432 research outputs found
Fast and High-Fidelity Entangling Gate through Parametrically Modulated Longitudinal Coupling
We investigate an approach to universal quantum computation based on the
modulation of longitudinal qubit-oscillator coupling. We show how to realize a
controlled-phase gate by simultaneously modulating the longitudinal coupling of
two qubits to a common oscillator mode. In contrast to the more familiar
transversal qubit-oscillator coupling, the magnitude of the effective
qubit-qubit interaction does not rely on a small perturbative parameter. As a
result, this effective interaction strength can be made large, leading to short
gate times and high gate fidelities. We moreover show how the gate infidelity
can be exponentially suppressed with squeezing and how the entangling gate can
be generalized to qubits coupled to separate oscillators. Our proposal can be
realized in multiple physical platforms for quantum computing, including
superconducting and spin qubits.Comment: 5 pages, 3 figures, Supplemental Materia
Rapid Steady State Convergence for Quantum Systems Using Time-Delayed Feedback Control
We propose a time-delayed feedback control scheme for open quantum systems
that can dramatically reduce the time to reach steady state. No measurement is
performed in the feedback loop, and we suggest a simple all-optical
implementation for a cavity QED system. We demonstrate the potential of the
scheme by applying it to a driven and dissipative Dicke model, as recently
realized in a quantum gas experiment. The time to reach steady state can then
reduced by two orders of magnitude for parameters taken from experiment, making
previously inaccessible long time attractors reachable within typical
experimental run times. The scheme also offers the possibility of slowing down
the dynamics, as well as qualitatively changing the phase diagram of the
corresponding physical system.Comment: 25 pages, 9 figures. Invited paper in "Focus on Coherent Control of
Complex Quantum Systems", Eds. B. Whaley and G. Milburn. PS: Preview on OSX
struggles with opening some of the figures with a lot of data in the
Anomalous Paths in Quantum Mechanical Path-Integrals
We investigate modifications of the discrete-time lattice action, for a
quantum mechanical particle in one spatial dimension, that vanish in the
na\"ive continuum limit but which, nevertheless, induce non-trivial effects due
to quantum fluctuations. These effects are seen to modify the geometry of the
paths contributing to the path-integral describing the time evolution of the
particle, which we investigate through numerical simulations. In particular, we
demonstrate the existence of a modified lattice action resulting in paths with
any fractal dimension, d_f, between one and two. We argue that d_f=2 is a
critical value, and we exhibit a type of lattice modification where the
fluctuations in the position of the particle becomes independent of the time
step, in which case the paths are interpreted as superdiffusive L\'{e}vy
flights. We also consider the jaggedness of the paths, and show that this gives
an independent classification of lattice theories.Comment: 7 pages,6 figure
- …