379 research outputs found
Entangling ability of a beam splitter in the presence of temporal which-path information
We calculate the amount of polarization-entanglement induced by two-photon
interference at a lossless beam splitter. Entanglement and its witness are
quantified respectively by concurrence and the Bell-CHSH parameter. In the
presence of a Mandel dip, the interplay of two kinds of which-path information
-- temporal and polarization -- gives rise to the existence of entangled
polarization-states that cannot violate the Bell-CHSH inequality.Comment: 8 pages including 2 figure
Quantum computing on encrypted data
The ability to perform computations on encrypted data is a powerful tool for
protecting privacy. Recently, protocols to achieve this on classical computing
systems have been found. Here we present an efficient solution to the quantum
analogue of this problem that enables arbitrary quantum computations to be
carried out on encrypted quantum data. We prove that an untrusted server can
implement a universal set of quantum gates on encrypted quantum bits (qubits)
without learning any information about the inputs, while the client, knowing
the decryption key, can easily decrypt the results of the computation. We
experimentally demonstrate, using single photons and linear optics, the
encryption and decryption scheme on a set of gates sufficient for arbitrary
quantum computations. Because our protocol requires few extra resources
compared to other schemes it can be easily incorporated into the design of
future quantum servers. These results will play a key role in enabling the
development of secure distributed quantum systems
Novel High-Speed Polarization Source for Decoy-State BB84 Quantum Key Distribution over Free Space and Satellite Links
To implement the BB84 decoy-state quantum key distribution (QKD) protocol
over a lossy ground-satellite quantum uplink requires a source that has high
repetition rate of short laser pulses, long term stability, and no phase
correlations between pulses. We present a new type of telecom optical
polarization and amplitude modulator, based on a balanced Mach-Zehnder
interferometer configuration, coupled to a polarization-preserving
sum-frequency generation (SFG) optical setup, generating 532 nm photons with
modulated polarization and amplitude states. The weak coherent pulses produced
by SFG meet the challenging requirements for long range QKD, featuring a high
clock rate of 76 MHz, pico-second pulse width, phase randomization, and 98%
polarization visibility for all states. Successful QKD has been demonstrated
using this apparatus with full system stability up to 160 minutes and channel
losses as high 57 dB [Phys. Rev. A, Vol. 84, p.062326]. We present the design
and simulation of the hardware through the Mueller matrix and Stokes vector
relations, together with an experimental implementation working in the telecom
wavelength band. We show the utility of the complete system by performing high
loss QKD simulations, and confirm that our modulator fulfills the expected
performance.Comment: 21 pages, 8 figures and 2 table
Experimental quantum key distribution with simulated ground-to-satellite photon losses and processing limitations
Quantum key distribution (QKD) has the potential to improve communications
security by offering cryptographic keys whose security relies on the
fundamental properties of quantum physics. The use of a trusted quantum
receiver on an orbiting satellite is the most practical near-term solution to
the challenge of achieving long-distance (global-scale) QKD, currently limited
to a few hundred kilometers on the ground. This scenario presents unique
challenges, such as high photon losses and restricted classical data
transmission and processing power due to the limitations of a typical satellite
platform. Here we demonstrate the feasibility of such a system by implementing
a QKD protocol, with optical transmission and full post-processing, in the
high-loss regime using minimized computing hardware at the receiver. Employing
weak coherent pulses with decoy states, we demonstrate the production of secure
key bits at up to 56.5 dB of photon loss. We further illustrate the feasibility
of a satellite uplink by generating secure key while experimentally emulating
the varying channel losses predicted for realistic low-Earth-orbit satellite
passes at 600 km altitude. With a 76 MHz source and including finite-size
analysis, we extract 3374 bits of secure key from the best pass. We also
illustrate the potential benefit of combining multiple passes together: while
one suboptimal "upper-quartile" pass produces no finite-sized key with our
source, the combination of three such passes allows us to extract 165 bits of
secure key. Alternatively, we find that by increasing the signal rate to 300
MHz it would be possible to extract 21570 bits of secure finite-sized key in
just a single upper-quartile pass.Comment: 12 pages, 7 figures, 2 table
Mitigating radiation damage of single photon detectors for space applications
Single-photon detectors in space must retain useful performance
characteristics despite being bombarded with sub-atomic particles. Mitigating
the effects of this space radiation is vital to enabling new space applications
which require high-fidelity single-photon detection. To this end, we conducted
proton radiation tests of various models of avalanche photodiodes (APDs) and
one model of photomultiplier tube potentially suitable for satellite-based
quantum communications. The samples were irradiated with 106 MeV protons at
doses approximately equivalent to lifetimes of 0.6 , 6, 12 and 24 months in a
low-Earth polar orbit. Although most detection properties were preserved,
including efficiency, timing jitter and afterpulsing probability, all APD
samples demonstrated significant increases in dark count rate (DCR) due to
radiation-induced damage, many orders of magnitude higher than the 200 counts
per second (cps) required for ground-to-satellite quantum communications. We
then successfully demonstrated the mitigation of this DCR degradation through
the use of deep cooling, to as low as -86 degrees C. This achieved DCR below
the required 200 cps over the 24 months orbit duration. DCR was further reduced
by thermal annealing at temperatures of +50 to +100 degrees C.Comment: The license has been corrected. Note that the license of v2 was
incorrect and not valid. No other changes since v
Nondegenerate parametric down conversion in coherently prepared two-level atomic gas
We describe parametric down conversion process in a two-level atomic gas,
where the atoms are in a superposition state of relevant energy levels. This
superposition results in splitting of the phase matching condition into three
different conditions. Another, more important, peculiarity of the system under
discussion is the nonsaturability of amplification coefficients with increasing
pump wave intensity, under "sideband" generation conditions
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