In the quest to reboot computing, quantum annealing (QA) is an interesting candidate for a new capability. While it has not demonstrated an advantage over classical computing on a real-worldapplication, many important regions of the QA design space have yet to be explored. In IARPA’s Quantum Enhanced Optimization (QEO) program, we have opened some new lines of inquiry to get to the heart of QA, and are designing testbed superconducting circuits and conducting key experiments. In this paper, we discuss recent experimental progress related to one of the key design dimensions: qubit coherence. Using MIT Lincoln Laboratory’s qubit fabrication process and extending recent progress in flux qubits, we are implementing and measuring QA-capable flux qubits. Achieving high coherence in a QA context presents significant new engineering challenges. We report on techniques and preliminary measurement results addressing two of the challenges: crosstalk calibration and qubit readout. This groundwork enables exploration of other promising features and provides a path to understanding the physics and the viability of quantum annealing as a computing resource.
We revisit the evidence for quantum annealing in the D-Wave One device (DW1) based on the study of random Ising instances. Using the probability distributions of finding the groundstates of such instances, previous work found agreement with both simulated quantum annealing (SQA) and a classical rotor model. Thus the DW1 ground state success probabilities are consistent with both models, and a different measure is needed to distinguish the data and the models. Here we consider measures that account for ground state degeneracy and the distributions of excited states, and present evidence that for these new measures neither SQA nor the classical rotor model correlate perfectly with the DW1 experiments. We thus provide evidence that SQA and the classical rotor model, both of which are classically efficient algorithms, do not satisfactorily explain all the DW1 data. A complete model for the DW1 remains an open problem. Using the same criteria we find that, on the other hand, SQA and the classical rotor model correlate closely with each other. To explain this we show that the rotor model can be derived as the semiclassical limit of the spin-coherent states path integral. We also find differences in which set of ground states is found by each method, though this feature is sensitive to calibration errors of the DW1 device and to simulation parameters.
Recently the question of whether the D-Wave processors exhibit large-scale quantum behavior or can be described by a classical model has attracted significant interest. In this workwe address this question by studying a 503 qubit D-Wave Two device as a „black box“, i.e., by studying its input-output behavior. We examine three candidate classical models and one quantum model, and compare their predictions to experiments we have performed on the device using groups of up to 40 qubits. The candidate classical models are simulated annealing, spin dynamics, a recently proposed hybrid O(2) rotor-Monte Carlo model, and three modified versions thereof. The quantum model is an adiabatic Markovian master equation derived in the weak coupling limit of an open quantum system. Our experiments realize an evolution from a transverse field to an Ising Hamiltonian, with a final-time degenerate ground state that splits into two types of states we call „isolated“ and „clustered“. We study the population ratio of the isolated and clustered states as a function of the overall energy scale of the Ising term, and the distance between the final state and the Gibbs state, and find that these are sensitive probes that distinguish the classical models from one another and from both the experimental data and the master equation. The classical models are all found to disagree with the data, while the master equation agrees with the experiment without fine-tuning, and predicts mixed state entanglement at intermediate evolution times. This suggests that an open system quantum dynamical description of the D-Wave device is well-justified even in the presence of relevant thermal excitations and fast single-qubit decoherence.
Quantum information processing offers dramatic speedups, yet is famously susceptible to decoherence, the process whereby quantum superpositions decay into mutually exclusive classicalalternatives, thus robbing quantum computers of their power. This has made the development of quantum error correction an essential and inescapable aspect of both theoretical and experimental quantum computing. So far little is known about protection against decoherence in the context of quantum annealing, a computational paradigm which aims to exploit ground state quantum dynamics to solve optimization problems more rapidly than is possible classically. Here we develop error correction for quantum annealing and provide an experimental demonstration using up to 344 superconducting flux qubits in processors which have recently been shown to physically implement programmable quantum annealing. We demonstrate a substantial improvement over the performance of the processors in the absence of error correction. These results pave a path toward large scale noise-protected adiabatic quantum optimization devices.
Quantum annealing is a general strategy for solving difficult optimization
problems with the aid of quantum adiabatic evolution. Both analytical and
numerical evidence suggests thatunder idealized, closed system conditions,
quantum annealing can outperform classical thermalization-based algorithms such
as simulated annealing. Do engineered quantum annealing devices effectively
perform classical thermalization when coupled to a decohering thermal
environment? To address this we establish, using superconducting flux qubits
with programmable spin-spin couplings, an experimental signature which is
consistent with quantum annealing, and at the same time inconsistent with
classical thermalization, in spite of a decoherence timescale which is orders
of magnitude shorter than the adiabatic evolution time. This suggests that
programmable quantum devices, scalable with current superconducting technology,
implement quantum annealing with a surprising robustness against noise and
imperfections.