1,484 research outputs found
Teleportation of the Relativistic Quantum Field
The process of teleportation of a completely unknown one-particle state of a
free relativistic quantum field is considered. In contrast to the
non-relativistic quantum mechanics, the teleportation of an unknown state of
the quantum field cannot be in principle described in terms of a measurement in
a tensor product of two Hilbert spaces to which the unknown state and the state
of the EPR-pair belong. The reason is of the existence of a cyclic (vacuum)
state common to both the unknown state and the EPR-pair. Due to the common
vacuum vector and the microcausality principle (commutation relations for the
field operators), the teleportation amplitude contains inevitably contributions
which are irrelevant to the teleportation process. Hence in the relativistic
theory the teleportation in the sense it is understood in the non-relativistic
quantum mechanics proves to be impossible because of the impossibility of the
realization of the appropriate measurement as a tensor product of the
measurements related to the individual subsystems so that one can only speak of
the amplitude of the propagation of the field as a whole.Comment: 11 page
Adiabatic Gate Teleportation
The difficulty in producing precisely timed and controlled quantum gates is a
significant source of error in many physical implementations of quantum
computers. Here we introduce a simple universal primitive, adiabatic gate
teleportation, which is robust to timing errors and many control errors and
maintains a constant energy gap throughout the computation above a degenerate
ground state space. Notably this construction allows for geometric robustness
based upon the control of two independent qubit interactions. Further, our
piecewise adiabatic evolution easily relates to the quantum circuit model,
enabling the use of standard methods from fault-tolerance theory for
establishing thresholds.Comment: 4 pages, 1 figure, with additional 3 pages and 2 figures in an
appendix. v2 Refs added. Video abstract available at
http://www.quantiki.org/video_abstracts/0905090
All Teleportation and Dense Coding Schemes
We establish a one-to-one correspondence between (1) quantum teleportation
schemes, (2) dense coding schemes, (3) orthonormal bases of maximally entangled
vectors, (4) orthonormal bases of unitary operators with respect to the
Hilbert-Schmidt scalar product, and (5) depolarizing operations, whose Kraus
operators can be chosen to be unitary. The teleportation and dense coding
schemes are assumed to be ``tight'' in the sense that all Hilbert spaces
involved have the same finite dimension d, and the classical channel involved
distinguishes d^2 signals. A general construction procedure for orthonormal
bases of unitaries, involving Latin Squares and complex Hadamard Matrices is
also presented.Comment: 21 pages, LaTe
Quantum nonlocality of four-qubit entangled states
Quantum nonlocality of several four-qubit states is investigated by
constructing a new Bell inequality. These include the
Greenberger-Zeilinger-Horne (GHZ) state, W state, cluster state, and the state
that has been recently proposed in [PRL, {\bf 96}, 060502 (2006)]. The
Bell inequality is optimally violated by but not violated by the GHZ
state. The cluster state also violates the Bell inequality though not
optimally. The state can thus be discriminated from the cluster state
by using the inequality. Different aspects of four-partite entanglement are
also studied by considering the usefulness of a family of four-qubit mixed
states as resources for two-qubit teleportation. Our results generalize those
in [PRL, {\bf 72}, 797 (1994)].Comment: 13 pages, 1 figur
Twisted Photons: New Quantum Perspectives in High Dimensions
Quantum information science and quantum information technology have seen a
virtual explosion world-wide. It is all based on the observation that
fundamental quantum phenomena on the individual particle or system-level lead
to completely novel ways of encoding, processing and transmitting information.
Quantum mechanics, a child of the first third of the 20th century, has found
numerous realizations and technical applications, much more than was thought at
the beginning. Decades later, it became possible to do experiments with
individual quantum particles and quantum systems. This was due to technological
progress, and for light in particular, the development of the laser. Hitherto,
nearly all experiments and also nearly all realizations in the fields have been
performed with qubits, which are two-level quantum systems. We suggest that
this limitation is again mainly a technological one, because it is very
difficult to create, manipulate and measure more complex quantum systems. Here,
we provide a specific overview of some recent developments with
higher-dimensional quantum systems. We mainly focus on Orbital Angular Momentum
(OAM) states of photons and possible applications in quantum information
protocols. Such states form discrete higher-dimensional quantum systems, also
called qudits. Specifically, we will first address the question what kind of
new fundamental properties exist and the quantum information applications which
are opened up by such novel systems. Then we give an overview of recent
developments in the field by discussing several notable experiments over the
past 2-3 years. Finally, we conclude with several important open questions
which will be interesting for investigations in the future.Comment: 15 pages, 7 figure
Generation of the Complete Four-dimensional Bell Basis
The Bell basis is a distinctive set of maximally entangled two-particle
quantum states that forms the foundation for many quantum protocols such as
teleportation, dense coding and entanglement swapping. While the generation,
manipulation, and measurement of two-level quantum states is well understood,
the same is not true in higher dimensions. Here we present the experimental
generation of a complete set of Bell states in a four-dimensional Hilbert
space, comprising of 16 orthogonal entangled Bell-like states encoded in the
orbital angular momentum of photons. The states are created by the application
of generalized high-dimensional Pauli gates on an initial entangled state. Our
results pave the way for the application of high-dimensional quantum states in
complex quantum protocols such as quantum dense coding.Comment: 4 pages, 4 figure
Controlled cyclic remote state preparation of arbitrary qubit states
Quantum secure communications could securely transmit quantum information by using quantum resource. Recently, novel applications such as bidirectional and asymmetric quantum protocols have been developed.
In this paper, we propose a new method for generating entanglement which is highly useful for multiparty quantum communications such as teleportation and Remote State Preparation (RSP). As one of its applications, we propose a new type of quantum secure communications, i.e. cyclic RSP protocols. Starting from a four-party controlled cyclic RSP protocol of one-qubit states, we show that this cyclic protocol can be generalized to a multiparty controlled cyclic RSP protocol for preparation of arbitrary qubit states. We point out that previous bidirectional and asymmetric protocols can be regarded as a simpler form of our cyclic RSP protocols
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