786 research outputs found
Improved Soundness for QMA with Multiple Provers
We present three contributions to the understanding of QMA with multiple
provers:
1) We give a tight soundness analysis of the protocol of [Blier and Tapp,
ICQNM '09], yielding a soundness gap Omega(1/N^2). Our improvement is achieved
without the use of an instance with a constant soundness gap (i.e., without
using a PCP).
2) We give a tight soundness analysis of the protocol of [Chen and Drucker,
ArXiV '10], thereby improving their result from a monolithic protocol where
Theta(sqrt(N)) provers are needed in order to have any soundness gap, to a
protocol with a smooth trade-off between the number of provers k and a
soundness gap Omega(k^2/N), as long as k>=Omega(log N). (And, when
k=Theta(sqrt(N)), we recover the original parameters of Chen and Drucker.)
3) We make progress towards an open question of [Aaronson et al., ToC '09]
about what kinds of NP-complete problems are amenable to sublinear
multiple-prover QMA protocols, by observing that a large class of such examples
can easily be derived from results already in the PCP literature - namely, at
least the languages recognized by a non-deterministic RAMs in quasilinear time.Comment: 24 pages; comments welcom
Directionally-unbiased unitary optical devices in discrete-time quantum walks
The optical beam splitter is a widely-used device in photonics-based quantum information processing. Specifically, linear optical networks demand large numbers of beam splitters for unitary matrix realization. This requirement comes from the beam splitter property that a photon cannot go back out of the input ports, which we call “directionally-biased”. Because of this property, higher dimensional information processing tasks suffer from rapid device resource growth when beam splitters are used in a feed-forward manner. Directionally-unbiased linear-optical devices have been introduced recently to eliminate the directional bias, greatly reducing the numbers of required beam splitters when implementing complicated tasks. Analysis of some originally directional optical devices and basic principles of their conversion into directionally-unbiased systems form the base of this paper. Photonic quantum walk implementations are investigated as a main application of the use of directionally-unbiased systems. Several quantum walk procedures executed on graph networks constructed using directionally-unbiased nodes are discussed. A significant savings in hardware and other required resources when compared with traditional directionally-biased beam-splitter-based optical networks is demonstrated.Accepted manuscriptPublished versio
Magnetization Transfer by a Quantum Ring Device
We show that a tight-binding model device consisting of a laterally connected
ring at half filling in a tangent time-dependent magnetic field can in
principle be designed to pump a purely spin current. The process exploits the
spin-orbit interaction in the ring. This behavior is understood analytically
and found to be robust with respect to temperature and small deviations from
half filling.Comment: 4 figures, 1 typo correcte
Physics as Information Processing
I review some recent advances in foundational research at Pavia QUIT group.
The general idea is that there is only Quantum Theory without quantization
rules, and the whole Physics---including space-time and relativity--is emergent
from the quantum-information processing. And since Quantum Theory itself is
axiomatized solely on informational principles, the whole Physics must be
reformulated in information-theoretical terms: this is the "It from Bit of J.
A. Wheeler. The review is divided into four parts: a) the informational
axiomatization of Quantum Theory; b) how space-time and relativistic covariance
emerge from quantum computation; c) what is the information-theoretical meaning
of inertial mass and of , and how the quantum field emerges; d) an
observational consequence of the new quantum field theory: a mass-dependent
refraction index of vacuum. I will conclude with the research lines that will
follow in the immediate future.Comment: Work presented at the conference "Advances in Quantum Theory" held on
14-17 June 2010 at the Linnaeus University, Vaxjo, Swede
Statistical analysis on testing of an entangled state based on Poisson distribution framework
A hypothesis testing scheme for entanglement has been formulated based on the
Poisson distribution framework instead of the POVM framework. Three designs
were proposed to test the entangled states in this framework. The designs were
evaluated in terms of the asymptotic variance. It has been shown that the
optimal time allocation between the coincidence and anti-coincidence
measurement bases improves the conventional testing method. The test can be
further improved by optimizing the time allocation between the anti-coincidence
bases.Comment: This paper is an extended version of the theoretical part of v1 of
quant-ph/0603254.quant-ph/0603254 is revised so that it is more familiar to
experimentalist
Investigating the Global Properties of a Resource Theory of Contextuality
Resource theories constitute a powerful theoretical framework and a tool that
captures, in an abstract structure, pragmatic aspects of the most varied
theories and processes. For physical theories, while this framework deals
directly with questions about the concrete possibilities of carrying out tasks
and processes, resource theories also make it possible to recast these already
established theories on a new language, providing not only new perspectives on
the potential of physical phenomena as valuable resources for technological
development, for example, but they also provide insights into the very
foundations of these theories. In this work, we will investigate some
properties of a resource theory for quantum contextuality, an essential
characteristic of quantum phenomena that ensures the impossibility of
interpreting the results of quantum measurements as revealing properties that
are independent of the set of measurements being made. We will present the
resource theory to be studied and investigate certain global properties of this
theory using tools and methods that, although already developed and studied by
the community in other resource theories, had not yet been used to characterize
resource theories of contextuality. In particular, we will use the so called
cost and yield monotones, extending the results of reference Quantum 4, 280
(2020) to general contextuality scenarios.Comment: Preliminary version, comments welcom
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