2,358 research outputs found
A high bandwidth quantum repeater
We present a physical- and link-level design for the creation of entangled
pairs to be used in quantum repeater applications where one can control the
noise level of the initially distributed pairs. The system can tune
dynamically, trading initial fidelity for success probability, from high
fidelity pairs (F=0.98 or above) to moderate fidelity pairs. The same physical
resources that create the long-distance entanglement are used to implement the
local gates required for entanglement purification and swapping, creating a
homogeneous repeater architecture. Optimizing the noise properties of the
initially distributed pairs significantly improves the rate of generating
long-distance Bell pairs. Finally, we discuss the performance trade-off between
spatial and temporal resources.Comment: 5 page
Upper limits of particle emission from high-energy collision and reaction near a maximally rotating Kerr black hole
The center-of-mass energy of two particles colliding near the horizon of a
maximally rotating black hole can be arbitrarily high if the angular momentum
of either of the incident particles is fine-tuned, which we call a critical
particle. We study particle emission from such high-energy collision and
reaction in the equatorial plane fully analytically. We show that the
unconditional upper limit of the energy of the emitted particle is given by
218.6% of that of the injected critical particle, irrespective of the details
of the reaction and this upper limit can be realized for massless particle
emission. The upper limit of the energy extraction efficiency for this emission
as a collisional Penrose process is given by 146.6%, which can be realized in
the collision of two massive particles with optimized mass ratio. Moreover, we
analyze perfectly elastic collision, Compton scattering, and pair annihilation
and show that net positive energy extraction is really possible for these three
reactions. The Compton scattering is most efficient among them and the
efficiency can reach 137.2%. On the other hand, our result is qualitatively
consistent with the earlier claim that the mass and energy of the emitted
particle are at most of order the total energy of the injected particles and
hence we can observe neither super-heavy nor super-energetic particles.Comment: 22 pages, 3 figures, typos corrected, reference updated, accepted for
publication in Physical Review D, typos correcte
Local Probabilistic Decoding of a Quantum Code
flip is an extremely simple and maximally local classical decoder which has
been used to great effect in certain classes of classical codes. When applied
to quantum codes there exist constant-weight errors (such as half of a
stabiliser) which are uncorrectable for this decoder, so previous studies have
considered modified versions of flip, sometimes in conjunction with other
decoders. We argue that this may not always be necessary, and present numerical
evidence for the existence of a threshold for flip when applied to the looplike
syndromes of a three-dimensional toric code on a cubic lattice. This result can
be attributed to the fact that the lowest-weight uncorrectable errors for this
decoder are closer (in terms of Hamming distance) to correctable errors than to
other uncorrectable errors, and so they are likely to become correctable in
future code cycles after transformation by additional noise. Introducing
randomness into the decoder can allow it to correct these "uncorrectable"
errors with finite probability, and for a decoding strategy that uses a
combination of belief propagation and probabilistic flip we observe a threshold
of under phenomenological noise. This is comparable to the best
known threshold for this code () which was achieved using belief
propagation and ordered statistics decoding [Higgott and Breuckmann, 2022], a
strategy with a runtime of as opposed to the ( when
parallelised) runtime of our local decoder. We expect that this strategy could
be generalised to work well in other low-density parity check codes, and hope
that these results will prompt investigation of other previously overlooked
decoders.Comment: 10 pages + 1 page appendix, 7 figures. Comments welcome.; v3
Published versio
SU(N)-symmetric quasi-probability distribution functions
We present a set of N-dimensional functions, based on generalized
SU(N)-symmetric coherent states, that represent finite-dimensional Wigner
functions, Q-functions, and P-functions. We then show the fundamental
properties of these functions and discuss their usefulness for analyzing
N-dimensional pure and mixed quantum states.Comment: 16 pages, 2 figures. Updated text to reflect referee comment
Weak non-linearities and cluster states
We propose a scalable approach to building cluster states of matter qubits
using coherent states of light. Recent work on the subject relies on the use of
single photonic qubits in the measurement process. These schemes have a low
initial success probability and low detector efficiencies cause a serious
blowup in resources. In contrast, our approach uses continuous variables and
highly efficient measurements. We present a two-qubit scheme, with a simple
homodyne measurement system yielding an entangling operation with success
probability 1/2. Then we extend this to a three-qubit interaction, increasing
this probability to 3/4. We discuss the important issues of the overhead cost
and the time scaling, showing how these can be vastly improved with access to
this new probability range.Comment: 5 pages, to appear in Phys. Rev.
Modified TAP equations for the SK spin glass
The stability of the TAP mean field equations is reanalyzed with the
conclusion that the exclusive reason for the breakdown at the spin glass
instability is an inconsistency for the value of the local susceptibility. A
new alternative approach leads to modified equations which are in complete
agreement with the original ones above the instability. Essentially altered
results below the instability are presented and the consequences for the
dynamical mean field equations are discussed.Comment: 7 pages, 2 figures, final revised version to appear in Europhys. Let
Efficient optical quantum information processing
Quantum information offers the promise of being able to perform certain
communication and computation tasks that cannot be done with conventional
information technology (IT). Optical Quantum Information Processing (QIP) holds
particular appeal, since it offers the prospect of communicating and computing
with the same type of qubit. Linear optical techniques have been shown to be
scalable, but the corresponding quantum computing circuits need many auxiliary
resources. Here we present an alternative approach to optical QIP, based on the
use of weak cross-Kerr nonlinearities and homodyne measurements. We show how
this approach provides the fundamental building blocks for highly efficient
non-absorbing single photon number resolving detectors, two qubit parity
detectors, Bell state measurements and finally near deterministic control-not
(CNOT) gates. These are essential QIP devicesComment: Accepted to the Journal of optics B special issue on optical quantum
computation; References update
Qudit Quantum State Tomography
Recently quantum tomography has been proposed as a fundamental tool for
prototyping a few qubit quantum device. It allows the complete reconstruction
of the state produced from a given input into the device. From this
reconstructed density matrix, relevant quantum information quantities such as
the degree of entanglement and entropy can be calculated. Generally orthogonal
measurements have been discussed for this tomographic reconstruction. In this
paper, we extend the tomographic reconstruction technique to two new regimes.
First we show how non-orthogonal measurement allow the reconstruction of the
state of the system provided the measurements span the Hilbert space. We then
detail how quantum state tomography can be performed for multi qudits with a
specific example illustrating how to achieve this in one and two qutrit
systems.Comment: 6 pages, 4 figures, submitted to PR
Modified Thouless-Anderson-Palmer equations for the Sherrington-Kirkpatrick spin glass: Numerical solutions
For large but finite systems the static properties of the infinite ranged
Sherrington-Kirkpatrick model are numerically investigated in the entire the
glass regime. The approach is based on the modified Thouless-Anderson-Palmer
equations in combination with a phenomenological relaxational dynamics used as
a numerical tool. For all temperatures and all bond configurations stable and
meta stable states are found. Following a discussion of the finite size
effects, the static properties of the state of lowest free energy are presented
in the presence of a homogeneous magnetic field for all temperatures below the
spin glass temperature. Moreover some characteristic features of the meta
stable states are presented. These states exist in finite temperature intervals
and disappear via local saddle node bifurcations. Numerical evidence is found
that the excess free energy of the meta stable states remains finite in the
thermodynamic limit. This implies a the `multi-valley' structure of the free
energy on a sub-extensive scale.Comment: Revtex 10 pages 13 figures included, submitted to Phys.Rev.B.
Shortend and improved version with additional numerical dat
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