166 research outputs found
Optimal multiqubit operations for Josephson charge qubits
We introduce a method for finding the required control parameters for a
quantum computer that yields the desired quantum algorithm without invoking
elementary gates. We concentrate on the Josephson charge-qubit model, but the
scenario is readily extended to other physical realizations. Our strategy is to
numerically find any desired double- or triple-qubit gate. The motivation is
the need to significantly accelerate quantum algorithms in order to fight
decoherence.Comment: 4 pages, 5 figure
Multiparty Quantum Secret Sharing
Based on a quantum secure direct communication (QSDC) protocol [Phys. Rev.
A69(04)052319], we propose a -threshold scheme of multiparty quantum
secret sharing of classical messages (QSSCM) using only single photons. We take
advantage of this multiparty QSSCM scheme to establish a scheme of multiparty
secret sharing of quantum information (SSQI), in which only all quantum
information receivers collaborate can the original qubit be reconstructed. A
general idea is also proposed for constructing multiparty SSQI schemes from any
QSSCM scheme
Multiparty Quantum Secret Sharing Based on Entanglement Swapping
A multiparty quantum secret sharing (QSS) protocol is proposed by using
swapping quantum entanglement of Bell states. The secret messages are imposed
on Bell states by local unitary operations. The secret messages are split into
several parts and each part is distributed to a party so that no action of a
subset of all the parties but their entire cooperation is able to read out the
secret messages. In addition, the dense coding is used in this protocol to
achieve a high efficiency. The security of the present multiparty QSS against
eavesdropping has been analyzed and confirmed even in a noisy quantum channel.Comment: 5 page
Quantum secret sharing between multi-party and multi-party without entanglement
We propose a quantum secret sharing protocol between multi-party ( members
in group 1) and multi-party ( members in group 2) using a sequence of single
photons. These single photons are used directly to encode classical information
in a quantum secret sharing process. In this protocol, all members in group 1
directly encode their respective keys on the states of single photons via
unitary operations, then the last one (the member of group 1) sends
of the resulting qubits to each of group 2. Thus the secret message
shared by all members of group 1 is shared by all members of group 2 in such a
way that no subset of each group is efficient to read the secret message, but
the entire set (not only group 1 but also group 2) is. We also show that it is
unconditionally secure. This protocol is feasible with present-day techniques.Comment: 6 pages, no figur
Long-range entanglement generation via frequent measurements
A method is introduced whereby two non-interacting quantum subsystems, that
each interact with a third subsystem, are entangled via repeated projective
measurements of the state of the third subsystem. A variety of physical
examples are presented. The method can be used to establish long range
entanglement between distant parties in one parallel measurement step, thus
obviating the need for entanglement swapping.Comment: 7 pages, incl. 2 figures. v2: added a few small clarifications and a
referenc
Eutactic quantum codes
We consider sets of quantum observables corresponding to eutactic stars.
Eutactic stars are systems of vectors which are the lower dimensional
``shadow'' image, the orthogonal view, of higher dimensional orthonormal bases.
Although these vector systems are not comeasurable, they represent redundant
coordinate bases with remarkable properties. One application is quantum secret
sharing.Comment: 6 page
The von Neumann Entropy of EPR Spin Correlation for the Relativistic Pairs
Variation of the von Neumann entropy by the Lorentz transformation is
discussed. Taking the spin-singlet state in the center of mass frame, the von
Neumann entropy in the laboratory frame is calculated from the reduced density
matrix obtained by taking the trace over 4-momentum after the Lorentz
transformation. As the model to discuss the EPR spin correlation, it is
supposed that one parent particle splits into a superposition state of various
pair states in various directions. Computing the von Neumann entropy and the
Shannon entropy, we have shown a global behavior of the entropy to see a
relativistic effect. We discuss also the super-relativistic limit,
distinguishability between the two particles of the pair and so on.Comment: 15 pages, 9 figures; changed the title, revised the manuscript, added
reference
Simple test for quantum channel capacity
Basing on states and channels isomorphism we point out that semidefinite
programming can be used as a quick test for nonzero one-way quantum channel
capacity. This can be achieved by search of symmetric extensions of states
isomorphic to a given quantum channel. With this method we provide examples of
quantum channels that can lead to high entanglement transmission but still have
zero one-way capacity, in particular, regions of symmetric extendibility for
isotropic states in arbitrary dimensions are presented. Further we derive {\it
a new entanglement parameter} based on (normalised) relative entropy distance
to the set of states that have symmetric extensions and show explicitly the
symmetric extension of isotropic states being the nearest to singlets in the
set of symmetrically extendible states. The suitable regularisation of the
parameter provides a new upper bound on one-way distillable entanglement.Comment: 6 pages, no figures, RevTeX4. Signifficantly corrected version. Claim
on continuity of channel capacities removed due to flaw in the corresponding
proof. Changes and corrections performed in the part proposing a new upper
bound on one-way distillable etanglement which happens to be not one-way
entanglement monoton
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