For randomly selected couplers and fields, the D-Wave device typically yields a highly Boltzmann like distribution [ indicating equilibration. These equilibrated data however do notcontain much useful information about the dynamics which lead to equilibration. To illuminate the dynamics, special Hamiltonians can be chosen which contain large energy barriers. In this paper we generalize this approach by considering a class of Hamiltonians which map clusters of spin-like qubits into ’superspins‘, thereby creating an energy landscape where local minima are separated by large energy barriers. These large energy barriers allow us to observe signatures of the transverse field frozen. To study these systems, we assume that the these frozen spins are describes by the Kibble-Zurek mechanism which was originally developed to describe formation of topological defects in the early universe. It was soon realized that it also has applications in analogous superconductor systems and later realized to also be important for the transverse field Ising model . We demonstrate that these barriers block equilibration and yield a non-trivial distribution of qubit states in a regime where quantum effects are expected to be strong, suggesting that these data should contain signatures of whether the dynamics are fundamentally classical or quantum. We exhaustively study a class of 3×3 square lattice superspin Hamiltonians and compare the experimental results with those obtained by exact diagonalisation. We find that the best fit to the data occurs at finite transverse field. We further demonstrate that under the right conditions, the superspins can be relaxed to equilibrium, erasing the signature of the transverse field. These results are interesting for a number of reasons. They suggest a route to detect signatures of quantum mechanics on the device on a statistical level.
Quantum annealing provides a way of solving optimization problems by encoding them as Ising spin models which are implemented using physical qubits. The solution of the optimisationproblem then corresponds to the ground state of the system. Quantum tunnelling is harnessed to enable the system to move to the ground state in a potentially highly non-convex energy landscape. A major difficulty in encoding optimization problems in physical quantum annealing devices is the fact that many real world optimisation problems require interactions of higher connectivity as well as multi-body terms beyond the limitations of the physical hardware. In this work we address the question of how to implement multi-body interactions using hardware which natively only provides two-body interactions. The main result is an efficient circuit design of such multi-body terms using superconducting flux qubits. It is then shown how this circuit can be used as a unit cell of a scalable architecture by applying it to a recently proposed embedding technique for constructing an architecture of logical qubits with arbitrary connectivity using physical qubits which have nearest-neighbour four-body interactions.
In this paper we examine the use of an adiabatic quantum data transfer protocol to build a universal quantum computer. Single qubit gates are realized by using a bus protocol to transferqubits of information down a spin chain with a unitary twist. This twist arises from altered couplings on the chain corresponding to unitary rotations performed on one region of the chain. We show how a controlled NOT gate can be realized by using a control qubit with Ising type coupling. The method discussed here can be extended to non-adiabatic quantum bus protocols. We also examine the potential of realizing such a quantum computer by using superconducting flux qubits.
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.
We present a concept for performing direct parity measurements on three or more qubits in microwave structures with superconducting resonators coupled to Josephson-junction qubits.We write the quantum-eraser conditions that must be fulfilled for the parity measurements as requirements for the scattering phase shift of our microwave structure. We show that these conditions can be fulfilled with present-day devices. We present one particular scheme, implemented with two-dimensional cavity techniques, in which each qubit should be coupled equally to two different microwave cavities. The magnitudes of the couplings that are needed are in the range that has been achieved in current experiments. A quantum calculation indicates that the measurement is optimal if the scattering signal can be measured with near single photon sensitivity. A comparison with an extension of a related proposal from cavity optics is presented. We present a second scheme, for which a scalable implementation of the four-qubit parities of the surface quantum error correction code can be envisioned. It uses three-dimensional cavity structures, using cavity symmetries to achieve the necessary multiple resonant modes within a single resonant structure.