Abstract

When the quantum nature of the optical field used to drive a qubit is taken into account, entanglement between the qubit and the field becomes a possible source of error which has been the subject of some attention lately.¹ For instance, it is intuitively clear that, if a transition from the qubit state |0〉 to the state |l〉 requires the absorption of a photon, the transition from the state |0〉 to the coherent superposition |0〉 + | 1) cannot be done "cleanly"—that is, without entanglement— with a quantized field; rather, if the initial state of the field is |ψ), one will always end up in a superposition state of the form | 0〉| ψ〉 + |1〉 |ψ_〉, where | ψ_) is a field state having, on average, one photon less than | ψ〉.

© 2002 Optical Society of America