115 research outputs found

### Squeezing based on nondegenerate frequency doubling internal to a realistic laser

We investigate theoretically the quantum fluctuations of the fundamental
field in the output of a nondegenerate second harmonic generation process
occuring inside a laser cavity. Due to the nondegenerate character of the
nonlinear medium, a field orthogonal to the laser field is for some operating
conditions indepedent of the fluctuations produced by the laser medium. We show
that this fact may lead to perfect squeezing for a certain polarization mode of
the fundamental field. The experimental feasibility of the system is also
discussed.Comment: 6 pages, 5 figure

### Proposal for optical parity state re-encoder

We propose a re-encoder to generate a refreshed parity encoded state from an
existing parity encoded state. This is the simplest case of the scheme by
Gilchrist et al. (Phys. Rev. A 75, 052328). We show that it is possible to
demonstrate with existing technology parity encoded quantum gates and
teleportation.Comment: 8 pages, 4 figure

### Input states for quantum gates

We examine three possible implementations of non-deterministic linear optical
cnot gates with a view to an in-principle demonstration in the near future. To
this end we consider demonstrating the gates using currently available sources
such as spontaneous parametric down conversion and coherent states, and current
detectors only able to distinguish between zero or many photons. The
demonstration is possible in the co-incidence basis and the errors introduced
by the non-optimal input states and detectors are analysed

### Non-deterministic Gates for Photonic Single Rail Quantum Logic

We discuss techniques for producing, manipulating and measureing qubits
encoded optically as vacuum and single photon states. We show that a universal
set of non-deterministic gates can be constructed using linear optics and
photon counting. We investigate the efficacy of a test gate given realistic
detector efficiencies.Comment: 8 pages, 6 figure

### Comparison of LOQC C-sign gates with ancilla inefficiency and an improvement to functionality under these conditions

We compare three proposals for non-deterministic C-sign gates implemented
using linear optics and conditional measurements with non-ideal ancilla mode
production and detection. The simplified KLM gate [Ralph et al, Phys.Rev.A {\bf
65}, 012314 (2001)] appears to be the most resilient under these conditions. We
also find that the operation of this gate can be improved by adjusting the
beamsplitter ratios to compensate to some extent for the effects of the
imperfect ancilla.Comment: to appear in PR

### Secure quantum key distribution using squeezed states

We prove the security of a quantum key distribution scheme based on
transmission of squeezed quantum states of a harmonic oscillator. Our proof
employs quantum error-correcting codes that encode a finite-dimensional quantum
system in the infinite-dimensional Hilbert space of an oscillator, and protect
against errors that shift the canonical variables p and q. If the noise in the
quantum channel is weak, squeezing signal states by 2.51 dB (a squeeze factor
e^r=1.34) is sufficient in principle to ensure the security of a protocol that
is suitably enhanced by classical error correction and privacy amplification.
Secure key distribution can be achieved over distances comparable to the
attenuation length of the quantum channel.Comment: 19 pages, 3 figures, RevTeX and epsf, new section on channel losse

### Experimental requirements for Grover's algorithm in optical quantum computation

The field of linear optical quantum computation (LOQC) will soon need a
repertoire of experimental milestones. We make progress in this direction by
describing several experiments based on Grover's algorithm. These experiments
range from a relatively simple implementation using only a single non-scalable
CNOT gate to the most complex, requiring two concatenated scalable CNOT gates,
and thus form a useful set of early milestones for LOQC. We also give a
complete description of basic LOQC using polarization-encoded qubits, making
use of many simplifications to the original scheme of Knill, Laflamme, and
Milburn.Comment: 9 pages, 8 figure

### Experimental investigation of continuous variable quantum teleportation

We report the experimental demonstration of quantum teleportation of the
quadrature amplitudes of a light field. Our experiment was stably locked for
long periods, and was analyzed in terms of fidelity, F; and with signal
transfer, T_{q}=T^{+}+T^{-}, and noise correlation, V_{q}=V_{in|out}^{+}
V_{in|out}^{-}. We observed an optimum fidelity of 0.64 +/- 0.02, T_{q}= 1.06
+/- 0.02 and V_{q} =0.96 +/- 0.10. We discuss the significance of both T_{q}>1
and V_{q}<1 and their relation to the teleportation no-cloning limit.Comment: 4 pages, 4 figure

### Asymmetric quantum channel for quantum teleportation

There are a few obstacles, which bring about imperfect quantum teleportation
of a continuous variable state, such as unavailability of maximally entangled
two-mode squeezed states, inefficient detection and imperfect unitary
transformation at the receiving station. We show that all those obstacles can
be understood by a combination of an {\it asymmetrically-decohered} quantum
channel and perfect apparatuses for other operations. For the
asymmetrically-decohered quantum channel, we find some counter-intuitive
results; one is that teleportation does not necessarily get better as the
channel is initially squeezed more and another is when one branch of the
quantum channel is unavoidably subject to some imperfect operations, blindly
making the other branch as clean as possible may not result in the best
teleportation result. We find the optimum strategy to teleport an unknown field
for a given environment or for a given initial squeezing of the channel.Comment: 4pages, 1figur

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