arXiv subjects

T. W. Lynn

Publications and source records attributed to T. W. Lynn.

7 recordsLinked to original sources

arXiv2011

Entanglement-secured single-qubit quantum secret-sharing

In single-qubit quantum secret sharing, a secret is shared between N parties via manipulation and measurement of one qubit at a time. Each qubit is sent to all N parties in sequence; the secret is encoded in the first participant's preparation of the qubit state and the subsequent participants' choices of state rotation or measurement basis. We present a protocol for single-qubit quantum secret sharing using polarization entanglement of photon pairs produced in type-I spontaneous parametric downconversion. We investigate the protocol's security against eavesdropping attack under common experimental conditions: a lossy channel for photon transmission, and imperfect preparation of the initial qubit state. A protocol which exploits entanglement between photons, rather than simply polarization correlation, is more robustly secure. We implement the entanglement-based secret-sharing protocol with 87% secret-sharing fidelity, limited by the purity of the entangled state produced by our present apparatus. We demonstrate a photon-number splitting eavesdropping attack, which achieves no success against the entanglement-based protocol while showing the predicted rate of success against a correlation-based protocol.

quant-ph↗

arXiv2011

Distinguishability of hyperentangled Bell state by linear evolution and local projective measurement

Measuring an entangled state of two particles is crucial to many quantum communication protocols. Yet Bell state distinguishability using a finite apparatus obeying linear evolution and local measurement is theoretically limited. We extend known bounds for Bell-state distinguishability in one and two variables to the general case of entanglement in $n$ two-state variables. We show that at most $2^{n+1}-1$ classes out of $4^n$ hyper-Bell states can be distinguished with one copy of the input state. With two copies, complete distinguishability is possible. We present optimal schemes in each case.

quant-ph↗

arXiv2005

Studies of Air Showers above 10^18 eV with the CHICOS Array

CHICOS (California HIgh school Cosmic ray ObServatory) is presently an array of more than 140 detectors distributed over a large area (~400 km^2) of southern California, and will consist of 180 detectors at 90 locations in the near future. These sites, located at area schools, are equipped with computerized data acquisition and automatic nightly data transfer (via internet) to our Caltech lab. The installed sites make up the largest currently operating ground array for ultra-high energy cosmic ray research in the northern hemisphere. The goal of CHICOS is to provide data related to the flux and distribution of arrival directions for ultra-high energy cosmic rays. We have performed detailed Monte-Carlo calculations to determine the density and arrival-time distribution of charged particles in extensive air showers for the CHICOS array. Calculations were performed for proton primaries with energies 10^18 to 10^21 eV and zenith angles out to 50 degrees. We have developed novel parameterizations for both distributions as functions of distance from the shower axis, primary energy, and incident zenith angle. These parameterizations are used in aperture calculations and reconstruction of shower data, enabling preliminary analysis of ultra-high energy shower data from CHICOS.

astro-ph↗

arXiv2005

Search for Correlated High Energy Cosmic Ray Events with CHICOS

We present the results of a search for time correlations in high energy cosmic ray data (primary E > 10^14 eV) collected by the California HIgh school Cosmic ray ObServatory (CHICOS) array. Data from 69 detector sites spread over an area of 400 km^2 were studied for evidence of isolated events separated by more than 1 km with coincidence times ranging from 1 microseconds up to 1 second. We report upper limits for the coincidence probability as a function of coincidence time.

astro-ph↗

arXiv2005

Strategies for Real-Time Position Control of a Single Atom in Cavity QED

Recent realizations of single-atom trapping and tracking in cavity QED open the door for feedback schemes which actively stabilize the motion of a single atom in real time. We present feedback algorithms for cooling the radial component of motion for a single atom trapped by strong coupling to single-photon fields in an optical cavity. Performance of various algorithms is studied through simulations of single-atom trajectories, with full dynamical and measurement noise included. Closed loop feedback algorithms compare favorably to open-loop "switching" analogs, demonstrating the importance of applying actual position information in real time. The high optical information rate in current experiments enables real-time tracking that approaches the standard quantum limit for broadband position measurements, suggesting that realistic active feedback schemes may reach a regime where measurement backaction appreciably alters the motional dynamics.

quant-ph↗

arXiv2005

Search for Correlated High Energy Cosmic Ray Events with CHICOS

We present the results of a search for time correlations in high energy cosmic ray data (primary E > 10^{14} eV) collected by the California HIgh school Cosmic ray ObServatory (CHICOS) array. Data from 60 detector sites spread over an area of 400 km^2 were studied for evidence of correlated events separated by more than 1 km with coincidence times ranging from 1 microsec up to 1 second. All searches were consistent with the absence of excess coincidences except for a 2.9 sigma excess observed for coincidence times less than 10 microsec. We report upper limits for the coincidence probability as a function of coincidence time.

astro-ph↗

arXiv2000

Trapping of Single Atoms with Single Photons in Cavity QED

Two recent experiments have reported the trapping of individual atoms inside optical resonators by the mechanical forces associated with single photons [Hood et al., Science 287, 1447 (2000) and Pinkse et al., Nature 404, 365 (2000)]. Here we analyze the trapping dynamics in these settings, focusing on two points of interest. Firstly, we investigate the extent to which light-induced forces in these experiments are distinct from their free-space counterparts. Secondly, we explore the quantitative features of the resulting atomic motion and how these dynamics are mapped onto variations of the intracavity field. Not surprisingly, qualitatively distinct atomic dynamics arise as the coupling and dissipative rates are varied. For the experiment of Hood et al., we show that atomic motion is largely conservative and is predominantly in radial orbits transverse to the cavity axis. A comparison with the free-space theory demonstrates that the fluctuations of the dipole force are suppressed by an order of magnitude. This effect is based upon the Jaynes-Cummings eigenstates of the atom-cavity system and represents qualitatively new physics for optical forces at the single-photon level. By contrast, even in a regime of strong coupling in the experiment of Pinkse et al., there are only small quantitative distinctions between the free-space theory and the quantum theory, so it is not clear that description of this experiment as a novel single-quantum trapping effect is necessary. The atomic motion is strongly diffusive, leading to an average localization time comparable to the time for an atom to transit freely through the cavity and to a reduction in the ability to infer aspects of the atomic motion from the intracavity photon number.

quant-ph↗