Optical Science Laboratory
Controlling matter qubits with coherent electromagnetic
field is regarded as the most promising building block of quantum information
processing systems. In practice, we can use the classical field theory
which enables us to control a qubit perfectly as described by the Bloch
rotation according to the pulse area of the field. However, classical fields
exist only as an infinitely strong intensity limit. In reality, strong
but finite strength coherent fields are available that will cause control
errors because of their lack of infiniteness.
In this work, we formulate the full quantum mechanical
interaction between a pure coherent field and a qubit in the general initial
state including mixed states. While the fidelity error accompanied by π/2
pulse control is shown to be inversely proportional to the average photon
number in a way similar to that revealed by the former results [1, 2] our
results show that the error depends strongly on the initial state of the
qubit [3]. Our result (Fig. 1) reveals that the the entanglement built
up between the field and the qubit is the origin of the error.
This work was partly supported by CREST of the Japan Science
Technology Agency.
[1] J.Gea-Banacloche, Phys. Rev. A 65 (2002) 022308.
[2] M.Ozawa, Phys. Rev. Lett. 89 (2002) 057902.
[3] K. Igeta, N. Imoto, and M. Koashi, Phys. Rev. A 87 (2013) 022321.
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