Fundamental limit to Qubit Control with Coherent Field

The ultimate accuracy as regards controlling a qubit with a coherent field is studied in terms of degradation of the fidelity by employing a fully quantum mechanical treatment. While the fidelity error accompanied by pi/2 pulse control is shown to be inversely proportional to the average photon number in a way similar to that revealed by the Gea-Banacloche's results. Our results show that the error depends strongly on the initial state of the qubit. When the initial state of the qubit is in the ground state, the error is about 20 times smaller than that of the control started from the exited state, no matter how large N is. This dependency is explained in the context of an exact quantum mechanical description of the pulse area theorem. By using the result, the error accumulation tendency of successive pulse controls is found to be both non-linear and initial state-dependent.Comment: 9 pages, 3 figures, to appear in PR

Boosting up quantum key distribution by learning statistics of practical single photon sources

We propose a simple quantum-key-distribution (QKD) scheme for practical single photon sources (SPSs), which works even with a moderate suppression of the second-order correlation $g^{(2)}$ of the source. The scheme utilizes a passive preparation of a decoy state by monitoring a fraction of the signal via an additional beam splitter and a detector at the sender's side to monitor photon number splitting attacks. We show that the achievable distance increases with the precision with which the sub-Poissonian tendency is confirmed in higher photon number distribution of the source, rather than with actual suppression of the multi-photon emission events. We present an example of the secure key generation rate in the case of a poor SPS with $g^{(2)} = 0.19$, in which no secure key is produced with the conventional QKD scheme, and show that learning the photon-number distribution up to several numbers is sufficient for achieving almost the same achievable distance as that of an ideal SPS.Comment: 11 pages, 3 figures; published version in New J. Phy