392 research outputs found
A bridge between the single-photon and squeezed-vacuum state
The two modes of the Einstein-Podolsky-Rosen quadrature entangled state
generated by parametric down-conversion interfere on a beam splitter of
variable splitting ratio. Detection of a photon in one of the beam splitter
output channels heralds preparation of a signal state in the other, which is
characterized using homodyne tomography. By controlling the beam splitting
ratio, the signal state can be chosen anywhere between the single-photon and
squeezed state
Maximum-likelihood coherent-state quantum process tomography
Coherent-state quantum process tomography (csQPT) is a method of completely
characterizing a quantum-optical "black box" by probing it with coherent states
and performing homodyne measurements on the output [M. Lobino et al, Science
322, 563 (2008)]. We present a technique for csQPT that is fully based on
statistical inference, specifically, quantum expectation-maximization. The
method relies on the Jamiolkowski isomorphism and iteratively reconstructs the
process tensor in the Fock basis directly from the experimental data. This
approach permits incorporation of a priori constraints into the reconstruction
procedure, thereby guaranteeing that the resulting process tensor is physically
consistent. Furthermore, our method is easier to implement and requires a
narrower range of coherent states than its predecessors. We test its
feasibility using simulations on several experimentally relevant processes.Comment: 17 pages, 4 figure
Interconvertibility of single-rail optical qubits
We show how to convert between partially coherent superpositions of a single
photon with the vacuum using linear optics and postselection based on homodyne
measurements. We introduce a generalized quantum efficiency for such states and
show that any conversion that decreases this quantity is possible. We also
prove that our scheme is optimal by showing that no linear optical scheme with
generalized conditional measurements, and with one single-rail qubit input can
improve the generalized efficiency.Comment: 3 pages, 2 figure
Linear-optical processing cannot increase photon efficiency
We answer the question whether linear-optical processing of the states
produced by one or multiple imperfect single-photon sources can improve the
single-photon fidelity. This processing can include arbitrary interferometers,
coherent states, feedforward, and conditioning on results of detections. We
show that without introducing multiphoton components, the single-photon
fraction in any of the single-mode states resulting from such processing cannot
be made to exceed the efficiency of the best available photon source. If
multiphoton components are allowed, the single-photon fidelity cannot be
increased beyond 1/2. We propose a natural general definition of the
quantum-optical state efficiency, and show that it cannot increase under
linear-optical processing.Comment: 4 pages, 3 figure
Remote preparation of a single-mode photonic qubit by measuring field quadrature noise
An electromagnetic field quadrature measurement, performed on one of the
modes of the nonlocal single-photon state , collapses it into a
superposition of the single-photon and vacuum states in the other mode. We use
this effect to implement remote preparation of arbitrary single-mode photonic
qubits conditioned on observation of a preselected quadrature value. The
quantum efficiency of the prepared qubit can be higher than that of the initial
single photon
Versatile Digital GHz Phase Lock for External Cavity Diode Lasers
We present a versatile, inexpensive and simple optical phase lock for
applications in atomic physics experiments. Thanks to all-digital phase
detection and implementation of beat frequency pre-scaling, the apparatus
requires no microwave-range reference input, and permits phase locking at
frequency differences ranging from sub-MHz to 7 GHz (and with minor extension,
to 12 GHz). The locking range thus covers ground state hyperfine splittings of
all alkali metals, which makes this system a universal tool for many
experiments on coherent interaction between light and atoms.Comment: 4.5 pages, 5 figures v3: fixed error in schematic: R10 connects to
other end of C
A Monolithic Filter Cavity for Experiments in Quantum Optics
By applying a high-reflectivity dielectric coating on both sides of a
commercial plano-convex lens, we produce a stable monolithic Fabry-Perot cavity
suitable for use as a narrow band filter in quantum optics experiments. The
resonant frequency is selected by means of thermal expansion. Owing to the long
term mechanical stability, no optical locking techniques are required. We
characterize the cavity performance as an optical filter, obtaining a 45 dB
suppression of unwanted modes while maintaining a transmission of 60%.Comment: 4 pages, 4 figure
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