636 research outputs found
Effects of systematic phase errors on optimized quantum random-walk search algorithm
This paper researches how the systematic errors in phase inversions affect
the success rate and the number of iterations in optimized quantum random-walk
search algorithm. Through geometric description of this algorithm, the model of
the algorithm with phase errors is established and the relationship between the
success rate of the algorithm, the database size, the number of iterations and
the phase error is depicted. For a given sized database, we give both the
maximum success rate of the algorithm and the required number of iterations
when the algorithm is in the presence of phase errors. Through analysis and
numerical simulations, it shows that optimized quantum random-walk search
algorithm is more robust than Grover's algorithm.Comment: Submitted to Chinese Physics B, Comments are welcom
Demonstration of the Essentiality of Entanglement in a Deutsch-like Quantum Algorithm
Quantum algorithms could efficiently solve certain classically intractable
problems by exploiting quantum parallelism. To date, whether the quantum
entanglement is useful or not for quantum computing is still a question of
debate. Here, we present a new quantum algorithm to show that entanglement
could help to gain advantage over classical algorithm and even the quantum
algorithm without entanglement. Furthermore, we implement experiments to
demonstrate our proposed algorithm using superconducting qubits. Our results
show the viability of the algorithm and suggest that entanglement is essential
in getting quantum speedup for certain problems in quantum computing, which
provide a reliable and clear guidance for developing useful quantum algorithms
in future.Comment: 6 pages, 6 figures and 2 tables, Theoretical work has been formulated
in earlier versions by Ashutosh K. Goswami and Prasanta K. Panigrah
Detector-decoy high-dimensional quantum key distribution
The decoy-state high-dimensional quantum key distribution provides a
practical secure way to share more private information with high
photon-information efficiency. In this paper, based on detector-decoy method,
we propose a detector-decoy high-dimensional quantum key distribution protocol.
Employing threshold detectors and a variable attenuator, we can estimate
single-photon fraction of postselected events and Eves Holevo information under
the Gaussian collective attack with much simpler operations in practical
implementation. By numerical evaluation, we show that without varying source
intensity and optimizing decoy-state intensity, our protocol could perform much
better than one-decoy-state protocol and as well as the two-decoy-state
protocol. Specially, when the detector efficiency is lower, the advantage of
the detector-decoy method becomes more prominent
Simulating the Dynamics of Single Photons in BosonSampling Devices with Matrix Product States
BosonSampling is a well-defined scheme for demonstrating quantum supremacy
with photons in near term. Although relying only on multi-photon interference
in nonadaptive linear-optical networks, it is hard to simulate classically.
Here we study BosonSampling using matrix product states, a powerful method from
quantum many-body physics. This method stores the instantaneous quantum state
during the evolution of photons in the optical quantum circuit, which allows us
to reveal the dynamical features of single photons in BosonSampling devices,
such as entanglement entropy growth. We show the flexiblility of this method by
also applying it to dissipative optical quantum circuits, as well as circuits
with fermionic particles. Our work shows that matrix product states is a
powerful platform to simulate optical quantum circuits. And it is readily
extended to study quantum dynamics in multi-particle quantum walks beyond
BosonSampling.Comment: 7 pages, 4 figure
Security of quantum key distribution with state-dependent imperfections
In practical quantum key distribution (QKD) system, the state preparation and
measurement are imperfect comparing with the ideal BB84 protocol, which are
always state-dependent in practical realizations. If the state-dependent
imperfections can not be regarded as an unitary transformation, it should not
be considered as part of quantum channel noise introduced by the eavesdropper,
the commonly used secret key rate formula GLLP can not be applied
correspondingly. In this paper, the unconditional security of quantum key
distribution with state-dependent imperfection has been analyzed by estimating
the upper bound of the phase error rate about the quantum channel
Security of practical phase-coding quantum key distribution
Security proof of practical quantum key distribution (QKD) has attracted a
lot of attentions in recent years. Most of real-life QKD implementations are
based on phase-coding BB84 protocol, which usually uses Unbalanced Mach-Zehnder
Interferometer (UMZI) as the information coder and decoder. However, the long
