149 research outputs found
Single-photon characteristics of superposed weak coherent states
We study a superposed weak coherent state that can fundamentally mimic an
ideal single photon not only with respect to the number of photons but also in
terms of an indeterminate phase. It is close to the single-photon state with
high fidelity and exhibits fundamental features of single photons such as
antibunching and Hong-Ou-Mandel interference. The emergence and vanishing of
single-photon characteristics can be directly observed by changing two
parameters, i.e., the mean photon number and number of phases. Our result shows
that the uncertainty between the photon number and phase indeed constitutes the
characteristics of single photons. Finally, we apply the superposed weak
coherent state to quantum key distribution and demonstrate that it outperforms
the typical approach using phase-randomized weak coherent states.Comment: 8 pages, 5 figures, title and abstract changed, published versio
Minimal control power of the controlled teleportation
We generalize the control power of a perfect controlled teleportation of an
entangled three-qubit pure state, suggested by Li and Ghose [Phys. Rev. A {\bf
90}, 052305 (2014)], to the control power of a general controlled teleportation
of a multiqubit pure state. Thus, we define the minimal control power, and
calculate the values of the minimal control power for a class of general
three-qubit Greenberger-Horne-Zeilinger (GHZ) states and the three-qubit W
class whose states have zero three-tangles. Moreover, we show that the standard
three-qubit GHZ state and the standard three-qubit W state have the maximal
values of the minimal control power for the two classes, respectively. This
means that the minimal control power can be interpreted as not only an
operational quantity of a three-qubit quantum communication but also a degree
of three-qubit entanglement. In addition, we calculate the values of the
minimal control power for general n-qubit GHZ states and the n-qubit W-type
states.Comment: 5 pages, No figures; Close to published versio
Entanglement of thermal scalar fields in a compact space space
Using the thermal Green's function approach we propose a general method to
investigate entanglement of the vacuum state or thermal ground states in an
arbitrary dimensional space-time. As an application we show quantum
separability of the massive thermal scalar field vacuum in the 1+1 dimensional
cylindrical space-time. Separability is demonstrated using the positive partial
transpose criterion for effective two-mode Gaussian states of collective
operators. In this case, for all mass and temperature values entanglement is
absent between the collective operators.Comment: title change
Entanglement swapping secures multiparty quantum communication
Extending the eavesdropping strategy devised by Zhang, Li and Guo [Phys. Rev.
A 63, 036301 (2001)], we show that the multiparty quantum communication
protocol based on entanglement swapping, which was proposed by Cabello
[quant-ph/0009025], is not secure. We modify the protocol so that entanglement
swapping can secure multiparty quantum communication, such as multiparty
quantum key distribution and quantum secret sharing of classical information,
and show that the modified protocol is secure against the Zhang-Li-Guo's
strategy for eavesdropping as well as the basic intercept-resend attack.Comment: 16 pages, 4 figure
Two-particle indistinguishability and identification of boson-and-fermion species: a Fisher information approach
We present a study on two-particle indistinguishability and particle-species
identification by introducing a Fisher-information (FI) approach---in which two
particles pass through a two-wave mixing operation and the number of particles
is counted in one of the output modes. In our study, we first show that FI can
reproduce the Hong-Ou-Mandel (HOM) effect with two bosons or two fermions. In
particular, it is found that even though bosons and fermions exhibit different
physical behavior (i.e., "bunching" or "anti-bunching") due to their
indistinguishability, the aspects of HOM-like dip are quantitatively same. We
then provide a simple method for estimating the degree of two-particle
indistinguishability in a Mach-Zehnder interferometer-type setup. The presented
method also enables us to identify whether the particles are bosons or
fermions. Our study will provide useful primitives for various study of boson
and fermion characteristics.Comment: 11+1 pages, 6 figures. Corrected typos in page
Effective Formalism for Open Quantum System Dynamics: Time-coarse-graining Approach
We formulate an effective-description framework for the dynamics of open
quantum systems by extending the time-coarse-graining formalism to open
systems. Our coarse-graining procedure efficiently removes high-frequency
processes which are responsible for coherences between lower- and
upper-manifold states and are irrelevant when considering only low-energy
dynamics. We investigate the regime of validity of the resulting coarse-grained
master equation by applying it to multi-level atoms driven by far-detuned
lasers. Except for such high-frequency coherences, we find good agreement
between the exact and coarse-grained dynamics unless the driving lasers are too
strong or the initial high-frequency coherences are sizable
Quantum Shift Register
We consider a quantum circuit in which shift and rotation operations on
qubits are performed by swap gates and controlled swap gates. These operations
can be useful for quantum computers performing elementary arithmetic operations
such as multiplication and a bit-wise comparison of qubits.Comment: 3 pages, 4 figures, revte
Deciding whether a quantum state has secret correlations is an NP-complete problem
From the NP-hardness of the quantum separability problem and the relation
between bipartite entanglement and the secret key correlations, it is shown
that the problem deciding whether a given quantum state has secret correlations
in it or not is in NP-complete.Comment: 3 pages, revtex, no fig, typos correcte
Gaussian private quantum channel with squeezed coherent states
While the objective of conventional quantum key distribution (QKD) is to
secretly generate and share the classical bits concealed in the form of
maximally mixed quantum states, that of private quantum channel (PQC) is to
secretly transmit individual quantum states concealed in the form of maximally
mixed states using shared one-time pad and it is called Gaussian private
quantum channel (GPQC) when the scheme is in the regime of continuous
variables. We propose a GPQC enhanced with squeezed coherent states (GPQCwSC),
which is a generalization of GPQC with coherent states only (GPQCo) [Phys. Rev.
A 72, 042313 (2005)]. We show that GPQCwSC beats the GPQCo for the upper bound
on accessible information. As a subsidiary example, it is shown that the
squeezed states take an advantage over the coherent states against a beam
splitting attack in a continuous variable QKD. It is also shown that a
squeezing operation can be approximated as a superposition of two different
displacement operations in the small squeezing regime.Comment: 7 pages, 2 figure
Correspondence between maximally entangled states in discrete and Gaussian regimes
We study a general corresponding principle between discrete-variable quantum
states and continuous-variable (especially, restricted on Gaussian) states via
quantum purification method. In the previous work, we have already investigated
an information-theoretic correspondence between the Gaussian maximally mixed
states (GMMSs) and their purifications known as Gaussian maximally entangled
states (GMESs) in [Phys. Lett. A {\bf 380}, 3607 (2016)]. We here compare an
-dimensional maximally entangled state to the GMES we proposed
previously, through an explicit calculation of quantum fidelity between those
entangled states. By exploiting the results, we naturally conclude that our
GMES is more suitable to the concept of \emph{maximally entangled} state in
Gaussian quantum information, and thus it might be useful or applicable for
quantum information tasks than the two-mode squeezed vacuum (TMSV) state in the
Gaussian regime.Comment: 5 pages, 2 figures; Minor changed and references update
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