Quantum dynamics of a dc-SQUID coupled to an asymmetric Cooper pair transistor

TL;DR

This paper provides a theoretical analysis of a superconducting circuit combining an asymmetric Cooper pair transistor and a dc-SQUID, modeling it as coupled quantum systems with tunable interactions, relevant for quantum computing applications.

Contribution

It introduces a Hamiltonian model for the circuit, showing how it can be reduced to coupled qubits or the Jaynes-Cummings model, and explains tunable coupling mechanisms.

Findings

The circuit can be modeled as a charge qubit coupled to an anharmonic oscillator.

The coupling strength can be tuned from strong to zero.

The model explains the 'quantronium' and adiabatic quantum transfer read-out.

Abstract

We present a theoretical analysis of the quantum dynamics of a superconducting circuit based on a highly asymmetric Cooper pair transistor (ACPT) in parallel to a dc-SQUID. Starting from the full Hamiltonian we show that the circuit can be modeled as a charge qubit (ACPT) coupled to an anharmonic oscillator (dc-SQUID). Depending on the anharmonicity of the SQUID, the Hamiltonian can be reduced either to one that describes two coupled qubits or to the Jaynes-Cummings Hamiltonian. Here the dc-SQUID can be viewed as a tunable micron-size resonator. The coupling term, which is a combination of a capacitive and a Josephson coupling between the two qubits, can be tuned from the very strong- to the zero-coupling regimes. It describes very precisely the tunable coupling strength measured in this circuit and explains the 'quantronium' as well as the adiabatic quantum transfer read-out.