arm and short arm of UMZI will introduce different loss in practical
experimental realizations, the state emitted by Alice's side is nolonger
standard BB84 states. In this paper, we will give a security analysis in this
situation. Counterintuitively, active compensation for this different loss will
only lower the secret key bit rate.Comment: 4 pages, 3 figures
Practical decoy-state round-robin differential-phaseshift quantum key distribution
To overcome the signal disturbance from the transmission process, recently, a
new type of protocol named round-robin differential-phase-shift(RRDPS) quantum
key distribution[Nature 509, 475(2014)] is proposed. It can estimate how much
information has leaked to eavesdropper without monitoring bit error rates. In
this paper, we compare the performance of RRDPS using different sources without
and with decoy-state method, such as weak coherent pulses(WCPs) and heralded
single photon source(HSPS). For practical implementations, we propose finite
decoy-state method for RRDPS, the performance of which is close to the infinite
one. Taking WCPs as an example, the three-intensity decoystate protocol can
distribute secret keys over a distance of 128 km when the length of pulses
packet is 32, which confirms the great practical interest of our method.Comment: 24 pages, 7 figure
Continuous-variable measurement-device-independent quantum key distribution with photon subtraction
It has been found that non-Gaussian operations can be applied to increase and
distill entanglement between Gaussian entangled states. We show the successful
use of the non-Gaussian operation, in particular, photon subtraction operation,
on the continuous-variable measurement-device-independent quantum key
distribution (CV-MDI-QKD) protocol. The proposed method can be implemented
based on existing technologies. Security analysis shows that the photon
subtraction operation can remarkably increase the maximal transmission distance
of the CV-MDI-QKD protocol, which precisely make up for the shortcoming of the
original CV-MDI-QKD protocol, and 1-photon subtraction operation has the best
performance. Moreover, the proposed protocol provides a feasible method for the
experimental implementation of the CV-MDI-QKD protocol.Comment: 8 pages, 9 figure
Ground state cooling in a hybrid optomechanical system with a three-level atomic ensemble
Cooling mechanical resonators is of great importance for both fundamental
study and applied science. We investigate the hybrid optomechanical cooling
with a three-level atomic ensemble fixed in a strong excited optical cavity. By
using the quantum noise approach, we find the upper bound of the noise spectrum
and further present three optimal parameter conditions, which can yield a small
heating coefficient, a large cooling coefficient, and thus a small final phonon
number. Moreover, through the covariance matrix approach, results of numerical
simulation are obtained, which are consistent with the theoretical
expectations. It is demonstrated that our scheme can achieve ground state
cooling in the highly unresolved sideband regime, within the current
experimental technologies. Compared with the previous cooling methods, in our
scheme, there are fewer constraints on the drive strength of atomic ensemble
and number of atoms in the ensemble. In addition, the tolerable ranges of
parameters for ground state cooling are extended. As a result, our scheme is
very suitable for experiments and can be a guideline for the research of hybrid
optomechanical cooling.Comment: 21 pages, 7 figures. Comments Welcome
Phase-encoded measurement device independent quantum key distribution with practical spontaneous parametric-down-conversion sources
Measurement-device-independent quantum key distribution (MDI-QKD) with weak
coherent sources has been widely and meticulously analyzed. However, the
analysis for MDI-QKD with spontaneous parametric-down-conversion sources
(SPDCS) is incomplete. In this paper, by accounting for practical parameters of
SPDCS with thermal distribution, we presents an investigation on the
performances of MDI-QKD under the active three-intensity decoy protocol and the
passive one-intensity decoy protocol respectively. Phase randomization,
inherently prerequisite for decoy protocol, is taken into consideration for
evaluating the overall quantum bit gain and quantum bit error rate. The
numerical simulations show that MDI-QKD using SPDCS with practical decoy
protocols can be demonstrated comparable to the asymptotical case with infinite
decoy states and has apparent superiority both in transmission distance and key
generation rate compared to the MDI-QKD using weak coherent sources. Our
results also indicate that MDI-QKD using thermal distributed SPDCS with active
three-intensity decoy protocol performs better than the one with passive
one-intensity decoy protocol.Comment: 13 pages, 6 figure
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