For a generic interferometer, the conditional probability density distribution, p(φ|m), for the phase φ given measurement outcome m, will generally have multiple peaks. Therefore, the phase sensitivity of an interferometer cannot be adequately characterized by the standard deviation, such as ∆φ ∼ 1/ √ N (the standard limit), or ∆φ ∼ 1/N (the Heisenberg limit). We propose an alternative measure of phase sensitivity-the fidelity of an interferometer-defined as the Shannon mutual information between the phase shift φ and the measurement outcomes m. As an example application of interferometer fidelity, we consider a generic optical Mach-Zehnder interferometer, used as a sensor of a classical field. We find the surprising result that an entangled N00N state input leads to a lower fidelity than a Fock state input, for the same photon number.
Introduction.Phase sensitivity of interferometers has been a topic of research for many years because of interest in the fundamental limitations of measurement [1, 2], gravitational-wave detection [3], and optical [4,5], atom [6], and Bose-Einstein condensate(BEC)-based gyroscopes [7,8,9]. Recently, applications to sensors are being explored [10,11].The phase sensitivity of interferometers is believed to be limited by quantum fluctuations [12], and the phase sensitivity of various interferometers has been explored for different types of input states, such as squeezed states [12,13], and number states [14,15,16,17,18,19,20,21,22]. In all the above cases, the phase sensitivity ∆φ has been discussed in terms of two limits, known as the standard limit, ∆φ SL = 1/ √ N , and the Heisenberg limit[23], ∆φ HL = 1/N , where N is the number of particles that enter the interferometer during each measurement cycle. These arguments are based on results of standard estimation theory [24] which connects an ensemble of measurement outcomes, m i , i = 1, 2, · · · , M , with corresponding phases, φ i , through a theoretical relation m = m(φ). An example of the theoretical relation associated with some quantum observable is m(φ) = φ| m|φ , where the state is parameterized by a single parameter φ. Standard estimation theory predicts that the standard deviation, ∆φ, of the probability distribution for the phase φ, is related to the standard deviation in the measurements, ∆m, by [24]