Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session G4: Quantum Measurement |
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Chair: David Hanneke, Amherst College Room: 554AB |
Wednesday, May 25, 2016 8:00AM - 8:12AM |
G4.00001: Approaching the Heisenberg Limit Without Single-Particle Detection Gregory Bentsen, Emily Davis, Monika Schleier-Smith Achieving Heisenberg-limited measurements with ensembles of more than a few particles remains a major outstanding challenge. The problem is two-fold: one must not only prepare a sufficiently sensitive state, but also be able to detect it. While it is commonly assumed that Heisenberg-limited measurement demands single-particle-resolved detection, we propose an alternative approach that bypasses this requirement. We show that the “one-axis twisting” interaction, well known for generating spin squeezing in atomic ensembles, can also amplify the output signal of an entanglement-enhanced interferometer to facilitate readout [1]. Even in the presence of dissipation, the protocol significantly relaxes the detection resolution required for spectroscopy beyond the standard quantum limit, and achieves near-Heisenberg-limited precision in a $\sqrt{N}$-times shorter evolution than is required to reach the GHZ state.\\ \vspace{0.5cm} [1] E. Davis, G. Bentsen, M. Schleier-Smith, arXiv:1508.04110 (2015) [Preview Abstract] |
Wednesday, May 25, 2016 8:12AM - 8:24AM |
G4.00002: Deterministic Squeezed States with Joint Measurements and Feedback Graham P. Greve, Kevin C. Cox, Baochen Wu, James K. Thompson Joint measurement of many qubits or atoms is a powerful way to create entanglement for precision measurement and quantum information science. However, the random quantum collapse resulting from the joint measurement also leads to randomness in \textit{which } entangled state is created. We present an experiment in which we apply real-time feedback to eliminate the randomness generated during the joint measurement of $5 \times 10^4$ laser-cooled Rb atoms. The feedback effectively steers the quantum state to a desired squeezed state. After feedback, the final state achieves a directly observed phase resolution variance up to 7.4(6) dB below the standard quantum limit for unentangled atoms. The entanglement and improved measurement capability of these states can be realized without retaining knowledge of the joint measurement’s outcome, possibly opening new applications for spin squeezed states generated via joint measurement. [Preview Abstract] |
Wednesday, May 25, 2016 8:24AM - 8:36AM |
G4.00003: Production of a planar squeezed state in a cold atomic ensemble Giorgio Colangelo, Ferran Martin Ciurana, Robert J. Sewell, Morgan W. Mitchell Production of squeezed states is of great interest for quantum metrology and allows production of exotic highly entangled spin states, a powerful resource for quantum simulators. However, while canonical variables such as quadratures of the radiation field can be squeezed in at most one component, a planar quantum squeezed (PQS) state, where two orthogonal spin components are simultaneously squeezed can be achieved due to the angular momentum commutation relations. Such states have recently attracted attention due to their potential applications in atomic interferometry and quantum information. Here we report the generation of a PQS state by coherently rotate the collective spin of a cold atomic ensemble of more than one million atoms . We induce spin squeezing through quantum non-demolition (QND) measurements and a coherent rotation by an external magnetic field that rotates a coherent spin state on a plane. This allows us to successively measure and squeeze two components of the atomic spin, while maintaining a large spin polarization (coherence) in the plane. We observe 3dB of spin squeezing and quantum enhanced sensitivity in the estimation of the magnetic field for any angle in the rotation plane, and detect entanglement by using generalized spin squeezing inequalities. [Preview Abstract] |
Wednesday, May 25, 2016 8:36AM - 8:48AM |
G4.00004: High density spin noise spectroscopy with squeezed light Vito Giovanni Lucivero, Ricardo Jiménez-Martínez, Jia Kong, Morgan Mitchell Spin noise spectroscopy (SNS) has recently emerged as a powerful technique for determining physical properties of an unperturbed spin system from its power noise spectrum both in atomic and solid state physics. In the presence of a transverse magnetic field, we detect spontaneous spin fluctuations of a dense Rb vapor via Faraday rotation of an off-resonance probe beam, resulting in the excess of spectral noise at the Larmor frequency over a white photon shot-noise background. We report quantum enhancement of the signal-to-noise ratio via polarization squeezing of the probe beam up to 3dB over the full density range up to n$=$10$^{\mathrm{13}}$ atoms cm$^{\mathrm{-3}}$, covering practical conditions used in optimized SNS experiments. Furthermore, we show that squeezing improves the trade-off between statistical sensitivity and systematic errors due to line broadening, a previously unobserved quantum advantage. Finally, we present a novel theoretical model on quantum limits of noise spectroscopies by defining a standard quantum limit under optimized regimes and by discussing the conditions of its overcoming due to squeezing. Reference: Lucivero, et al. arXiv:1509.05653 (2015) [Preview Abstract] |
Wednesday, May 25, 2016 8:48AM - 9:00AM |
G4.00005: High sensitivity ancilla assisted nanoscale DC magnetometry YiXiang Liu, Ashok Ajoy, Luca Marseglia, Kasturi Saha, Paola Cappellaro Sensing slowly varying magnetic fields are particularly relevant to many real world scenarios, where the signals of interest are DC or close to static. Nitrogen Vacancy (NV) centers in diamond are a versatile platform for such DC magnetometry on nanometer length scales. Using NV centers, the standard technique for measuring DC magnetic fields is via the Ramsey protocol, where sensitivities can approach better than $1\mu$T/vHz, but are limited by the sensor fast dephasing time $T_2^*$. In this work we instead present a method of sensing DC magnetic fields that is intrinsically limited by the much longer $T_2$ coherence time. The method exploits a strongly-coupled ancillary nuclear spin to achieve high DC field sensitivities potentially exceeding that of the Ramsey method. In addition, through this method we sense the perpendicular component of the DC magnetic field, which in conjunction with the parallel component sensed by the Ramsey method provides a valuable tool for vector DC magnetometry at the nanoscale. [Preview Abstract] |
