531 research outputs found
Quantum copying: A network
We present a network consisting of quantum gates which produces two imperfect
copies of an arbitrary qubit. The quality of the copies does not depend on the
input qubit. We also show that for a restricted class of inputs it is possible
to use a very similar network to produce three copies instead of two. For
qubits in this class, the copy quality is again independent of the input and is
the same as the quality of the copies produced by the two-copy network.Comment: 10 pages LaTeX, with 1 figure, submitted to the Physical Review
Bounds for state-dependent quantum cloning
Due to the no-cloning theorem, the unknown quantum state can only be cloned
approximately or exactly with some probability. There are two types of cloners:
universal and state-dependent cloner. The optimal universal cloner has been
found and could be viewed as a special state-dependent quantum cloner which has
no information about the states. In this paper, we investigate the
state-dependent cloning when the state-set contains more than two states. We
get some bounds of the global fidelity for these processes. This method is not
dependent on the number of the states contained in the state-set. It is also
independent of the numbers of copying.Comment: 13 pages, 1 figure, to appear in Phys. Rev.
Quantum disentanglers
It is not possible to disentangle a qubit in an unknown state from a
set of (N-1) ancilla qubits prepared in a specific reference state . That
is, it is not possible to {\em perfectly} perform the transformation
. The question is then how well we can do? We consider a number of
different methods of extracting an unknown state from an entangled state formed
from that qubit and a set of ancilla qubits in an known state. Measuring the
whole system is, as expected, the least effective method. We present various
quantum ``devices'' which disentangle the unknown qubit from the set of ancilla
qubits. In particular, we present the optimal universal disentangler which
disentangles the unknown qubit with the fidelity which does not depend on the
state of the qubit, and a probabilistic disentangler which performs the perfect
disentangling transformation, but with a probability less than one.Comment: 8 pages, 1 eps figur
Multipartite Classical and Quantum Secrecy Monotones
In order to study multipartite quantum cryptography, we introduce quantities
which vanish on product probability distributions, and which can only decrease
if the parties carry out local operations or carry out public classical
communication. These ``secrecy monotones'' therefore measure how much secret
correlations are shared by the parties. In the bipartite case we show that the
mutual information is a secrecy monotone. In the multipartite case we describe
two different generalisations of the mutual information, both of which are
secrecy monotones. The existence of two distinct secrecy monotones allows us to
show that in multipartite quantum cryptography the parties must make
irreversible choices about which multipartite correlations they want to obtain.
Secrecy monotones can be extended to the quantum domain and are then defined on
density matrices. We illustrate this generalisation by considering tri-partite
quantum cryptography based on the Greenberger-Horne-Zeilinger (GHZ) state. We
show that before carrying out measurements on the state, the parties must make
an irreversible decision about what probability distribution they want to
obtain
Entanglement of coherent states and decoherence
A possibility to produce entangled superpositions of strong coherent states
is discussed. A recent proposal by Howell and Yazell [Phys. Rev. A 62, 012102
(2000)] of a device which entangles two strong coherent coherent states is
critically examined. A serious flaw in their design is found. New modified
scheme is proposed and it is shown that it really can generate non-classical
states that can violate Bell inequality. Moreover, a profound analysis of the
effect of losses and decoherence on the degree of entanglement is accomplished.
It reveals the high sensitivity of the device to any disturbances and the
fragility of generated states
Undetermined states: how to find them and their applications
We investigate the undetermined sets consisting of two-level, multi-partite
pure quantum states, whose reduced density matrices give absolutely no
information of their original states. Two approached of finding these quantum
states are proposed. One is to establish the relation between codewords of the
stabilizer quantum error correction codes (SQECCs) and the undetermined states.
The other is to study the local complementation rules of the graph states. As
an application, the undetermined states can be exploited in the quantum secret
sharing scheme. The security is guaranteed by their undetermineness.Comment: 6 pages, no figur
Universal cloning of continuous quantum variables
The cloning of quantum variables with continuous spectra is analyzed. A
universal - or Gaussian - quantum cloning machine is exhibited that copies
equally well the states of two conjugate variables such as position and
momentum. It also duplicates all coherent states with a fidelity of 2/3. More
generally, the copies are shown to obey a no-cloning Heisenberg-like
uncertainty relation.Comment: 4 pages, RevTex. Minor revisions, added explicit cloning
transformation, added reference
Optimal manipulations with qubits: Universal quantum entanglers
We analyze various scenarios for entangling two initially unentangled qubits.
In particular, we propose an optimal universal entangler which entangles a
qubit in unknown state with a qubit in a reference (known) state
. That is, our entangler generates the output state which is as close as
possible to the pure (symmetrized) state . The most
attractive feature of this entangling machine, is that the fidelity of its
performance (i.e. the distance between the output and the ideally entangled --
symmetrized state) does not depend on the input and takes the constant value
. We also analyze how to optimally generate
from a single qubit initially prepared in an unknown state |\Psi\r a two
qubit entangled system which is as close as possible to a Bell state
, where \l\Psi|\Psi^\perp\r =0.Comment: 11 pages, 3 eps figures, accepted for publication in Phys. Rev.
Security of quantum cryptography using balanced homodyne detection
In this paper we investigate the security of a quantum cryptographic scheme
which utilizes balanced homodyne detection and weak coherent pulse (WCP). The
performance of the system is mainly characterized by the intensity of the WCP
and postselected threshold. Two of the simplest intercept/resend eavesdropping
attacks are analyzed. The secure key gain for a given loss is also discussed in
terms of the pulse intensity and threshold.Comment: RevTeX4, 8pages, 7 figure
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