230,703 research outputs found
Quantum state transfer between photons preloaded with quantum information
Quantum mechanics provides a ``disembodied'' way to transfer an unknown
quantum state from one quantum system to another. However, all experiments of
quantum state transfer to date are limited to cases where the target quantum
system contains no prior quantum information. Here we propose a scheme for
transferring a quantum state to a quantum system preloaded with quantum
information. By using an optical qubit-ququart entangling gate, we have
experimentally demonstrated this new protocol -- transferring a qubit to a
photon preloaded with one qubit of quantum information. After the state
transfer, the target photon contains two qubits of quantum information, one
from the qubit being transferred and the other from the pre-existing qubit.
Furthermore, we have also experimentally realized the inverse operation of the
aforementioned quantum state transfer, which is called the partial quantum
state transfer, namely transferring one qubit of quantum information from a
photon preloaded with two qubits of quantum information to another photon. The
fidelities of the quantum state transfer range from to , all
above the classical limit of . Our work sheds light on a new direction for
quantum state transfer and demonstrates our ability to implement entangling
operations beyond two-level quantum systems.Comment: 22pages, 9 figure
Long-range adiabatic quantum state transfer through a linear array of quantum dots
We introduce an adiabatic long-range quantum communication proposal based on
a quantum dot array. By adiabatically varying the external gate voltage applied
on the system, the quantum information encoded in the electron can be
transported from one end dot to another. We numerically solve the Schr\"odinger
equation for a system with a given number of quantum dots. It is shown that
this scheme is a simple and efficient protocol to coherently manipulate the
population transfer under suitable gate pulses. The dependence of the energy
gap and the transfer time on system parameters is analyzed and shown
numerically. We also investigate the adiabatic passage in a more realistic
system in the presence of inevitable fabrication imperfections. This method
provides guidance for future realizations of adiabatic quantum state transfer
in experiments.Comment: 7 pages, 7 figure
Coherent information analysis of quantum channels in simple quantum systems
The coherent information concept is used to analyze a variety of simple
quantum systems. Coherent information was calculated for the information decay
in a two-level atom in the presence of an external resonant field, for the
information exchange between two coupled two-level atoms, and for the
information transfer from a two-level atom to another atom and to a photon
field. The coherent information is shown to be equal to zero for all
full-measurement procedures, but it completely retains its original value for
quantum duplication. Transmission of information from one open subsystem to
another one in the entire closed system is analyzed to learn quantum
information about the forbidden atomic transition via a dipole active
transition of the same atom. It is argued that coherent information can be used
effectively to quantify the information channels in physical systems where
quantum coherence plays an important role.Comment: 24 pages, 7 figs; Final versiob after minor changes, title changed;
to be published in Phys. Rev. A, September 200
Raman Adiabatic Transfer of Optical States
We analyze electromagnetically induced transparency and light storage in an
ensemble of atoms with multiple excited levels (multi-Lambda configuration)
which are coupled to one of the ground states by quantized signal fields and to
the other one via classical control fields. We present a basis transformation
of atomic and optical states which reduces the analysis of the system to that
of EIT in a regular 3-level configuration. We demonstrate the existence of dark
state polaritons and propose a protocol to transfer quantum information from
one optical mode to another by an adiabatic control of the control fields
Experimental Quantum Teleportation of a Two-Qubit Composite System
Quantum teleportation, a way to transfer the state of a quantum system from
one location to another, is central to quantum communication and plays an
important role in a number of quantum computation protocols. Previous
experimental demonstrations have been implemented with photonic or ionic
qubits. Very recently long-distance teleportation and open-destination
teleportation have also been realized. Until now, previous experiments have
only been able to teleport single qubits. However, since teleportation of
single qubits is insufficient for a large-scale realization of quantum
communication and computation2-5, teleportation of a composite system
containing two or more qubits has been seen as a long-standing goal in quantum
information science. Here, we present the experimental realization of quantum
teleportation of a two-qubit composite system. In the experiment, we develop
and exploit a six-photon interferometer to teleport an arbitrary polarization
state of two photons. The observed teleportation fidelities for different
initial states are all well beyond the state estimation limit of 0.40 for a
two-qubit system. Not only does our six-photon interferometer provide an
important step towards teleportation of a complex system, it will also enable
future experimental investigations on a number of fundamental quantum
communication and computation protocols such as multi-stage realization of
quantum-relay, fault-tolerant quantum computation, universal quantum
error-correction and one-way quantum computation.Comment: 16pages, 4 figure
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