Collapse-Revival and Entanglement in a Single-Cooper-Pair Box with a Single-Mode Cavity Field

A single-Cooper-pair box biased by a classical voltage and also irradiated by a single-mode quantized field is considered. The degree of entanglement between the quantized field and the single-Cooper-pair box is examined by using the field entropy. It is found that the collapse and revival phenomenon can exist in this system, the entropy reaches its local minimum at one-half of the quiescent period, and the larger the entropy, the greater the entanglement is.