1,602 research outputs found
Apply current exponential de Finetti theorem to realistic quantum key distribution
In the realistic quantum key distribution (QKD), Alice and Bob respectively
get a quantum state from an unknown channel, whose dimension may be unknown.
However, while discussing the security, sometime we need to know exact
dimension, since current exponential de Finetti theorem, crucial to the
information-theoretical security proof, is deeply related with the dimension
and can only be applied to finite dimensional case. Here we address this
problem in detail. We show that if POVM elements corresponding to Alice and
Bob's measured results can be well described in a finite dimensional subspace
with sufficiently small error, then dimensions of Alice and Bob's states can be
almost regarded as finite. Since the security is well defined by the smooth
entropy, which is continuous with the density matrix, the small error of state
actually means small change of security. Then the security of
unknown-dimensional system can be solved. Finally we prove that for heterodyne
detection continuous variable QKD and differential phase shift QKD, the
collective attack is optimal under the infinite key size case.Comment: 11 pages, 2 figures, detailed version, applications adde
Building quantum neural networks based on swap test
Artificial neural network, consisting of many neurons in different layers, is
an important method to simulate humain brain. Usually, one neuron has two
operations: one is linear, the other is nonlinear. The linear operation is
inner product and the nonlinear operation is represented by an activation
function. In this work, we introduce a kind of quantum neuron whose inputs and
outputs are quantum states. The inner product and activation operator of the
quantum neurons can be realized by quantum circuits. Based on the quantum
neuron, we propose a model of quantum neural network in which the weights
between neurons are all quantum states. We also construct a quantum circuit to
realize this quantum neural network model. A learning algorithm is proposed
meanwhile. We show the validity of learning algorithm theoretically and
demonstrate the potential of the quantum neural network numerically.Comment: 10 pages, 13 figure
To what extent does the self-consistent mean-field exist?
A non-convergent difficulty near level-repulsive region is discussed within
the self-consistent mean-field theory. It is shown by numerical and analytic
studies that the mean-field is not realized in the many-fermion system when
quantum fluctuations coming from two-body residual interaction and quadrupole
deformation are larger than an energy difference between two avoided crossing
orbits. An analytic condition indicating a limitation of the mean-field concept
is derived for the first time
Security proof of differential phase shift quantum key distribution in the noiseless case
Differential phase shift quantum key distribution systems have a high
potential for achieving high speed key generation. However, its unconditional
security proof is still missing, even though it has been proposed for many
years. Here, we prove its security against collective attacks with a weak
coherent light source in the noiseless case (i.e. no bit error). The only
assumptions are that quantum theory is correct, the devices are perfect and
trusted and the key size is infinite. Our proof works on threshold detectors.
We compute the lower bound of the secret key generation rate using the
information-theoretical security proof method. Our final result shows that the
lower bound of the secret key generation rate per pulse is linearly
proportional to the channel transmission probability if Bob's detection counts
obey the binomial distribution.Comment: Published version, 13 pages, 4 figures, minor changes, references
added, acknowledgement adde
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