Thursday, May 26, 2011

Quantum annealing with manufactured spins

M. W. Johnson,M. H. S. Amin,S. Gildert,T. Lanting,F. Hamze,N. Dickson,R. Harris,A. J. Berkley,J. Johansson,P. Bunyk,E. M. Chapple,C. Enderud,J. P. Hilton,K. Karimi,E. Ladizinsky,N. Ladizinsky,T. Oh,I. Perminov,C. Rich,M. C. Thom,E. Tolkacheva,C. J. S. Truncik,S. Uchaikin,J. Wang,B. Wilson& G. Rose

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 difficult1. It is believed that the ground state of some naturally occurring spin systems can be effectively attained through a process called quantum annealing2, 3. If it could be harnessed, quantum annealing might improve on known methods for solving certain types of problem4, 5. However, physical investigation of quantum annealing has been largely confined to microscopic spins in condensed-matter systems6, 7, 8, 9, 10, 11, 12. 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 spin–spin 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 low-energy configuration13, 14. 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 more-effective approaches to solving certain classes of hard combinatorial optimization problems.

Tuesday, May 24, 2011

Single-ion quantum lock-in amplifier

Shlomi Kotler, Nitzan Akerman, Yinnon Glickman, Anna Keselman & Roee Ozeri

Quantum metrology1 uses tools from quantum information science to improve measurement signal-to-noise ratios. The challenge is to increase sensitivity while reducing susceptibility to noise, tasks that are often in conflict. Lock-in measurement is a detection scheme designed to overcome this difficulty by spectrally separating signal from noise. Here we report on the implementation of a quantum analogue to the classical lock-in amplifier. All the lock-in operations—modulation, detection and mixing—are performed through the application of non-commuting quantum operators to the electronic spin state of a single, trapped Sr+ ion. We significantly increase its sensitivity to external fields while extending phase coherence by three orders of magnitude, to more than one second. Using this technique, we measure frequency shifts with a sensitivity of 0.42 Hz Hz−1/2 (corresponding to a magnetic field measurement sensitivity of 15 pT Hz−1/2), obtaining an uncertainty of less than 10 mHz (350 fT) after 3,720 seconds of averaging. These sensitivities are limited by quantum projection noise and improve on other single-spin probe technologies2, 3 by two orders of magnitude. Our reported sensitivity is sufficient for the measurement of parity non-conservation4, as well as the detection of the magnetic field of a single electronic spin one micrometre from an ion detector with nanometre resolution. As a first application, we perform light shift spectroscopy of a narrow optical quadrupole transition. Finally, we emphasize that the quantum lock-in technique is generic and can potentially enhance the sensitivity of any quantum sensor.

Thursday, May 12, 2011

Shortcut to adiabaticity for an interacting Bose-Einstein condensate

Jean-Fran├žois Schaff, Xiao-Li Song, Pablo Capuzzi, Patrizia Vignolo, Guillaume Labeyrie

We present an investigation of the fast decompression of a three-dimensional (3D) Bose-Einstein condensate (BEC) at finite temperature using an engineered trajectory for the harmonic trapping potential. Taking advantage of the scaling invariance properties of the time-dependent Gross-Pitaevskii equation, we exhibit a solution yielding a final state identical to that obtained through a perfectly adiabatic transformation, in a much shorter time. Experimentally, we perform a large trap decompression and displacement within a time comparable to the final radial trapping period. By simultaneously monitoring the BEC and the non-condensed fraction, we demonstrate that our specific trap trajectory is valid both for a quantum interacting many-body system and a classical ensemble of non-interacting particles.

Self contained quantum heat engines

Noah Linden, Sandu Popescu, Paul Skrzypczyk

We construct the smallest possible self contained heat engines; one composed of only two qubits, the other of only a single qutrit. The engines are self-contained as they do not require external sources of work and/or control. They are able to produce work which is used to continuously lift a weight. Despite the dimension of the engine being small, it is still able to operate at the Carnot efficiency.