1,212 research outputs found
Dispersion and fidelity in quantum interferometry
We consider Mach-Zehnder and Hong-Ou-Mandel interferometers with nonclassical
states of light as input, and study the effect that dispersion inside the
interferometer has on the sensitivity of phase measurements. We study in detail
a number of different one- and two-photon input states, including Fock, dual
Fock, N00N states, and photon pairs from parametric downconversion. Assuming
there is a phase shift in one arm of the interferometer, we compute
the probabilities of measurement outcomes as a function of , and then
compute the Shannon mutual information between and the measurements.
This provides a means of quantitatively comparing the utility of various input
states for determining the phase in the presence of dispersion. In addition, we
consider a simplified model of parametric downconversion for which
probabilities can be explicitly computed analytically, and which serves as a
limiting case of the more realistic downconversion model.Comment: 12 pages, 14 figures. Submitted to Physical Review
Symmetric Autocompensating Quantum Key Distribution
We present quantum key distribution schemes which are autocompensating
(require no alignment) and symmetric (Alice and Bob receive photons from a
central source) for both polarization and time-bin qubits. The primary benefit
of the symmetric configuration is that both Alice and Bob may have passive
setups (neither Alice nor Bob is required to make active changes for each run
of the protocol). We show that both the polarization and the time-bin schemes
may be implemented with existing technology. The new schemes are related to
previously described schemes by the concept of advanced waves.Comment: 4 pages, 2 figur
Performance of Photon-Pair Quantum Key Distribution Systems
We analyze the quantitative improvement in performance provided by a novel
quantum key distribution (QKD) system that employs a correlated photon source
(CPS) and a photon-number resolving detector (PNR). Our calculations suggest
that given current technology, the CPR implementation offers an improvement of
several orders of magnitude in secure bit rate over previously described
implementations
Goldstone-Mode Phonon Dynamics in the Pyrochlore Cd2Re2O7
We have measured the polarized Raman scattering spectra of Cd2Re2O7, the
first superconducting pyrochlore, as a function of temperature. For
temperatures below the cubic-to-tetragonal structural phase transition (SPT) at
200K, a peak with B1 symmetry develops at zero frequency with divergent
intensity. We identify this peak as the first observation of the Goldstone
phonon in a crystalline solid. The Goldstone phonon is a collective excitation
that exists due to the breaking of the continuous symmetry with the SPT. Its
emergence coincides with that of a Raman-active soft mode. The order parameter
for both features derives from an unstable doubly-degenerate vibration (with Eu
symmetry) of the O1 atoms which drives the SPT.Comment: 4+ pages, 4 figures. Updated figures and text. Accepted to PR
Metallic "Ferroelectricity" in the Pyrochlore Cd2Re2O7
A class of materials known as ``ferroelectric metals'' was discussed
theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)],
but to date no examples of this class have been reported. Here we present
measurements of the elastic moduli of Cd2Re2O7 through the 200 K
cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals
that the transition is consistent with Cd2Re2O7 being classified as a
``ferroelectric metal'' in the weaker sense described by Anderson and Blount
(loss of a center of symmetry). First-principles calculations of the lattice
instabilities indicate that the dominant lattice instability corresponds to a
two-fold degenerate mode with Eu symmetry, and that motions of the O ions
forming the O octahedra dominate the energetics of the transition.Comment: 4 pages, 2 figure
Multi-Particle Interferometry Based on Double Entangled States
A method for producing a 4-photon entangled state based on the use of two independent pair sources is discussed. Of particular interest is that each of the pair sources produces a two-photon state which is simultaneously entangled in both polarization and space-time variables. Performing certain measurements which exploit this double entanglement provides an opportunity for verifying the recent demonstration of nonlocality by Greenberger, Horne, and Zeilinger
Two-Photon Entanglement and EPR Experiments Using Type-2 Spontaneous Parametric Down Conversion
Simultaneous entanglement in spin and space-time of a two-photon quantum state generated in type-2 spontaneous parametric down-conversion is demonstrated by the observation of quantum interference with 98% visibility in a simple beam-splitter (Hanburry Brown-Twiss) anticorrelation experiment. The nonlocal cancellation of two-photon probability amplitudes as a result of this double entanglement allows us to demonstrate two different types of Bell's inequality violations in one experimental setup
Two-Photon Ghost Image and Interference-Diffraction
One of the most surprising consequences of quantum mechanics is entanglement of two or more distance particles. The two-particle entangled state was mathematically formulated by Schrodinger. Based on this unusual quantum behavior, EPR defined their 'physical reality' and then asked the question: 'Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?' One may not appreciate EPR's criterion of physical reality and insist that 'no elementary quantum phenomenon is a phenomenon until it is a recorded phenomenon'. Optical spontaneous parametric down conversion (SPDC) is the most effective mechanism to generate an EPR type entangled two-photon state. In SPDC, an optical beam, called the pump, is incident on a birefringent crystal. The pump is intense enough so that nonlinear effects lead to the conversion of pump photons into pairs of photons, historically called signal and idler. Technically, the SPDC is said to be type-1 or type-2, depending on whether the signal and idler beams have parallel or orthogonal polarization. The SPDC conversion efficiency is typically on the order of 10(exp -9) to 10(exp -11), depending on the SPDC nonlinear material. The signal and idler intensities are extremely low, only single photon detection devices can register them. The quantum entanglement nature of SPDC has been demonstrated in EPR-Bohm experiments and Bell's inequality measurements. The following two experiments were recently performed in our laboratory, which are more closely related to the original 1935 EPR gedankenezperiment. The first experiment is a two-photon optical imaging type experiment, which has been named 'ghost image' by the physics community. The signal and idler beams of SPDC are sent in different directions, so that the detection of the signal and idler photons can be performed by two distant photon counting detectors. An aperture object (mask) is placed in front of the signal photon detector and illuminated by the signal beam through a convex lens. Surprisingly, an image of this aperture is observed in the idler beam, by scanning the idler photon detector in the transverse plane of the idler beam, if we are sure that the idler photon detector 'catches' the 'twin brother' of the signal, which can be easily performed by a coincidence measurement. This effect is even more striking when we found that the object-lens-image relationship satisfies the Gaussian thin lens equation. The second experiment demonstrates two-photon 'ghost' interference-diffraction. The experimental set up is similar to the image experiment, except that rather than a lens and an aperture it is a Young's double-slit (or a single-slit) inserted into the path of the signal beam. We could not find any interference (or diffraction) pattern behind the slit. Surprisingly, an interference (or diffraction) pattern is observed when scanning the detector in the idler beam, if we are sure that the idler photon detector 'catches' the 'twin brother' of the signal
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