Cavity-enabled spin squeezing for a quantum-enhanced atomic clock

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 127-135).For the past decade, the stability of microwave atomic clocks has stood at the standard quantum limit, set by the projection noise inherent in measurements on ensembles of uncorrelated particles. Here, I demonstrate an atomic clock that surpasses this limit by operating with atoms in a particular type of entangled state called a "squeezed spin state." The generation of non-classical spin correlations in a dilute cloud of atoms is facilitated by an optical cavity, which allows for strong collective coupling of the atomic ensemble to a single mode of light. Since the light exiting the cavity is entangled with the atoms, an appropriate measurement performed on the light field can project the atomic ensemble into a squeezed spin state. I demonstrate 3.0(8) dB of spin squeezing by this method of quantum non-demolition measurement. I further introduce a new method, cavity feedback squeezing, which uses the light field circulating in the resonator to mediate an effective interaction among the atoms. The light-mediated interaction mimics the spin dynamics of the one-axis twisting Hamiltonian, under which a coherent spin state evolves deterministically into a squeezed spin state. The states prepared by cavity feedback are intrinsically squeezed by up to 10(1) dB and detectably squeezed by up to 5.6(6) dB. Applied in an atomic clock, they produce an Allan variance 4.7(5) dB below the standard quantum limit for averaging times of up to 50 s. In a detour from engineering collective spin dynamics, I present direct observations of collective motional dynamics of atoms under the influence of cavity cooling. I demonstrate cooperatively enhanced cooling of a single collective motional mode down to a mean occupation number of 2.0 (-0.3/+0.9) phonons. The cooling is quantitatively well described by a simple, analytic quantum optomechanical model.by Monika Helene Schleier-Smith.Ph.D

Producing Squeezed Input States for an Atomic Clock Using an Optical Cavity

Spin squeezing, the generation of collective states of atomic ensembles with reduced spin noise by exploiting non-classical correlations between particles, is a promising approach to overcoming the standard quantum limit set by projection noise of independent atoms. We present two implementations of spin squeezing in ensembles of [superscript 87]Rb confined within an optical resonator, and discuss some of the decoherence mechanisms, both technical and fundamental, that we encounter.Harvard University - MIT Center for Ultracold AtomsDefense Advanced Research Projects AgencyNational Science Foundatio

Orientation-Dependent Entanglement Lifetime in a Squeezed Atomic Clock

We study experimentally the application of a class of entangled states, squeezed spin states, to the improvement of atomic-clock precision. In the presence of anisotropic noise, the entanglement lifetime is strongly dependent on squeezing orientation. We measure the Allan deviation spectrum of a clock operated with a phase-squeezed input state. For averaging times up to 50 s the squeezed clock achieves a given precision 2.8(3) times faster than a clock operating at the standard quantum limit.National Science Foundation (U.S.)Hertz FoundationUnited States. Defense Advanced Research Projects Agency (DARPA)Natural Sciences and Engineering Research Council of Canada (NSERC

Preparation of reduced-quantum-uncertainty input states for an atomic clock

Atomic clocks have reached the Standard Quantum Limit (SQL) of precision,1 set by the projection noise inherent in measurements on uncorrelated atoms. It is possible to overcome this limit by entangling the atoms to generate a "squeezed state" of the atomic ensemble. We use the collective interaction of an atomic ensemble with a far-detuned light field in an optical resonator to prepare squeezed states by two different methods: quantum non-demolition (QND) measurement and Hamiltonian evolution. We apply both methods to an ensemble of 5 x 10[superscript 4] [superscript 87]Rb atoms in a superposition of hyperfine clock states. We measure the suppression of projection noise and compare it to the accompanying reduction in signal, thereby quantifying the net gain in spectroscopic sensitivity. By QND measurement, with resolution up to 9 dB below the projection noise level, we achieve 3.0(8) dB of metrologically relevant squeezing. Whereas the measurement-based approach relies on knowledge of the (randomly distributed) measurement outcome to produce a conditionally squeezed state, the method of Hamiltonian evolution produces a known squeezed state independent of detector performance. We mimic the dynamics of the one-axis twisting Hamiltonian, proposed as a generator of squeezed states by Kitagawa and Ueda, by using the atom-induced frequency shift of the resonator mode and the corresponding resonator-field-induced shift of the atomic transition frequency to introduce an effective interaction among the atoms. The resulting deterministic squeezing is sufficient to allow a 6.0(4) dB improvement in spectroscopic sensitivity over the SQLNational Science Foundation, Center for Ultracold AtomsDefense Advanced Research Projects AgencyNational Science Foundatio