27 research outputs found
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
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
Joint eavesdropping on the BB84 decoy state protocol with an arbitrary passive light-source side channel
Passive light-source side channel in quantum key distribution (QKD) makes the
quantum signals more distinguishable thus provides additional information about
the quantum signal to an eavesdropper. The explicit eavesdropping strategies
aimed at the passive side channel known to date were limited to the separate
measurement of the passive side channel in addition to the operational degree
of freedom. Here we show how to account for the joint eavesdropping on both
operational degree of freedom and the passive side channel of the generic form.
In particular, we use the optimal phase-covariant cloning of the signal photon
state, which is the most effective attack on the BB84 protocol without side
channels, followed by a joint collective measurement of the side channel and
the operational degree of freedom. To estimate QKD security under this attack,
we develop an effective error method and show its applicability to the BB84
decoy-state protocol
Efficient generation of temporally shaped photons using nonlocal spectral filtering
We study the generation of single-photon pulses with the tailored temporal
shape via nonlocal spectral filtering. A shaped photon is heralded from a
time-energy entangled photon pair upon spectral filtering and time-resolved
detection of its entangled counterpart. We show that the temporal shape of the
heralded photon is defined by the time-inverted impulse response of the
spectral filter and does not depend on the heralding instant. Thus one can
avoid post-selection of particular heralding instants and achieve substantially
higher heralding rate of shaped photons as compared to the generation of
photons via nonlocal temporal modulation. Furthermore, the method can be used
to generate shaped photons with a coherence time in the ns-s range and is
particularly suitable to produce photons with the exponentially rising temporal
shape required for efficient interfacing to a single quantum emitter in free
space
Quantum uniqueness
In the classical world one can construct two identical systems which have
identical behavior and give identical measurement results. We show this to be
impossible in the quantum domain. We prove that after the same quantum
measurement two different quantum systems cannot yield always identical
results, provided the possible measurement results belong to a non orthogonal
set. This is interpreted as quantum uniqueness - a quantum feature which has no
classical analog. Its tight relation with objective randomness of quantum
measurements is discussed.Comment: Presented at 4th Feynman festival, June 22-26, 2009, in Olomouc,
Czech Republic
Coherent State Quantum Key Distribution with Multi Letter Phase-Shift Keying
We present a protocol for quantum key distribution using discrete modulation
of coherent states of light. Information is encoded in the variable phase of
coherent states which can be chosen from a regular discrete set ranging from
binary to continuous modulation, similar to phase-shift-keying in classical
communication. Information is decoded by simultaneous homodyne measurement of
both quadratures and requires no active choice of basis. The protocol utilizes
either direct or reverse reconciliation, both with and without postselection.
We analyze the security of the protocol and show how to enhance it by the
optimal choice of all variable parameters of the quantum signal.Comment: 11 pages, 2 figures, 1 table. Comments welcom
Triplet-like correlation symmetry of continuous variable entangled states
We report on a continuous variable analogue of the triplet two-qubit Bell
states. We theoretically and experimentally demonstrate a remarkable similarity
of two-mode continuous variable entangled states with triplet Bell states with
respect to their correlation patterns. Borrowing from the two qubit language,
we call these correlations triplet-like.Comment: 7 pages, 5 figures. Comments are welcom