Non-Markovian dynamics of single- and two-qubit systems interacting with Gaussian and non-Gaussian fluctuating transverse environments

We address the interaction of single- and two-qubit systems with an external transverse fluctuating field and analyze in detail the dynamical decoherence induced by Gaussian noise and random telegraph noise (RTN). Upon exploiting the exact RTN solution of the time-dependent von Neumann equation, we analyze in detail the behavior of quantum correlations and prove the non-Markovianity of the dynamical map in the full parameter range, i.e., for either fast or slow noise. The dynamics induced by Gaussian noise is studied numerically and compared to the RTN solution, showing the existence of (state dependent) regions of the parameter space where the two noises lead to very similar dynamics. We show that the effects of RTN noise and of Gaussian noise are different, i.e., the spectrum alone is not enough to summarize the noise effects, but the dynamics under the effect of one kind of noise may be simulated with high fidelity by the other one

Quantum computing with quantum-Hall edge state interferometry

Electron interferometers based on Hall edge states (ESs) proved to be robust demonstrators of the coherent quantum dynamics of carriers. Several proposals to expose their capability to build and control quantum entanglement and to exploit them as building block for quantum computing devices has been presented. Here, we review the time-dependent numerical modeling of Hall interferometers operating at the single-carrier level at integer filling factor (FF). By defining the qubit state either as the spatial localization (at FF 1) or the Landau index (at FF 2) of a single carrier propagating in the ES, we show how a generic one-qubit rotation can be realized. By a proper design of the two-dimensional electron gas potential landscape, an entangling two-qubit gate can be implemented by exploiting Coulomb interaction, thus realizing a universal set of quantum gates. We also assess how the shape of the edge confining potential affects the visibility of the quantum transformations

Superconducting Qubits: Dephasing and Quantum Chemistry

One of the most exciting potential applications of a quantum computer is the abilityto efficiently simulate quantum systems, a task that is out of the reach of even thelargest classical supercomputers. Such simulations require a quantum algorithm capableof efficiently representing and manipulating a quantum system, as well as a device withsufficient coherence to execute it. In this work, we describe experiments advancing bothof these goals. First, we discuss dephasing—currently a leading cause of decoherencein superconducting qubits—and present measurements accurately quantifying both low-and high-frequency phase noise sources. We then discuss two quantum algorithms forthe simulation of chemical Hamiltonians, and experimentally contrast their performance.These results show that with continuing improvement in quantum devices we may soonbe able to apply quantum computers to practical chemistry problems

Coherence characterization with a superconducting flux qubit through NMR approaches

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 191-200).This thesis discusses a series of experimental studies that investigate the coherence properties of a superconducting persistent-current or flux qubit, a promising candidate for developing a scalable quantum processor. A collection of coherence characterization experiments and techniques that originate from the field of nuclear magnetic resonance (NMR) are implemented. In particular, one type of dynamical decoupling techniques that uses refocusing pulses to recover coherence is successfully realized for the first time. This technique is further utilized as a noise spectrum analyzer in the megahertz range, by which a 1/f-type dependence is observed for the flux noise. Then, a novel method of performing low-frequency noise spectroscopy is developed and successfully implemented. New techniques used in the readout scheme and data processing result in an improved spectral range and signal visibility over conventional methods. The observed power law dependence below kilohertz agrees with separate measurements at higher frequencies. Also, the noise is found to be temperature independent. Finally, a robust noise spectroscopy method is presented, where the spin-locking technique is employed to extract noise information by measuring the driven-evolution longitudinal relaxation. This technique shows improved accuracy over other methods, due to its insensitivity to low-frequency noise. Spectral signatures of coherent fluctuators are resolved, and further confirmed in a time-domain spin-echo experiment.by Fei Yan.Ph.D