74 research outputs found
Estimation of Output Channel Noise for Continuous Variable Quantum Key Distribution
Estimation of channel parameters is important for extending the range and
increasing the key rate of continuous variable quantum key distribution
protocols. We propose a new estimator for the channel noise parameter based on
the method of moments. The method of moments finds an estimator from the
moments of the output distribution of the protocol. This estimator has the
advantage of being able to use all of the states shared between Alice and Bob.
Other estimators are limited to a smaller publicly revealed subset of the
states. The proposed estimator has a lower variance for high loss channel than
what has previously been proposed. We show that the method of moments estimator
increases the key rate by up to an order of magnitude at the maximum
transmission of the protocol.Comment: 5 pages, 3 figure
Security of Continuous Variable Quantum Cryptography
We discuss a quantum key distribution scheme in which small phase and
amplitude modulations of CW light beams carry the key information. The presence
of EPR type correlations provides the quantum protection. We identify universal
constraints on the level of shared information between the intended receiver
(Bob) and any eavesdropper (Eve) and use this to make a general evaluation of
security. We identify teleportation as an optimum eavesdropping technique.Comment: 6 figure
Quantum Cloning of Continuous Variable Entangled States
We consider the quantum cloning of continuous variable entangled states. This
is achieved by introducing two symmetric entanglement cloning machines (or
e-cloners): a local e-cloner and a global e-cloner; where we look at the
preservation of entanglement in the clones under the condition that the
fidelity of the clones is maximized. These cloning machines are implemented
using simple linear optical elements such as beam splitters and homodyne
detection along with squeeze gates. We show that the global e-cloner
out-performs the local e-cloner both in terms of the fidelity of the cloned
states as well as the strength of the entanglement of the clones. There is a
minimum strength of entanglement (3dB for the inseparability criterion and
5.7dB for the EPR paradox criterion) of the input state of the global e-cloner
that is required to preserve the entanglement in the clones.Comment: 11 pages, 6 figure
Gaussian Post-selection for Continuous Variable Quantum Cryptography
We extend the security proof for continuous variable quantum key distribution
protocols using post selection to account for arbitrary eavesdropping attacks
by employing the concept of an equivalent protocol where the post-selection is
implemented as a series of quantum operations including a virtual distillation.
We introduce a particular `Gaussian' post selection and demonstrate that the
security can be calculated using only experimentally accessible quantities.
Finally we explicitly evaluate the performance for the case of a noisy Gaussian
channel in the limit of unbounded key length and find improvements over all
pre-existing continuous variable protocols in realistic regimes.Comment: 4+4 pages. arXiv admin note: substantial text overlap with
arXiv:1106.082
Conditional quantum-state engineering using ancillary squeezed-vacuum states
We investigate an optical scheme to conditionally engineer quantum states
using a beam splitter, homodyne detection and a squeezed vacuum as an ancillar
state. This scheme is efficient in producing non-Gaussian quantum states such
as squeezed single photons and superpositions of coherent states (SCSs). We
show that a SCS with well defined parity and high fidelity can be generated
from a Fock state of , and conjecture that this can be generalized for
an arbitrary Fock state. We describe our experimental demonstration of this
scheme using coherent input states and measuring experimental fidelities that
are only achievable using quantum resources.Comment: 10 pages, 14 figures, use pdf version, high quality figures available
on reques
Measuring photon anti-bunching from continuous variable sideband squeezing
We present a technique for measuring the second-order coherence function
of light using a Hanbury-Brown Twiss intensity interferometer
modified for homodyne detection. The experiment was performed entirely in the
continuous variable regime at the sideband frequency of a bright carrier field.
We used the setup to characterize for thermal and coherent
states, and investigated its immunity to optical loss. We measured
of a displaced squeezed state, and found a best anti-bunching
statistic of .Comment: 4 pages, 4 figure
Quantum State Engineering with Continuous-Variable Post-Selection
We present a scheme to conditionally engineer an optical quantum system via
continuous-variable measurements. This scheme yields high-fidelity squeezed
single photon and superposition of coherent states, from input single and two
photon Fock states respectively. The input Fock state is interacted with an
ancilla squeezed vacuum state using a beam-splitter. We transform the quantum
system by post-selecting on the continuous-observable measurement outcome of
the ancilla state. We experimentally demonstrate the principles of this scheme
using displaced coherent states and measure experimentally fidelities that are
only achievable using quantum resources.Comment: 4 pages, 5 figures, publishe
Measurement-Based Noiseless Linear Amplification for Quantum Communication
Entanglement distillation is an indispensable ingredient in extended quantum
communication networks. Distillation protocols are necessarily
non-deterministic and require advanced experimental techniques such as
noiseless amplification. Recently it was shown that the benefits of noiseless
amplification could be extracted by performing a post-selective filtering of
the measurement record to improve the performance of quantum key distribution.
We apply this protocol to entanglement degraded by transmission loss of up to
the equivalent of 100km of optical fibre. We measure an effective entangled
resource stronger than that achievable by even a maximally entangled resource
passively transmitted through the same channel. We also provide a
proof-of-principle demonstration of secret key extraction from an otherwise
insecure regime. The measurement-based noiseless linear amplifier offers two
advantages over its physical counterpart: ease of implementation and near
optimal probability of success. It should provide an effective and versatile
tool for a broad class of entanglement-based quantum communication protocols.Comment: 7+3 pages, 5+1 figures, close to published versio
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