13,047 research outputs found
Noise-enhanced classical and quantum capacities in communication networks
The unavoidable presence of noise is thought to be one of the major problems
to solve in order to pave the way for implementing quantum information
technologies in realistic physical platforms. However, here we show a clear
example in which noise, in terms of dephasing, may enhance the capability of
transmitting not only classical but also quantum information, encoded in
quantum systems, through communication networks. In particular, we find
analytically and numerically the quantum and classical capacities for a large
family of quantum channels and show that these information transmission rates
can be strongly enhanced by introducing dephasing noise in the complex network
dynamics.Comment: 4 pages, 4 figures; See Video Abstract at
http://www.quantiki.org/video_abstracts/1003587
Entanglement Distillation Protocols and Number Theory
We show that the analysis of entanglement distillation protocols for qudits
of arbitrary dimension benefits from applying basic concepts from number
theory, since the set \zdn associated to Bell diagonal states is a module
rather than a vector space. We find that a partition of \zdn into divisor
classes characterizes the invariant properties of mixed Bell diagonal states
under local permutations. We construct a very general class of recursion
protocols by means of unitary operations implementing these local permutations.
We study these distillation protocols depending on whether we use twirling
operations in the intermediate steps or not, and we study them both
analitically and numerically with Monte Carlo methods. In the absence of
twirling operations, we construct extensions of the quantum privacy algorithms
valid for secure communications with qudits of any dimension . When is a
prime number, we show that distillation protocols are optimal both
qualitatively and quantitatively.Comment: REVTEX4 file, 7 color figures, 2 table
Enhancement of entanglement in one-dimensional disordered systems
The pairwise quantum entanglement of sites in disordered electronic
one-dimensional systems (rings) is studied. We focus on the effect of diagonal
and off diagonal disorder on the concurrence between electrons on
neighbor and non neighbor sites as a function of band filling. In the
case of diagonal disorder, increasing the degree of disorder leads to a
decrease of the concurrence with respect to the ordered case. However,
off-diagonal disorder produces a surprisingly strong enhancement of
entanglement. This remarkable effect occurs near half filling, where the
concurrence becomes up to 15% larger than in the ordered system.Comment: 21 pages, 9 figure
Thresholds for Linear Optics Quantum Computing with Photon Loss at the Detectors
We calculate the error threshold for the linear optics quantum computing
proposal by Knill, Laflamme and Milburn [Nature 409, pp. 46--52 (2001)] under
an error model where photon detectors have efficiency <100% but all other
components -- such as single photon sources, beam splitters and phase shifters
-- are perfect and introduce no errors. We make use of the fact that the error
model induced by the lossy hardware is that of an erasure channel, i.e., the
error locations are always known. Using a method based on a Markov chain
description of the error correction procedure, our calculations show that, with
the 7 qubit CSS quantum code, the gate error threshold for fault tolerant
quantum computation is bounded below by a value between 1.78% and 11.5%
depending on the construction of the entangling gates.Comment: 7 pages, 6 figure
Classical simulatability, entanglement breaking, and quantum computation thresholds
We investigate the amount of noise required to turn a universal quantum gate
set into one that can be efficiently modelled classically. This question is
useful for providing upper bounds on fault tolerant thresholds, and for
understanding the nature of the quantum/classical computational transition. We
refine some previously known upper bounds using two different strategies. The
first one involves the introduction of bi-entangling operations, a class of
classically simulatable machines that can generate at most bipartite
entanglement. Using this class we show that it is possible to sharpen
previously obtained upper bounds in certain cases. As an example, we show that
under depolarizing noise on the controlled-not gate, the previously known upper
bound of 74% can be sharpened to around 67%. Another interesting consequence is
that measurement based schemes cannot work using only 2-qubit non-degenerate
projections. In the second strand of the work we utilize the Gottesman-Knill
theorem on the classically efficient simulation of Clifford group operations.
The bounds attained for the pi/8 gate using this approach can be as low as 15%
for general single gate noise, and 30% for dephasing noise.Comment: 12 pages, 3 figures. v2: small typos changed, no change to result
Theory of Spin Relaxation in Two-Electron Lateral Coupled Si/SiGe Quantum Dots
Highly accurate numerical results of phonon-induced two-electron spin
relaxation in silicon double quantum dots are presented. The relaxation,
enabled by spin-orbit coupling and the nuclei of Si (natural or purified
abundance), are investigated for experimentally relevant parameters, the
interdot coupling, the magnetic field magnitude and orientation, and the
detuning. We calculate relaxation rates for zero and finite temperatures (100
mK), concluding that our findings for zero temperature remain qualitatively
valid also for 100 mK. We confirm the same anisotropic switch of the axis of
prolonged spin lifetime with varying detuning as recently predicted in GaAs.
Conditions for possibly hyperfine-dominated relaxation are much more stringent
in Si than in GaAs. For experimentally relevant regimes, the spin-orbit
coupling, although weak, is the dominant contribution, yielding anisotropic
relaxation rates of at least two order of magnitude lower than in GaAs.Comment: 11 pages, 10 figure
Preparing projected entangled pair states on a quantum computer
We present a quantum algorithm to prepare injective PEPS on a quantum
computer, a class of open tensor networks representing quantum states. The
run-time of our algorithm scales polynomially with the inverse of the minimum
condition number of the PEPS projectors and, essentially, with the inverse of
the spectral gap of the PEPS' parent Hamiltonian.Comment: 5 pages, 1 figure. To be published in Physical Review Letters.
Removed heuristics, refined run-time boun
Manifestly supersymmetric M-theory
In this paper, the low-energy effective dynamics of M-theory,
eleven-dimensional supergravity, is taken off-shell in a manifestly
supersymmetric formulation. We show that a previously proposed relaxation of
the superspace torsion constraints does indeed accommodate a current
supermultiplet which lifts the equations of motion corresponding to the
ordinary second order derivative supergravity lagrangian. Whether the auxiliary
fields obtained this way can be used to construct an off-shell lagrangian is
not yet known. We comment on the relation and application of this completely
general formalism to higher-derivative (R^4) corrections. Some details of the
calculation are saved for a later publication.Comment: 13 pages, plain tex. v2: minor changes, one ref. adde
The Bravyi-Kitaev transformation for quantum computation of electronic structure
Quantum simulation is an important application of future quantum computers
with applications in quantum chemistry, condensed matter, and beyond. Quantum
simulation of fermionic systems presents a specific challenge. The
Jordan-Wigner transformation allows for representation of a fermionic operator
by O(n) qubit operations. Here we develop an alternative method of simulating
fermions with qubits, first proposed by Bravyi and Kitaev [S. B. Bravyi, A.Yu.
Kitaev, Annals of Physics 298, 210-226 (2002)], that reduces the simulation
cost to O(log n) qubit operations for one fermionic operation. We apply this
new Bravyi-Kitaev transformation to the task of simulating quantum chemical
Hamiltonians, and give a detailed example for the simplest possible case of
molecular hydrogen in a minimal basis. We show that the quantum circuit for
simulating a single Trotter time-step of the Bravyi-Kitaev derived Hamiltonian
for H2 requires fewer gate applications than the equivalent circuit derived
from the Jordan-Wigner transformation. Since the scaling of the Bravyi-Kitaev
method is asymptotically better than the Jordan-Wigner method, this result for
molecular hydrogen in a minimal basis demonstrates the superior efficiency of
the Bravyi-Kitaev method for all quantum computations of electronic structure
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