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arXiv:1701.06544 · Physical Review Applied

Coherent Coupled Qubits for Quantum Annealing

Steven J. Weber, Gabriel Samach, David J. Hover, Simon Gustavsson, David K. Kim, Alexander J. Melville, Danna Rosenberg, Adam Sears, Fei Yan, Jonilyn Yoder, William D. Oliver, Andrew J. Kerman

Published Jul 10, 2017• 115 citations
arXivHTMLPDF

Abstract

Although quantum annealing has received considerable interest as a potential computing paradigm, it has not met the key criterion of improved scaling over classical methods. Qubit coherence is currently a major limitation. Starting from a detailed theoretical understanding of the system, the authors demonstrate tunably coupled flux qubits with a circuit geometry similar to those used in existing annealers, but with coherence times that are roughly two orders of magnitude longer. Their design concepts are not limited to superconducting qubits, and could be generalized to other types being considered for quantum annealing.

DOI
10.1103/physrevapplied.8.014004

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Although quantum annealing has received considerable interest as a potential computing paradigm, it has not met the key criterion of improved scaling over classical methods. Qubit coherence is currently a major limitation. Starting from a detailed theoretical understanding of the system, the authors demonstrate tunably coupled flux qubits with a circuit geometry similar to those used in existing annealers, but with coherence times that are roughly two orders of magnitude longer. Their design concepts are not limited to superconducting qubits, and could be generalized to other types being considered for quantum annealing.
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Quantum annealing is a method to solve optimization problems that leverages quantum tunneling in a coupled qubit system. We present a detailed study of the coherence of a tunable capacitively-shunted flux qubit, designed for coherent quantum annealing applications. We find that for high qubit frequencies, thermal noise in the bias line makes a significant contribution to the relaxation, arising from the design choice to experimentally explore both fast annealing and high-frequency control. The measured dephasing rate is primarily due to intrinsic low-frequency flux noise in the two qubit loops, with additional contribution from the low-frequency noise of control electronics used for fast annealing. Our results characterize decoherence in a realistic setup for quantum annealing and are relevant for ongoing efforts toward building superconducting quantum annealers with increased coherence. Quantum annealers hold promise of outperforming classical computers in solving hard optimization problems, but one main challenge is understanding the role of noise in quantum annealing. Here, the authors characterize the relevant noise sources in a tunable flux qubit, a building block for quantum annealers, and provide a benchmark for future work on highly-coherent quantum annealers.
Flux qubits (also known as persistent current qubits), one kind of superconducting tunnel junction circuits, are recently demonstrated as a promising candidate to realize quantum computations.In order to do large-scale quantum information processing, it is necessary to switch on and off the couplings between individual flux qubits in the time-domain while those qubits retain coherence.This report contains two parts: one is to describe my understanding of models, fabrications and measurements for one single flux qubit; the other is to compare two proposed schemes and the corresponding experimental realizations of coherent tunable couplings between flux qubits.
The superconducting quantum annealing circuit is constructed with four Nb layers and Josephson junctions with a critical current density of 1 μA/μm2. Three key points exist to perform the quantum annealing in experiments. First, the shape of the energy potential of the qubit should be accurately designed. Second, the current pulses for modulation of the shape of the energy potential must propagate to the qubit accurately. Third is the suppression of noise in the environment where the qubit is placed. Furthermore, calibration of the measurement data is necessary with consideration of the experimental environment. In this paper, I will present a mechanism for realizing quantum annealing from two perspectives: a theoretical model and an experiment. Experimental findings on quantum annealing will contribute to study of the surrounding circuit of a gate-type quantum computer.
The field of quantum information has been a fast growing field of research in the last few decades. The reason for this development is the potentially immense power of quantum computers, capable of solving hard problems in a fraction of the time it would take a classical computer. Quantum annealers are a type of quantum computer that seek to solve computational problems by finding the ground state of a quantum system. This type of quantum computer has been implemented on a large scale using superconducting flux quantum bits, but the computational power is limited. One potential improvement to help in this context is the implementation of multi-body interactions between quantum bits. Many-body interactions would be useful in allowing quantum annealers to solve more complex problems that are relevant in various areas of quantum information and science and implementing quantum error correction. \n \nWe present a superconducting device that implements a strong and tunable in sign and magnitude three-body interaction between superconducting flux quantum bits. The circuit design proposed has vanishing two-body interactions and robustness against noise and parameter variations. This circuit behaves as an ideal computational basis ZZZ coupler in a simulated three-qubit quantum annealing experiment. These properties are confirmed by calculations based on the Born-Oppenheimer approximation, a two-level spin model for the coupling circuit, and full numerical diagonalization. \n \nThe proposed circuit is based on already available technology and therefore could be readily implemented in the next generation of quantum annealing hardware. This work will be relevant for advanced quantum annealing protocols and future developments of high-order many-body interactions in quantum computers and simulators.
Abstract Quantum annealing (QA) provides us with a way to solve combinatorial optimization problems. In the previous demonstration of the QA, a superconducting flux qubit (FQ) was used. However, the FQs in these demonstrations have a short coherence time such as tens of nano seconds. For the purpose to utilize quantum properties, it is necessary to use another qubit with a better coherence time. Here, we propose to use a capacitive-shunted flux qubit (CSFQ) for the implementation of the QA. The CSFQ has a few order of magnitude better coherence time than the FQ used in the QA. We theoretically show that, although it is difficult to perform the conventional QA with the CSFQ due to the form and strength of the interaction between the CSFQs, a spin-lock based QA can be implemented with the CSFQ even with the current technology. Our results pave the way for the realization of the practical QA that exploits quantum advantage with long-lived qubits.