Terence D. Clark

Quantum mechanical flux band dynamics of a superconducting weak link constriction ring

Abstract We present a quantum mechanical model of the dynamical behaviour of an ac-biased superconducting weak link constriction (SQUID) magnetometer. We show that the predictions of this model correspond closely to the observed characteristics of an ultra low noise UHF-biased SQUID magnetometer.

Signatures of chaoticlike and nonchaoticlike behavior in a nonlinear quantum oscillator through photon detection

The driven nonlinear Duffing oscillator is a very good, and standard, example of a quantum mechanical system from which classical-like orbits can be recovered from unravelings of the master equation. In order to generate such trajectories in the phase space of this oscillator, in this paper we use the quantum jump unraveling together with a suitable application of the correspondence principle. We analyze the measured readout by considering the power spectra of photon counts produced by the quantum jumps. Here we show that localization of the wave packet from the measurement of the oscillator by the photon detector produces a concomitant structure in the power spectra of the measured output. Furthermore, we demonstrate that this spectral analysis can be used to distinguish between different modes of the underlying dynamics of the oscillator.

Magnetic flux quantization in the repulsive electron correlation model for high Tc cuprate superconductivity

Abstract We discuss the condensate electrodynamics in the repulsive electronic correlation model of high T c superconductivity. We show that the electronic current density obeys the London equation, and that this leads to the Meissner effect and magnetic flux quantization in units of h /2 e , in agreement with widely accepted experimental observations. We also remark on the relationship between these effects and the condensate fraction in this model.

Rapid purification of a solid state charge qubit

This paper discusses the use of continuous weak measurement and quantum feedback for the rapid purification of the quantum state of a model solid state qubit: a superconducting Cooper pair box. The feedback algorithm uses Jacobs rapid purification protocol, which starts with a completely mixed state and applies controls to rotate the qubit Bloch vector onto the plane orthogonal to the measurement axis. This rotation maximises the rate of increase of the average purity of the state but can require large changes in the control fields to produce the required rotation. Since solid state qubits have finite controls and feedback channels have limited bandwidth, such rotations may not be practical. This paper applies Jacobs protocol to the Cooper pair box with realistic control fields.

Characterizing a superconducting charge qubit via environmental noise

In this paper, we propose a technique to characterise the energy level structure of a superconducting charge qubit. The technique relies on the backreaction of a solid-state qubit on its environment and the incoherent transfer of energy from a high frequency mode to a low frequency mode due to the stochastic transitions of the qubit between energy eigenstates. We consider a coupled system consisting of a model charge qubit and several classical degrees of freedom. The qubit is coupled to three electromagnetic modes: a low frequency bias field, a higher frequency mode (which is used to pump the qubit from the ground state to an excited state), and a lossy reservoir (which represents the cavity that contains the qubit and control fields). The reservoir provides a mechanism to allow the qubit to dissipate energy and to induce spontaneous decays from an excited state into the ground state. We show that these spontaneous decays can have a significant effect on the noise in the classical bias field, and that this noise can be used to characterise the energy level structure of the qubit.

A proportional guidance law for quantum information processing

In this paper, we consider a natural generalisation of classical proportional navigation guidance for quantum information processing devices. We demonstrate how standard guidance laws can be modified to allow the efficient control of the quantum state of an example qubit. We consider an example experimental system: a Josephson charge qubit (Cooper pair box). The quantum guidance algorithm is assessed in an open-loop control system based on the standard bias fields present in the device, without the need for any additional external fields (such as microwave pump fields, which are often used to drive these charge devices into excited states).