32 research outputs found
Discrimination of binary coherent states using a homodyne detector and a photon number resolving detector
We investigate quantum measurement strategies capable of discriminating two
coherent states probabilistically with significantly smaller error
probabilities than can be obtained using non- probabilistic state
discrimination. We apply a postselection strategy to the measurement data of a
homodyne detector as well as a photon number resolving detector in order to
lower the error probability. We compare the two different receivers with an
optimal intermediate measurement scheme where the error rate is minimized for a
fixed rate of inconclusive results. The photon number resolving (PNR) receiver
is experimentally demonstrated and compared to an experimental realization of a
homodyne receiver with postselection. In the comparison it becomes clear, that
the perfromance of the new PNR receiver surpasses the performance of the
homodyne receiver, which we proof to be optimal within any Gaussian operations
and conditional dynamics.Comment: 10 pages, 6 figure
Demonstration of Coherent State Discrimination Using a Displacement Controlled Photon Number Resolving Detector
We experimentally demonstrate a new measurement scheme for the discrimination
of two coherent states. The measurement scheme is based on a displacement
operation followed by a photon number resolving detector, and we show that it
outperforms the standard homodyne detector which we, in addition, proof to be
optimal within all Gaussian operations including conditional dynamics. We also
show that the non-Gaussian detector is superior to the homodyne detector in a
continuous variable quantum key distribution scheme.Comment: 5 pages, 3 figure
Electronic noise-free measurements of squeezed light
We study the implementation of a correlation measurement technique for the
characterization of squeezed light. We show that the sign of the covariance
coefficient revealed from the time resolved correlation data allow us to
distinguish between squeezed, coherent and thermal states. In contrast to the
traditional method of characterizing squeezed light, involving measurement of
the variation of the difference photocurrent, the correlation measurement
method allows to eliminate the contribution of the electronic noise, which
becomes a crucial issue in experiments with dim sources of squeezed light.Comment: submitted for publicatio
Atmospheric continuous-variable quantum communication
We present a quantum communication experiment conducted over a point-to-point
free-space link of 1.6 km in urban conditions. We study atmospheric influences
on the capability of the link to act as a continuous-variable (CV) quantum
channel. Continuous polarization states (that contain the signal encoding as
well as a local oscillator in the same spatial mode) are prepared and sent over
the link in a polarization multiplexed setting. Both signal and local
oscillator undergo the same atmospheric fluctuations. These are intrinsically
auto-compensated which removes detrimental influences on the interferometric
visibility. At the receiver, we measure the Q-function and interpret the data
using the framework of effective entanglement. We compare different state
amplitudes and alphabets (two-state and four-state) and determine their optimal
working points with respect to the distributed effective entanglement. Based on
the high entanglement transmission rates achieved, our system indicates the
high potential of atmospheric links in the field of CV QKD.Comment: 13 pages, 7 figure
Optimal working points for continuous-variable quantum channels
The most important ability of a quantum channel is to preserve the quantum
properties of transmitted quantum states. We experimentally demonstrate a
continuous-variable system for efficient benchmarking of quantum channels. We
probe the tested quantum channels for a wide range of experimental parameters
such as amplitude, phase noise and channel lengths up to 40 km. The data is
analyzed using the framework of effective entanglement. We subsequently are
able to deduce an optimal point of operation for each quantum channel with
respect to the rate of distributed entanglement. This procedure is a promising
candidate for benchmarking quantum nodes and individual links in large quantum
networks of different physical implementations.Comment: 5 pages, 4 (colour) figures; v2 changes: Added PACS numbers,
corrections to citations/page numbers, minor rephrasin
The polarization properties of a tilted polarizer
Polarizers are key components in optical science and technology. Thus,
understanding the action of a polarizer beyond oversimplifying approximations
is crucial. In this work, we study the interaction of a polarizing interface
with an obliquely incident wave experimentally. To this end, a set of Mueller
matrices is acquired employing a novel procedure robust against experimental
imperfections. We connect our observation to a geometric model, useful to
predict the effect of polarizers on complex light fields.Comment: 11 pages, 5 figure
After-gate attack on a quantum cryptosystem
We present a method to control the detection events in quantum key
distribution systems that use gated single-photon detectors. We employ bright
pulses as faked states, timed to arrive at the avalanche photodiodes outside
the activation time. The attack can remain unnoticed, since the faked states do
not increase the error rate per se. This allows for an intercept-resend attack,
where an eavesdropper transfers her detection events to the legitimate receiver
without causing any errors. As a side effect, afterpulses, originating from
accumulated charge carriers in the detectors, increase the error rate. We have
experimentally tested detectors of the system id3110 (Clavis2) from ID
Quantique. We identify the parameter regime in which the attack is feasible
despite the side effect. Furthermore, we outline how simple modifications in
the implementation can make the device immune to this attack.Comment: 14 pages, 8 figure
Discrimination of Optical Coherent States using a Photon Number Resolving Detector
The discrimination of non-orthogonal quantum states with reduced or without
errors is a fundamental task in quantum measurement theory. In this work, we
investigate a quantum measurement strategy capable of discriminating two
coherent states probabilistically with significantly smaller error
probabilities than can be obtained using non-probabilistic state
discrimination. We find that appropriate postselection of the measurement data
of a photon number resolving detector can be used to discriminate two coherent
states with small error probability. We compare our new receiver to an optimal
intermediate measurement between minimum error discrimination and unambiguous
state discrimination.Comment: 5 pages, 4 figure
QPSK coherent state discrimination via a hybrid receiver
We propose and experimentally demonstrate a near-optimal discrimination
scheme for the quadrature phase shift keying protocol (QPSK). We show in theory
that the performance of our hybrid scheme is superior to the standard scheme -
heterodyne detection - for all signal amplitudes and underpin the predictions
with our experimental results. Furthermore, our scheme provides the hitherto
best performance in the domain of highly attenuated signals. The discrimination
is composed of a quadrature measurement, a conditional displacement and a
threshold detector
Probabilistic Cloning of Coherent States without a Phase Reference
We present a probabilistic cloning scheme operating independently of any
phase reference. The scheme is based solely on a phase-randomized displacement
and photon counting, omitting the need for non-classical resources and
non-linear materials. In an experimental implementation, we employ the scheme
to clone coherent states from a phase covariant alphabet and demonstrate that
the cloner is capable of outperforming the hitherto best-performing
deterministic scheme. An analysis of the covariances between the output states
shows that uncorrelated clones can be approached asymptotically. An intriguing
feature is that the trade-off between success rate and achieved fidelity can be
optimized even after the cloning procedure