The feasibility of a Heisenberg-limited phase measurement using a Mach-Zehnder interferometer fed with twin photon correlated light is investigated theoretically. To take advantage of the Heisenberg limit, 1/N, for the phase sensitivity, one wants the number of correlated photons, N, to be high. This favors the use of parametric oscillation rather than the weaker but better correlated source given by parametric downconversion. In real systems, decorrelation arising from photon absorption, mode mismatch, and nonideal detectors must be considered. In this paper we address the problem of detection when correlated photons are used as the input. We study the influence of photon statistics and of imperfect quantum correlation of the input light, and show that it is still possible to break the classical 1/ͱN phase sensitivity limit in nonideal experimental conditions. All the results are valid in the general case of quantum correlated bosons.

We propose an optical parametric down conversion (PDC) scheme that does not suffer a trade-off between the state-purity of single-photon wave-packets and the rate of packet production. This is accomplished by modifying the PDC process by using a microcavity to engineer the density of states of the optical field at the PDC frequencies. The high-finesse cavity mode occupies a spectral interval much narrower than the bandwidth of the pulsed pump laser field, suppressing the spectral correlation, or entanglement, between signal and idler photons.Spectral filtering of the field occurs prior to photon creation rather than afterward as in most other schemes. Operator-Maxwell equations are solved to find the Schmidt-mode decomposition of the two-photon states produced. Greater than 99% pure-state packet production is predicted to be achievable.

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