14 research outputs found
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Dephasing with strings attached
Motivated by the existence of mobile low-energy excitations like domain walls
in one dimension or gauge-charged fractionalized particles in higher
dimensions, we compare quantum dynamics in the presence of weak Markovian
dephasing for a particle hopping on a chain and for an Ising domain wall whose
motion leaves behind a string of flipped spins. Exact solutions show that the
two models have near identical transport responses in the bulk. On the other
hand, in finite-length chains, the broadening of discrete spectral lines is
much more noticeable in the case of a domain wall. These results may be of
relevance to a broad class of systems including quasi-1D antiferromagnets,
polymer chains, and even retinal systems
Characterizing quantum supremacy in near-term devices
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. A critical question for quantum computing in the near future is whether quantum devices without error correction can perform a well-defined computational task beyond the capabilities of supercomputers. Such a demonstration of what is referred to as quantum supremacy requires a reliable evaluation of the resources required to solve tasks with classical approaches. Here, we propose the task of sampling from the output distribution of random quantum circuits as a demonstration of quantum supremacy. We extend previous results in computational complexity to argue that this sampling task must take exponential time in a classical computer. We introduce cross-entropy benchmarking to obtain the experimental fidelity of complex multiqubit dynamics. This can be estimated and extrapolated to give a success metric for a quantum supremacy demonstration. We study the computational cost of relevant classical algorithms and conclude that quantum supremacy can be achieved with circuits in a two-dimensional lattice of 7 × 7 qubits and around 40 clock cycles. This requires an error rate of around 0.5% for two-qubit gates (0.05% for one-qubit gates), and it would demonstrate the basic building blocks for a fault-tolerant quantum computer
Quantum Computing
Quantum mechanics---the theory describing the fundamental workings of
nature---is famously counterintuitive: it predicts that a particle can be in
two places at the same time, and that two remote particles can be inextricably
and instantaneously linked. These predictions have been the topic of intense
metaphysical debate ever since the theory's inception early last century.
However, supreme predictive power combined with direct experimental observation
of some of these unusual phenomena leave little doubt as to its fundamental
correctness. In fact, without quantum mechanics we could not explain the
workings of a laser, nor indeed how a fridge magnet operates. Over the last
several decades quantum information science has emerged to seek answers to the
question: can we gain some advantage by storing, transmitting and processing
information encoded in systems that exhibit these unique quantum properties?
Today it is understood that the answer is yes. Many research groups around the
world are working towards one of the most ambitious goals humankind has ever
embarked upon: a quantum computer that promises to exponentially improve
computational power for particular tasks. A number of physical systems,
spanning much of modern physics, are being developed for this task---ranging
from single particles of light to superconducting circuits---and it is not yet
clear which, if any, will ultimately prove successful. Here we describe the
latest developments for each of the leading approaches and explain what the
major challenges are for the future.Comment: 26 pages, 7 figures, 291 references. Early draft of Nature 464, 45-53
(4 March 2010). Published version is more up-to-date and has several
corrections, but is half the length with far fewer reference
Recommended from our members
Dephasing with strings attached
Motivated by the existence of mobile low-energy excitations like domain walls in one dimension (1D) or gauge-charged fractionalized particles in higher dimensions, we compare quantum dynamics in the presence of weak Markovian dephasing for a particle hopping on a chain and for an Ising domain wall whose motion leaves behind a string of flipped spins. Exact solutions show that the two models have near identical transport responses in the bulk. On the other hand, in finite-length chains, the broadening of discrete spectral lines is much more noticeable in the case of a domain wall. These results may be of relevance to a broad class of systems including quasi-1D antiferromagnets, polymer chains, and even retinal systems
Thermally activated escape of driven systems: the activation energy
Thermally activated escape in the presence of a periodic external field is investigated theoretically and through analogue experiments and digital simulations. The observed variation of the activation energy for escape with driving force parameters is accurately described by the logarithmic susceptibility (LS). The frequency dispersion of the LS is shown to differ markedly from the standard linear susceptibility. Experimental data on the dispersion are in quantitative agreement with the theory. Switching between different branches of the activation energy is demonstrated for a nonsinusoidal (biharmonic) force