Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session W6: Focus Session: Quantum Measurements and Tomography |
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Sponsoring Units: GQI Chair: Olivier Pfister, University of Virginia Room: Minor Hall 125 |
Saturday, May 23, 2009 8:00AM - 8:30AM |
W6.00001: Quantum Measurements Based on Photon Number Resolved Detection Invited Speaker: The characterization of any quantum system requires measurements, which allows an observer to gain information about a performed experiment. The theory of quantum measurements connects the properties of a quantum state, which is typically defined by its density matrix \t{$\rho$}, and the description of the measurement devices, represented by a positive-operator-valued measure (POVM), with the probabilities of obtaining specific detection outcomes. The way on how we interpret our results depends, on the one hand, on the technical limitations of available detectors, and on the other hand, on our knowledge about the measurement apparatus. Up to recently no practical photon-number resolving detectors were available. Hence most research dealing with multi-photon states is based on homodyne tomography schemes. A time-multiplexing detector (TMD) that is capable to resolve photon statistics can be built from a fiber network followed by avalanche photo-detection. TMDs enable the direct measurement of count statistics, but their moderate efficiency hampers identifying the photon number of each signal state on a single-shot basis. The POVMs describing this detector correspond to loss-degraded photon number measurements, and a precise calibration of the losses can be utilized to recover the original photon number statistics in ensemble measurements by a loss inversion method. However, the knowledge of the photon statistics is not sufficient to completely characterize a state, because photon counting annihilates any information about the coherences between photon numbers. Nevertheless, TMD measurements can render a complete characterization of a density matrix \t{$\rho$}, if the statistics of the displaced states are analyzed. We investigate the capabilities of detector tomography and loss-tolerant detection of photon statistics for the complete characterization of photonic states. [Preview Abstract] |
Saturday, May 23, 2009 8:30AM - 9:00AM |
W6.00002: Coherent State Quantum Process Tomography Invited Speaker: Any quantum information device or circuit can be described as a quantum black box that maps input density matrices into the corresponding outputs. In this context, it is important to understand how the black box transforms a generic quantum state. This task requires a full characterization of the quantum process associated with the device by means of quantum process tomography (QPT). Recently, we developed a protocol, called coherent state QPT (csQPT), which recovers the process superoperator tensor by measuring, through homodyne tomography, the process outputs from a set of input coherent states. Hence, using a common laser source, at different intensities, we are able to reconstruct any quantum-optical process. Our technique is based on the fact that any input state density matrix can be decomposed in a superposition of coherent state density matrices using an approximated Glauber Sudarshan P-function. Then using the linearity of the process in the density matrix space, we can recover the effect of the process on the input state from the coherent state mapping. We applied our procedure to a quantum memory for light based on electromagnetically-induced transparency in warm Rubidium atoms and recovered the superoperator tensor in Fock basis. From this analysis we were able to predict how an arbitrary quantum state of light will be affected by storage in a memory apparatus. We tested our characterization by experimentally storing and retrieving squeezed vacuum under different experimental conditions, and comparing the results with the csQPT prediction. We observed a quantum mechanical fidelity greater then 99{\%}. The process tomography also offers important insights into the detrimental effects that affect the storage performance and provide important feedback for the device optimization. Furthermore, we can check the memory performance against any theoretical benchmark. [Preview Abstract] |
Saturday, May 23, 2009 9:00AM - 9:12AM |
W6.00003: Toward Quantum Non-demolition of nitrogen-vacancy centers in diamond Jonathan Hodges, Liang Jiang, Jeronimo Maze, Mikhail Lukin The nitrogen-vacancy color center (NVC) in diamond, which possesses a long-lived electronic spin (S=1) ground state with optical addressability, is a promising platform for quantum networks, single-photon sources, and nanoscale magnetometers. Here, we make use of a nuclear spin based quantum memory to demonstrate quantum non-demolition measurement of a solid-state spin qubit. By entangling the electron spin with a polarized carbon-13 spin (I=1/2) in the lattice, we have repeated optical measurement of the electron spin for the polarization lifetime of the nuclear spin. We show relative improvements in signal-to-noise of greater than 300\%. These techniques can be used to improve the sensitivity of NVC magnetometers. [Preview Abstract] |
Saturday, May 23, 2009 9:12AM - 9:24AM |
W6.00004: Improving Sensitivity and Bandwidth of an Atomic Magnetometer using Quantum Non-Demolition Measurement Vishal Shah, Georgios Vasilakis, Michael Romalis The fundamental sensitivity of an atomic magnetometer is limited by spin projection noise. In the case of uniform spin relaxation, it is well understood that it is not possible to improve the sensitivity using spin squeezing induced by quantum non-demolition (QND) measurement for measurement time scales longer than spin relaxation time [1, 2]. It is however possible to increase the bandwidth of the magnetometer using QND measurement. Here we experimentally demonstrate, in excellent agreement with the theory, an improvement in the bandwidth of our scalar alkali vapor atomic magnetometer using continuous QND measurement. We also investigate the possibility of improving sensitivity of our magnetometer in the special case in which the spin relaxation is time dependent. The case of time dependent spin relaxation naturally arises in high polarization regime in an alkali-alkali spin-exchange relaxation dominated atomic sample. [1] S. F. Huelga, Phys. Rev. Lett. 79, 3865 -- 3868, 1997. [2] M. Auzinsh, Phys. Rev. Lett. 93, 173002, 2004. [Preview Abstract] |
Saturday, May 23, 2009 9:24AM - 9:36AM |
W6.00005: Entanglement-based Free Space Quantum Cryptography in Daylight Ilja Gerhardt, Matthew P. Peloso, Caleb Ho, Antia Lamas-Linares, Christian Kurtsiefer In quantum key distribution (QKD) two families of protocols are established: One, based on preparing and sending approximations of single photons, the other based on measurements on entangled photon pairs, which allow to establish a secret key using less assumptions on the size of a Hilbert space. The larger optical bandwidth of photon pairs in comparison with light used for the first family makes establishing a free space link challenging. We present a complete entanglement based QKD system following the BBM92 protocol, which generates a secure key continuously 24 hours a day between distant parties. Spectral, spatial and temporal filtering schemes were introduced to a previous setup, suppressing more than 30\,dB of background. We are able to establish the link during daytime, and have developed an algorithm to start and maintain time synchronization with simple crystal oscillators. [Preview Abstract] |
Saturday, May 23, 2009 9:36AM - 9:48AM |
W6.00006: Quantum State Reconstruction and Random Evolution Carlos Riofrio, Seth Merkel, Steven Flammia, Ivan Deutsch In order to perform quantum state reconstruction, the set of measured observables must be informationally complete. In this work, we explore the performance of the reconstruction algorithm developed by Silberfarb et al. (PRL 95, 030402(2005)) under the asumption that the quantum system undergoes random evolution. We show that in that case, although the measurements do not span the space of all density matrices, we are able to reconstruct the set of all pure states and almost-all mixed states with very high fidelities. We find that this is only possible after the inclusion of the physical constraint of positivity. Using as an example the quantum states stored in ground-electronic hyperfine manifold (F=3) of an ensemble of Cs atoms controlled by time-varying magnetic fields and nonlinear light-shift, we give a possible physical realization of this protocol provided that the dynamics exhibits a classically chaotic phase space. For this purpose, we chose the well studied quantum kicked top dynamics. [Preview Abstract] |
Saturday, May 23, 2009 9:48AM - 10:00AM |
W6.00007: ABSTRACT WITHDRAWN |
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