Wednesday, May 25, 2016 9:00AM - 9:12AM |
G4.00006: Double Resonance Schemes for Nanoscale NV-NMR Emma Rosenfeld, Linh Pham, Chinmay Belthangady, Dominik Bucher, Trevor David Rhone, Francesco Casola, Ronald Walsworth Nitrogen-Vacancy (NV) centers in diamond enable promising applications in nanoscale magnetic resonance and manipulation of spins. In this talk, dressed state schemes for nanoscale magnetic resonance using single NV centers and neighboring spins will be discussed. Such schemes are $T_{1}^{\rho }$ limited and transfer polarization, enabling simultaneous manipulation and sensitive detection of both electronic and nuclear spins at the nanoscale. [Preview Abstract] |
Wednesday, May 25, 2016 9:12AM - 9:24AM |
G4.00007: Universal decoherence due to gravitational time dilation Igor Pikovski, Magdalena Zych, Fabio Costa, Caslav Brukner The absence of quantum behavior on macroscopic scales is usually attributed to decoherence --- the suppression of quantum superpositions due to interaction with an environment. Here we show that time dilation provides a universal decoherence mechanism for any complex system (1). The effect takes place even for isolated particles that do not interact with any external environment and causes decoherence of position and momentum of the center of mass of the system. While time dilation is very weak on earth, it is already sufficient to decohere gram-scale objects and complex molecules. The results show that novel phenomena arise at the interplay between quantum theory and general relativity even in the low energy limit. Possible experimental verifications of the effect are briefly discussed. \\ \\ (1) I. Pikovski, M. Zych, F. Costa, and \v{C}. Brukner, {\it Nature Physics} \textbf{11}, 668-672 (2015). [Preview Abstract] |
Wednesday, May 25, 2016 9:24AM - 9:36AM |
G4.00008: Quantum Measurement of Spin Correlations in a Symmetric Many-Body State$\backslash $f1 Ezad Shojaee, Amir Kalev, Ivan Deutsch h $-abstract-$\backslash $pard Continuous (nonprojective) measurement on a quantum system has been employed previously for fast, robust, and high-fidelity quantum state tomography (QST) on qudits [1]. We expand this protocol to many-body systems in order to perform QST on the reduced one-body and two-body density matrices of a symmetric many-body state of multiple qubits. Such QST will characterize the spin correlations in the system. In this protocol, a continuous measurement is done collectively on many copies of the reduced state at the same time, and therefore, while it is weakly perturbative on each copy, yields high signal-to-noise. Simultaneously, we subject the system to an external collective control in order to generate an informationally complete measurement record. We characterize the information-gain measurement disturbance tradeoff in terms of parameters in the problem (number of qubits, control parameters, shot-noise bandwidth, and the measurement strength).$\backslash $pard1] C. Riofr\'{\i}o,, P. S. Jessen, and I. H. Deutsch, ``Quantum tomography of the full hyperfine manifold of atomic spins via continuous measurement on an ensemble'', J. Phys. B 15, 154007 (2011).$\backslash $pard$\backslash $pard$\backslash $pard-/abstract-$\backslash $\tex [Preview Abstract] |
Wednesday, May 25, 2016 9:36AM - 9:48AM |
G4.00009: Generation and multi-pass propagation of a squeezed vacuum field in hot Rb vapor Mi Zhang, R. Nicholas Lanning, Zhihao Xiao, Jonathan P. Dowling, Irina Novikova, Eugeniy E. Mikhailov We study a squeezed vacuum field generated in hot Rb vapor via the polarization self-rotation effect. By propagating the strong laser beam through a vapor cell once, we were able to achieve a noise suppression of 2 dB below shot noise. Our previous experiments showed that the amount of observed squeezing may be limited by the contamination of the squeezed vacuum output with higher-order spatial modes, also generated inside the cell. Here, we investigate whether or not the squeezing can be improved by making the light interact several times with a less dense atomic ensemble. We carry out a comparison of various conditions, e.g. injection power, atomic density, passing numbers etc., and studied their effect on squeezing level and the spatial structure of the output squeezed vacuum field. We observe that multiple passages of beam through the medium can lead to an improvement of squeezing, and minimum noise occurs at almost the same effective atomic density for all setups. We show optimization of the conditions can lead to higher achievable squeezing which would be very useful for precision metrology and quantum memory applications. [Preview Abstract] |
Wednesday, May 25, 2016 9:48AM - 10:00AM |
G4.00010: Coherence-path duality relations for N paths Mark Hillery, Emilio Bagan, Janos Bergou, Seth Cottrell For an interferometer with two paths, there is a relation between the information about which path the particle took and the visibility of the interference pattern at the output. The more path information we have, the smaller the visibility, and vice versa. We generalize this relation to a multi-path interferometer, and we substitute two recently defined measures of quantum coherence for the visibility, which results in two duality relations. The path information is provided by attaching a detector to each path. In the first relation, which uses an $l_1$ measure of coherence, the path information is obtained by applying the minimum-error state discrimination procedure to the detector states. In the second, which employs an entropic measure of coherence, the path information is the mutual information between the detector states and the result of measuring them. Both approaches are quantitative versions of complementarity for $N$-path interferometers. [Preview Abstract] |
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