1 research outputs found
A quantum fluctuation description of charge qubits
We consider a specific instance of a superconducting circuit, the so-called
charge-qubit, consisting of a capacitor and a Josephson junction. Starting from
the microscopic description of the latter in terms of two tunneling BCS models
in the strong-coupling quasi-spin formulation, we derive the Hamiltonian
governing the quantum behavior of the circuit in the limit of a large number
of quasi-spins. Our approach relies on the identification of suitable
quantum fluctuations, i.e. of collective quasi-spin operators, which account
for the presence of fluctuation operators in the superconducting phase that
retain a quantum character in spite of the large- limit. We show indeed that
these collective quantum fluctuations generate the Heisenberg algebra on the
circle and that their dynamics reproduces the one of the quantized
charge-qubit, without the need of a phenomenological ``third quantization'' of
a semiclassically inspired model. As a byproduct of our derivation, we
explicitly obtain the temperature dependence of the junction critical Josephson
current in the strong coupling regime, a result which is not directly
accessible using standard approximation techniques.Comment: 34 page