Thermally assisted adiabatic quantum computation

We study the effect of a thermal environment on adiabatic quantum computation using the Bloch-Redfield formalism. We show that in certain cases the environment can enhance the performance in two different ways: (i) by introducing a time scale for thermal mixing near the anticrossing that is smaller than the adiabatic time scale, and (ii) by relaxation after the anticrossing. The former can enhance the scaling of computation when the environment is superohmic, while the latter can only provide a prefactor enhancement. We apply our method to the case of adiabatic Grover search and show that performance better than classical is possible with a superohmic environment, with no a priori knowledge of the energy spectrum.Comment: 4 pages, 2 figures, Final version to appear in PR

Role of Single Qubit Decoherence Time in Adiabatic Quantum Computation

We have studied numerically the evolution of an adiabatic quantum computer in the presence of a Markovian ohmic environment by considering Ising spin glass systems with up to 20 qubits independently coupled to this environment via two conjugate degrees of freedom. The required computation time is demonstrated to be of the same order as that for an isolated system and is not limited by the single-qubit decoherence time $T_2^*$, even when the minimum gap is much smaller than the temperature and decoherence-induced level broadening. For small minimum gap, the system can be described by an effective two-state model coupled only longitudinally to environment.Comment: 4 pages, 3 figures, published versio

On the construction of model Hamiltonians for adiabatic quantum computation and its application to finding low energy conformations of lattice protein models

In this report, we explore the use of a quantum optimization algorithm for obtaining low energy conformations of protein models. We discuss mappings between protein models and optimization variables, which are in turn mapped to a system of coupled quantum bits. General strategies are given for constructing Hamiltonians to be used to solve optimization problems of physical/chemical/biological interest via quantum computation by adiabatic evolution. As an example, we implement the Hamiltonian corresponding to the Hydrophobic-Polar (HP) model for protein folding. Furthermore, we present an approach to reduce the resulting Hamiltonian to two-body terms gearing towards an experimental realization.Comment: 35 pages, 8 figure

In-plane superfluid density and microwave conductivity of the organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br: evidence for d-wave pairing and resilient quasiparticles

We report the in-plane microwave surface impedance of a high-quality single crystal of kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. In the superconducting state, we find three independent signatures of d-wave pairing: (i) a strong, linear temperature dependence of superfluid density; (ii) deep in the superconducting state the quasiparticle scattering rate Gamma similar to T-3; and (iii) no BCS coherence peak is observed in the quasiparticle conductivity. Above T-c, the Kadowaki-Woods ratio and the temperature dependence of the in-plane conductivity show that the normal state is a Fermi liquid below similar or equal to 23 K, yet resilient quasiparticles dominate the transport up to similar or equal to 50 K

Classical Ising model test for quantum circuits

We exploit a recently constructed mapping between quantum circuits and graphs in order to prove that circuits corresponding to certain planar graphs can be efficiently simulated classically. The proof uses an expression for the Ising model partition function in terms of quadratically signed weight enumerators (QWGTs), which are polynomials that arise naturally in an expansion of quantum circuits in terms of rotations involving Pauli matrices. We combine this expression with a known efficient classical algorithm for the Ising partition function of any planar graph in the absence of an external magnetic field, and the Robertson-Seymour theorem from graph theory. We give as an example a set of quantum circuits with a small number of non-nearest neighbor gates which admit an efficient classical simulation.Comment: 17 pages, 2 figures. v2: main result strengthened by removing oracular settin

Precision microwave spectroscopy of the heavy fermion superconductor CeCoIn5

The heavy fermion superconductor CeCoIn5 demonstrates remarkable similarities to the high-Tc cuprates in many of its properties including proximity to antiferromagnetism, quasi-two-dimensionality, d-wave superconductivity, and departures from Fermi liquid behaviour in the normal state. It is also a “high-Tc” superconductor in the context of the heavy fermions. The experimental technique of microwave cavity perturbation has been used to measure the electrodynamics of a single crystal of CeCoIn5 over a range of temperatures, from 80 mK to 35 K, in a dilution refrigerator. Measurements at multiple frequencies required the development of an in-situ technique for the bolometric detection of the surface resistance. This has allowed conductivity spectra to be acquired, resulting in several important results. First, the resolution of an unexplained fractional power law in the penetration depth has been achieved by properly isolating the nodal quasiparticle contribution, revealing a previously unseen linear temperature dependence in CeCoIn5, as expected for a d-wave superconductor. Second, the temperature evolution of the microwave conductivity spectra implies that the effective mass of the quasiparticles continues to change below Tc, hinting that quantum criticality remains important even in the superconducting state. Third, conductivity spectra that are strikingly similar to those from YBa2Cu3O6+y suggest a strong connection in the underlying charge dynamics, as both CeCoIn5 and YBa2Cu3O6+y show a collapse in the quasiparticle scattering rate below Tc. Finally, the spectra indicate the presence of multiband effects