90 research outputs found
Open system quantum annealing in mean field models with exponential degeneracy
Real life quantum computers are inevitably affected by intrinsic noise
resulting in dissipative non-unitary dynamics realized by these devices. We
consider an open system quantum annealing algorithm optimized for a realistic
analog quantum device which takes advantage of noise-induced thermalization and
relies on incoherent quantum tunneling at finite temperature. We analyze the
performance of this algorithm considering a p-spin model which allows for a
mean field quasicalssical solution and at the same time demonstrates the 1st
order phase transition and exponential degeneracy of states. We demonstrate
that finite temperature effects introduced by the noise are particularly
important for the dynamics in presence of the exponential degeneracy of
metastable states. We determine the optimal regime of the open system quantum
annealing algorithm for this model and find that it can outperform simulated
annealing in a range of parameters.Comment: 11 pages, 5 figure
Non-universal weak antilocalization effect in cubic topological Kondo insulators
We study the quantum correction to conductivity on the surface of cubic
topological Kondo insulators with multiple Dirac bands. We consider the model
of time-reversal invariant disorder which induces the scattering of the
electrons within the Dirac bands as well as between the bands. When only
intraband scattering is present we find three long-range diffusion modes which
lead to weak antilocalization correction to conductivity, which remains
independent of the microscopic details such as Fermi velocities and relaxation
times. Interband scattering gaps out two diffusion modes leaving only one
long-range mode. We find that depending on the value of the phase coherence
time, either three or only one long-range diffusion modes contribute to weak
localization correction rendering the quantum correction to conductivity
non-universal. We provide an interpretation for the results of the recent
transport experiments on samarium hexaboride where weak antilocalization has
been observed.Comment: 15 pages, 7 figure
A Blueprint for Demonstrating Quantum Supremacy with Superconducting Qubits
Long coherence times and high fidelity control recently achieved in scalable superconducting circuits paved the way for the growing number of experimental studies of many-qubit quantum coherent phenomena in these devices. Albeit full implementation of quantum error correction and fault tolerant quantum computation remains a challenge the near term pre-error correction devices could allow new fundamental experiments despite inevitable accumulation of errors. One such open question foundational for quantum computing is achieving the so called quantum supremacy, an experimental demonstration of a computational task that takes polynomial time on the quantum computer whereas the best classical algorithm would require exponential time and/or resources. It is possible to formulate such a task for a quantum computer consisting of less than a 100 qubits. The computational task we consider is to provide approximate samples from a non-trivial quantum distribution. This is a generalization for the case of superconducting circuits of ideas behind boson sampling protocol for quantum optics introduced by Arkhipov and Aaronson. In this presentation we discuss a proof-of-principle demonstration of such a sampling task on a 9-qubit chain of superconducting gmon qubits developed by Google. We discuss theoretical analysis of the driven evolution of the device resulting in output approximating samples from a uniform distribution in the Hilbert space, a quantum chaotic state. We analyze quantum chaotic characteristics of the output of the circuit and the time required to generate a sufficiently complex quantum distribution. We demonstrate that the classical simulation of the sampling output requires exponential resources by connecting the task of calculating the output amplitudes to the sign problem of the Quantum Monte Carlo method. We also discuss the detailed theoretical modeling required to achieve high fidelity control and calibration of the multi-qubit unitary evolution in the device. We use a novel cross-entropy statistical metric as a figure of merit to verify the output and calibrate the device controls. Finally, we demonstrate the statistics of the wave function amplitudes generated on the 9-gmon chain and verify the quantum chaotic nature of the generated quantum distribution. This verifies the implementation of the quantum supremacy protocol
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