
quantum information processing, first quantum cryptography and then, more
dramatically, quantum computing, that turned the tables and showed that these
oddities could be put to practical effect. It is these application we describe
next. We conclude with a section describing some of the many questions left for
future work, especially the mysteries surrounding where the power of quantum
information ultimately comes from.Comment: Invited book chapter for Computation for Humanity - Information
Technology to Advance Society to be published by CRC Press. Concepts
clarified and style made more uniform in version 2. Many thanks to the
referees for their suggestions for improvement
Analyzing three-player quantum games in an EPR type setup
We use the formalism of Clifford Geometric Algebra (GA) to develop an
analysis of quantum versions of three-player non-cooperative games. The quantum
games we explore are played in an Einstein-Podolsky-Rosen (EPR) type setting.
In this setting, the players' strategy sets remain identical to the ones in the
mixed-strategy version of the classical game that is obtained as a proper
subset of the corresponding quantum game. Using GA we investigate the outcome
of a realization of the game by players sharing GHZ state, W state, and a
mixture of GHZ and W states. As a specific example, we study the game of
three-player Prisoners' Dilemma.Comment: 21 pages, 3 figure
Analyzing three-player quantum games in an EPR type setup
We use the formalism of Clifford Geometric Algebra (GA) to develop an
analysis of quantum versions of three-player non-cooperative games. The quantum
games we explore are played in an Einstein-Podolsky-Rosen (EPR) type setting.
In this setting, the players' strategy sets remain identical to the ones in the
mixed-strategy version of the classical game that is obtained as a proper
subset of the corresponding quantum game. Using GA we investigate the outcome
of a realization of the game by players sharing GHZ state, W state, and a
mixture of GHZ and W states. As a specific example, we study the game of
three-player Prisoners' Dilemma.Comment: 21 pages, 3 figure
Stochastic resonance in Gaussian quantum channels
We determine conditions for the presence of stochastic resonance in a lossy
bosonic channel with a nonlinear, threshold decoding. The stochastic resonance
effect occurs if and only if the detection threshold is outside of a "forbidden
interval". We show that it takes place in different settings: when transmitting
classical messages through a lossy bosonic channel, when transmitting over an
entanglement-assisted lossy bosonic channel, and when discriminating channels
with different loss parameters. Moreover, we consider a setting in which
stochastic resonance occurs in the transmission of a qubit over a lossy bosonic
channel with a particular encoding and decoding. In all cases, we assume the
addition of Gaussian noise to the signal and show that it does not matter who,
between sender and receiver, introduces such a noise. Remarkably, different
results are obtained when considering a setting for private communication. In
this case the symmetry between sender and receiver is broken and the "forbidden
interval" may vanish, leading to the occurrence of stochastic resonance effects
for any value of the detection threshold.Comment: 17 pages, 6 figures. Manuscript improved in many ways. New results on
private communication adde
Quantum Cryptography Beyond Quantum Key Distribution
Quantum cryptography is the art and science of exploiting quantum mechanical
effects in order to perform cryptographic tasks. While the most well-known
example of this discipline is quantum key distribution (QKD), there exist many
other applications such as quantum money, randomness generation, secure two-
and multi-party computation and delegated quantum computation. Quantum
cryptography also studies the limitations and challenges resulting from quantum
adversaries---including the impossibility of quantum bit commitment, the
difficulty of quantum rewinding and the definition of quantum security models
for classical primitives. In this review article, aimed primarily at
cryptographers unfamiliar with the quantum world, we survey the area of
theoretical quantum cryptography, with an emphasis on the constructions and
limitations beyond the realm of QKD.Comment: 45 pages, over 245 reference
Continuous Variable Quantum Key Distribution with a Noisy Laser
Existing experimental implementations of continuous-variable quantum key
distribution require shot-noise limited operation, achieved with shot-noise
limited lasers. However, loosening this requirement on the laser source would
allow for cheaper, potentially integrated systems. Here, we implement a
theoretically proposed prepare-and-measure continuous-variable protocol and
experimentally demonstrate the robustness of it against preparation noise
stemming for instance from technical laser noise. Provided that direct
reconciliation techniques are used in the post-processing we show that for
small distances large amounts of preparation noise can be tolerated in contrast
to reverse reconciliation where the key rate quickly drops to zero. Our
experiment thereby demonstrates that quantum key distribution with
non-shot-noise limited laser diodes might be feasible.Comment: 10 pages, 6 figures. Corrected plots for reverse reconciliatio
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