15,151 research outputs found
8x8 Reconfigurable quantum photonic processor based on silicon nitride waveguides
The development of large-scale optical quantum information processing
circuits ground on the stability and reconfigurability enabled by integrated
photonics. We demonstrate a reconfigurable 8x8 integrated linear optical
network based on silicon nitride waveguides for quantum information processing.
Our processor implements a novel optical architecture enabling any arbitrary
linear transformation and constitutes the largest programmable circuit reported
so far on this platform. We validate a variety of photonic quantum information
processing primitives, in the form of Hong-Ou-Mandel interference, bosonic
coalescence/anticoalescence and high-dimensional single-photon quantum gates.
We achieve fidelities that clearly demonstrate the promising future for
large-scale photonic quantum information processing using low-loss silicon
nitride.Comment: Added supplementary materials, extended introduction, new figures,
results unchange
Peak-to-average power ratio of good codes for Gaussian channel
Consider a problem of forward error-correction for the additive white
Gaussian noise (AWGN) channel. For finite blocklength codes the backoff from
the channel capacity is inversely proportional to the square root of the
blocklength. In this paper it is shown that codes achieving this tradeoff must
necessarily have peak-to-average power ratio (PAPR) proportional to logarithm
of the blocklength. This is extended to codes approaching capacity slower, and
to PAPR measured at the output of an OFDM modulator. As a by-product the
convergence of (Smith's) amplitude-constrained AWGN capacity to Shannon's
classical formula is characterized in the regime of large amplitudes. This
converse-type result builds upon recent contributions in the study of empirical
output distributions of good channel codes
Robust Control of Quantum Information
Errors in the control of quantum systems may be classified as unitary,
decoherent and incoherent. Unitary errors are systematic, and result in a
density matrix that differs from the desired one by a unitary operation.
Decoherent errors correspond to general completely positive superoperators, and
can only be corrected using methods such as quantum error correction.
Incoherent errors can also be described, on average, by completely positive
superoperators, but can nevertheless be corrected by the application of a
locally unitary operation that ``refocuses'' them. They are due to reproducible
spatial or temporal variations in the system's Hamiltonian, so that information
on the variations is encoded in the system's spatiotemporal state and can be
used to correct them. In this paper liquid-state nuclear magnetic resonance
(NMR) is used to demonstrate that such refocusing effects can be built directly
into the control fields, where the incoherence arises from spatial
inhomogeneities in the quantizing static magnetic field as well as the
radio-frequency control fields themselves. Using perturbation theory, it is
further shown that the eigenvalue spectrum of the completely positive
superoperator exhibits a characteristic spread that contains information on the
Hamiltonians' underlying distribution.Comment: 14 pages, 6 figure
Dynamic selection and estimation of the digital predistorter parameters for power amplifier linearization
© © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This paper presents a new technique that dynamically estimates and updates the coefficients of a digital predistorter (DPD) for power amplifier (PA) linearization. The proposed technique is dynamic in the sense of estimating, at every iteration of the coefficient's update, only the minimum necessary parameters according to a criterion based on the residual estimation error. At the first step, the original basis functions defining the DPD in the forward path are orthonormalized for DPD adaptation in the feedback path by means of a precalculated principal component analysis (PCA) transformation. The robustness and reliability of the precalculated PCA transformation (i.e., PCA transformation matrix obtained off line and only once) is tested and verified. Then, at the second step, a properly modified partial least squares (PLS) method, named dynamic partial least squares (DPLS), is applied to obtain the minimum and most relevant transformed components required for updating the coefficients of the DPD linearizer. The combination of the PCA transformation with the DPLS extraction of components is equivalent to a canonical correlation analysis (CCA) updating solution, which is optimum in the sense of generating components with maximum correlation (instead of maximum covariance as in the case of the DPLS extraction alone). The proposed dynamic extraction technique is evaluated and compared in terms of computational cost and performance with the commonly used QR decomposition approach for solving the least squares (LS) problem. Experimental results show that the proposed method (i.e., combining PCA with DPLS) drastically reduces the amount of DPD coefficients to be estimated while maintaining the same linearization performance.Peer ReviewedPostprint (author's final draft
Lightcone renormalization and quantum quenches in one-dimensional Hubbard models
The Lieb-Robinson bound implies that the unitary time evolution of an
operator can be restricted to an effective light cone for any Hamiltonian with
short-range interactions. Here we present a very efficient renormalization
group algorithm based on this light cone structure to study the time evolution
of prepared initial states in the thermodynamic limit in one-dimensional
quantum systems. The algorithm does not require translational invariance and
allows for an easy implementation of local conservation laws. We use the
algorithm to investigate the relaxation dynamics of double occupancies in
fermionic Hubbard models as well as a possible thermalization. For the
integrable Hubbard model we find a pure power-law decay of the number of doubly
occupied sites towards the value in the long-time limit while the decay becomes
exponential when adding a nearest neighbor interaction. In accordance with the
eigenstate thermalization hypothesis, the long-time limit is reasonably well
described by a thermal average. We point out though that such a description
naturally requires the use of negative temperatures. Finally, we study a
doublon impurity in a N\'eel background and find that the excess charge and
spin spread at different velocities, providing an example of spin-charge
separation in a highly excited state.Comment: published versio
Experimental demonstration of quantum effects in the operation of microscopic heat engines
The heat engine, a machine that extracts useful work from thermal sources, is
one of the basic theoretical constructs and fundamental applications of
classical thermodynamics. The classical description of a heat engine does not
include coherence in its microscopic degrees of freedom. By contrast, a quantum
heat engine might possess coherence between its internal states. Although the
Carnot efficiency cannot be surpassed, and coherence can be performance
degrading in certain conditions, it was recently predicted that even when using
only thermal resources, internal coherence can enable a quantum heat engine to
produce more power than any classical heat engine using the same resources.
Such a power boost therefore constitutes a quantum thermodynamic signature. It
has also been shown that the presence of coherence results in the thermodynamic
equivalence of different quantum heat engine types, an effect with no classical
counterpart. Microscopic heat machines have been recently implemented with
trapped ions, and proposals for heat machines using superconducting circuits
and optomechanics have been made. When operated with standard thermal baths,
however, the machines implemented so far have not demonstrated any inherently
quantum feature in their thermodynamic quantities. Here we implement two types
of quantum heat engines by use of an ensemble of nitrogen-vacancy centres in
diamond, and experimentally demonstrate both the coherence power boost and the
equivalence of different heat-engine types. This constitutes the first
observation of quantum thermodynamic signatures in heat machines
How quickly can anyons be braided? Or: How I learned to stop worrying about diabatic errors and love the anyon
Topological phases of matter are a potential platform for the storage and
processing of quantum information with intrinsic error rates that decrease
exponentially with inverse temperature and with the length scales of the
system, such as the distance between quasiparticles. However, it is less
well-understood how error rates depend on the speed with which non-Abelian
quasiparticles are braided. In general, diabatic corrections to the holonomy or
Berry's matrix vanish at least inversely with the length of time for the braid,
with faster decay occurring as the time-dependence is made smoother. We show
that such corrections will not affect quantum information encoded in
topological degrees of freedom, unless they involve the creation of
topologically nontrivial quasiparticles. Moreover, we show how measurements
that detect unintentionally created quasiparticles can be used to control this
source of error.Comment: 33 pages, 18 figures, version 3: extended results to general anyon
braidin
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