We show how in an N-atom collective state atomic interferometer, it is possible to detect the interference between the ground state and all orders of collective states, producing an enhancement of sensitivity scaling as root(N).
In a conventional atomic interferometer employing N atoms, the phase sensitivity is at the standard quantum limit: 1/N. Using spin-squeezing, the sensitivity can be increased, either by lowering the quantum noise or via phase amplification, or a combination thereof. Here, we show how to increase the sensitivity, to the Heisenberg limit of 1/N, while increasing the quantum noise by N, thereby suppressing by the same factor the effect of excess noise. The protocol uses a Schrodinger Cat state representing a superposition of two collective states of N atoms, behaving as a single entity with an N-fold increase in Compton frequency. The resulting N-fold phase magnification is revealed by using atomic state detection instead of collective state detection. We also show how to realize an atomic clock based on such a Schrodinger Cat state, with an N-fold increase in the effective transition frequency. We discuss potential experimental constraints for implementing this scheme, using one axis twist squeezing employing the cavity feedback scheme, and show that the effects of cavity decay and spontaneous emission are highly suppressed. We find that the maximum improvement in sensitivity can be close to the ideal limit, for as many as ten million atoms.
This Letter presents a nonclassical mechanism of two-atom interferometry. The two-atom interferometer introduces two different yet indistinguishable alternatives for randomly paired atoms to create a joint atom-detection event. The superposition of two-atom amplitudes yields two-atom interference with peculiar features which outshine the classic atom interferometer: (1) two-atom interference is still observable when the time delay of the interferometer is greater than the coherence time of the atom beam and (2) two-atom interference may eliminate phase noises, including background, variation, turbulence, and Raman laser induced phase noises, thereby allowing for higher sensitivity and stability sensing than is achievable by classic atom interferometers. These features are crucial in high sensitivity-accuracy acceleration and rotation measurements. The presented concept and mechanism of two-atom interference can be adapted to other matter-wave interferometers, such as two-neutron interferometer and two-electron interferometer.
Quantum metrology promises greater sensitivity for optical phase measurements than could ever be achieved classically. Here, we present a theory of the phase sensitivity for the general case where the detection probability is given by an N photon interference fringe. We find that the phase sensitivity has a complicated dependence on both the intrinsic efficiency of detection η and the interference fringe visibility V . Most importantly, the phase that gives maximum phase sensitivity is in general not the same as the phase at which the slope of the interference fringe is a maximum, as has previously been assumed. We determine the parameter range where quantum enhanced sensitivity can be achieved. In order to illustrate these theoretical results, we perform a four-photon experiment with η=3/4 and V =82±6% (an extension of our previous work (Nagata et al 2007 Science 316 726)) and find a phase sensitivity 1.3 times greater than the standard quantum limit at a phase different to that which gives maximum slope of the interference fringe.