Southwest Quantum Information and Technology

Twelfth Annual Meeting, February 18-21, 2010
Santa Fe, New Mexico

Full Program | Thursday | Friday | Saturday | Sunday

SESSION 9: Breakout II - Metrology and Measurement
Session Chair:
3:30-4:00	Alexandre Tacla, University of New Mexico
	Practical quantum metrology with Bose-Einstein condensates
	Abstract. We analyze in detail the recently proposed experiment [Boixo et al., Phys. Rev. Lett. 101 , 040403 (2008)] for achieving better than 1/N scaling in a quantum metrology protocol using a two mode Bose-Einstein condensate of N atoms. There were several simplifying assumptions in the original proposal that made it easy to see how a scaling approaching 1/N^(3/2) may be obtained. We look at these assumptions in detail to see when they may be justified. We present numerical results that confirm our theoretical predictions for the effect of the spreading of the BEC wave function with increasing N on the scaling. Numerical integration of the coupled Gross-Pitaevskii equations for the two mode BEC also shows that the assumption that the two modes share the same spatial wave function is justified for a length of time that is sufficient to run the metrology scheme.
4:00-4:30	Collin Trail, University of New Mexico
	Quantum Eraser and Phase-Matching for Exponential Spin-Squeezing via Coherent Optical Feedback
	Abstract. A scheme for squeezing collective atomic spin states via coherent optical feedback was proposed by M. Takeuchi et. al., Phys. Rev. Lett. 94, 023003, 2005. In the first pass, the Faraday effect acts to entangle the light with the atoms. In a coherent second pass, this information is imprinted back onto the atoms, creating an effective nonlinear interaction and entanglement between atoms. However, the light is still entangled to the atoms when it escapes, leading to substantial decoherence, and moreover, the interaction slowly rotates the system out of sync with the squeezing axis, both of which result in suboptimal squeezing. We show how the addition of a quantum eraser and phase matching can lead to radically improved exponential scaling. We analyze this system in the presence of realistic imperfections such as photon scattering, optical pumping, losses in transmission and reflection, finite detector efficiency, and nonprojective measurements, and show that spin squeezing near 10 dB should be possible.
4:30-5:00	Mankei Tsang, University of New Mexico
	Time-Symmetric Quantum Smoothing: A General Theory of Optimal Quantum Sensing
	Abstract. In real-world sensing applications, the signal to be estimated, such as the position of an aircraft, a gravitational wave, or a magnetic field, is seldom a parameter constant in time but a fluctuating random process. Drawing insights from Bayesian estimation theory, I shall demonstrate how the optimal estimation of a random process coupled to a quantum sensor can be done using the recently proposed quantum smoothing theory. The theory calls for the use of not one but two density operators, one to be solved forward in time and one backward in time, and can out-perform conventional quantum filtering methods if delay is permitted in the estimation. Potential applications include gravitational wave sensing and atomic magnetometry. The accuracy improvement of quantum optical phase estimation due to smoothing has recently been experimentally demonstrated by an Australian-Japanese collaboration [Wheatley et al., arXiv:0912.1162].
5:00-5:30	Francois Mallet, Joint Institute for Laboratory Astrophysics
	Tomographic reconstruction of the Wigner function of an itinerant microwave field.
	Abstract. Francois Mallet, Manuel Castellanos-Beltran, Hsiang-Sheng Ku, Kent Irwin, Leila Vale, Gene Hilton, Konrad Lehnert. In an increasing number of experiments, the desired information (for example the state of nanomechanical resonators or of superconducting qubits) is successfully encoded into the state of a coherent microwave field. However these experiments suffer from the lack of high efficiency detectors at microwave frequencies: the best commercially available amplifiers add twenty times more noise than the intrinsic quantum fluctuations of the field. Our group has made a crucial step to overcome this important limitation by developing quantum limited Josephson Parametric Amplifiers (JPAs) [1]. In this talk I will show how we dramatically increase the performance of the Quantum State Tomography of a squeezed state of the microwave field by using our JPAs. The achieved degree of squeezing and the quantum efficiency of the state tomography will be presented from the point of view of using these squeezed states as building blocks of a more global strategy to perform Quantum Information experiments. Indeed it has been shown in the field of Continuous Variables Quantum Information that theses squeezed states, can be combined to create EPR-like entangled states. Conveniently, the non-classical squeezed states are themselves created by the JPAs. [1] Amplification and squeezing of quantum noise with a tunable Josephson metamaterial, M. Castellanos-Beltran et al., Nat. Phys. 4, 929-931 (2008).