Publications (5)52.66 Total impact
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ABSTRACT: Many interesting but practically intractable problems can be reduced to that of finding the ground state of a system of interacting spins; however, finding such a ground state remains computationally difficult. It is believed that the ground state of some naturally occurring spin systems can be effectively attained through a process called quantum annealing. If it could be harnessed, quantum annealing might improve on known methods for solving certain types of problem. However, physical investigation of quantum annealing has been largely confined to microscopic spins in condensedmatter systems. Here we use quantum annealing to find the ground state of an artificial Ising spin system comprising an array of eight superconducting flux quantum bits with programmable spinspin couplings. We observe a clear signature of quantum annealing, distinguishable from classical thermal annealing through the temperature dependence of the time at which the system dynamics freezes. Our implementation can be configured in situ to realize a wide variety of different spin networks, each of which can be monitored as it moves towards a lowenergy configuration. This programmable artificial spin network bridges the gap between the theoretical study of ideal isolated spin networks and the experimental investigation of bulk magnetic samples. Moreover, with an increased number of spins, such a system may provide a practical physical means to implement a quantum algorithm, possibly allowing moreeffective approaches to solving certain classes of hard combinatorial optimization problems. 
Article: Experimental Investigation of an Eight Qubit Unit Cell in aSuperconducting Optimization Processor
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ABSTRACT: A superconducting chip containing a regular array of flux qubits, tunable interqubit inductive couplers, an XYaddressable readout system, onchip programmable magnetic memory, and a sparse network of analog control lines has been studied. The architecture of the chip and the infrastructure used to control it were designed to facilitate the implementation of an adiabatic quantum optimization algorithm. The performance of an eightqubit unit cell on this chip has been characterized by measuring its success in solving a large set of random Ising spin glass problem instances as a function of temperature. The experimental data are consistent with the predictions of a quantum mechanical model of an eightqubit system coupled to a thermal environment. These results highlight many of the key practical challenges that we have overcome and those that lie ahead in the quest to realize a functional large scale adiabatic quantum information processor.  [Show abstract] [Hide abstract]
ABSTRACT: We report measurements of macroscopic resonant tunneling between the two lowest energy states of a pair of magnetically coupled rfSQUID flux qubits. This technique provides a direct means of observing twoqubit dynamics and a probe of the environment coupled to the pair of qubits. Measurements of the tunneling rate as a function of qubit flux bias show a Gaussian line shape that is well matched to theoretical predictions. Moreover, the peak widths indicate that each qubit is coupled to a local environment whose fluctuations are uncorrelated with that of the other qubit. Comment: 4 pages, 5 figures  [Show abstract] [Hide abstract]
ABSTRACT: A novel rfSQUID flux qubit that is robust against fabrication variations in Josephson junction critical currents and device inductance has been implemented. Measurements of the persistent current and of the tunneling energy between the two lowest lying states, both in the coherent and incoherent regime, are presented. These experimental results are shown to be in agreement with predictions of a quantum mechanical Hamiltonian whose parameters were independently calibrated, thus justifying the identification of this device as a flux qubit. In addition, measurements of the flux and critical current noise spectral densities are presented that indicate that these devices with Nb wiring are comparable to the best Al wiring rfSQUIDs reported in the literature thusfar, with a $1/f$ flux noise spectral density at $1 $Hz of $1.3^{+0.7}_{0.5} \mu\Phi_0/\sqrt{\text{Hz}}$. An explicit formula for converting the observed flux noise spectral density into a free induction decay time for a flux qubit biased to its optimal point and operated in the energy eigenbasis is presented.  [Show abstract] [Hide abstract]
ABSTRACT: We report upon experimental results from a system consisting of four flux qubits linked via insitu sign and magnitude tunable coupling elements. The device was operated as an adiabatic quantum computer to solve NPcomplete problems whose solutions are encoded in the groundstate configuration of the qubits. Each qubit was coupled to its own dedicated dcSQUID in order to measure the state of each qubit, thus allowing for unambiguous identification of the groundstate of the coupled qubit system at the end of a computation. Results will be compared to a quantum model of the system's evolution.
Publication Stats
250  Citations  
52.66  Total Impact Points  
Top Journals
 Physical Review B (3)
 Nature (1)
Institutions

20102011

DWave Systems Inc.
Burnaby, British Columbia, Canada
