5,571 research outputs found
Photonic Entanglement for Fundamental Tests and Quantum Communication
Entanglement is at the heart of fundamental tests of quantum mechanics like
tests of Bell-inequalities and, as discovered lately, of quantum computation
and communication. Their technological advance made entangled photons play an
outstanding role in entanglement physics. We give a generalized concept of
qubit entanglement and review the state of the art of photonic experiments.Comment: 54 pages, 33 figures. Review article submitted to QIC (Rinton
The Fraunhofer Quantum Computing Portal - www.qc.fraunhofer.de - A web-based Simulator of Quantum Computing Processes
Fraunhofer FIRST develops a computing service and collaborative workspace
providing a convenient tool for simulation and investigation of quantum
algorithms. To broaden the twenty qubit limit of workstation-based simulations
to the next qubit decade we provide a dedicated high memorized Linux cluster
with fast Myrinet interconnection network together with a adapted parallel
simulator engine. This simulation service supplemented by a collaborative
workspace is usable everywhere via web interface and integrates both hardware
and software as collaboration and investigation platform for the quantum
community. The beta test version realizes all common one, two and three qubit
gates, arbitrary one and two bit gates, orthogonal measurements as well as
special gates like Oracle, Modulo function, Quantum Fourier Transformation and
arbitrary Spin-Hamiltonians up to 31 qubits. For a restricted gate set it
feasible to investigate circuits with up to sixty qubits. URL:
http://www.qc.fraunhofer.d
Estimating the privacy of quantum-random numbers
We analyze the information an attacker can obtain on the numbers generated by
a user by measurements on a subsystem of a system consisting of two entangled
two-level systems. The attacker and the user make measurements on their
respective subsystems, only. Already the knowledge of the density matrix of the
subsystem of the user completely determines the upper bound on the information
accessible to the attacker. We compare and contrast this information to the
appropriate bounds provided by quantum state discrimination.Comment: 26 pages, 4 figure
Multiple Particle Interference and Quantum Error Correction
The concept of multiple particle interference is discussed, using insights
provided by the classical theory of error correcting codes. This leads to a
discussion of error correction in a quantum communication channel or a quantum
computer. Methods of error correction in the quantum regime are presented, and
their limitations assessed. A quantum channel can recover from arbitrary
decoherence of x qubits if K bits of quantum information are encoded using n
quantum bits, where K/n can be greater than 1-2 H(2x/n), but must be less than
1 - 2 H(x/n). This implies exponential reduction of decoherence with only a
polynomial increase in the computing resources required. Therefore quantum
computation can be made free of errors in the presence of physically realistic
levels of decoherence. The methods also allow isolation of quantum
communication from noise and evesdropping (quantum privacy amplification).Comment: Submitted to Proc. Roy. Soc. Lond. A. in November 1995, accepted May
1996. 39 pages, 6 figures. This is now the final version. The changes are
some added references, changed final figure, and a more precise use of the
word `decoherence'. I would like to propose the word `defection' for a
general unknown error of a single qubit (rotation and/or entanglement). It is
useful because it captures the nature of the error process, and has a verb
form `to defect'. Random unitary changes (rotations) of a qubit are caused by
defects in the quantum computer; to entangle randomly with the environment is
to form a treacherous alliance with an enemy of successful quantu
Measurement-Based Noiseless Linear Amplification for Quantum Communication
Entanglement distillation is an indispensable ingredient in extended quantum
communication networks. Distillation protocols are necessarily
non-deterministic and require advanced experimental techniques such as
noiseless amplification. Recently it was shown that the benefits of noiseless
amplification could be extracted by performing a post-selective filtering of
the measurement record to improve the performance of quantum key distribution.
We apply this protocol to entanglement degraded by transmission loss of up to
the equivalent of 100km of optical fibre. We measure an effective entangled
resource stronger than that achievable by even a maximally entangled resource
passively transmitted through the same channel. We also provide a
proof-of-principle demonstration of secret key extraction from an otherwise
insecure regime. The measurement-based noiseless linear amplifier offers two
advantages over its physical counterpart: ease of implementation and near
optimal probability of success. It should provide an effective and versatile
tool for a broad class of entanglement-based quantum communication protocols.Comment: 7+3 pages, 5+1 figures, close to published versio
Unconditionally verifiable blind computation
Blind Quantum Computing (BQC) allows a client to have a server carry out a
quantum computation for them such that the client's input, output and
computation remain private. A desirable property for any BQC protocol is
verification, whereby the client can verify with high probability whether the
server has followed the instructions of the protocol, or if there has been some
deviation resulting in a corrupted output state. A verifiable BQC protocol can
be viewed as an interactive proof system leading to consequences for complexity
theory. The authors, together with Broadbent, previously proposed a universal
and unconditionally secure BQC scheme where the client only needs to be able to
prepare single qubits in separable states randomly chosen from a finite set and
send them to the server, who has the balance of the required quantum
computational resources. In this paper we extend that protocol with new
functionality allowing blind computational basis measurements, which we use to
construct a new verifiable BQC protocol based on a new class of resource
states. We rigorously prove that the probability of failing to detect an
incorrect output is exponentially small in a security parameter, while resource
overhead remains polynomial in this parameter. The new resource state allows
entangling gates to be performed between arbitrary pairs of logical qubits with
only constant overhead. This is a significant improvement on the original
scheme, which required that all computations to be performed must first be put
into a nearest neighbour form, incurring linear overhead in the number of
qubits. Such an improvement has important consequences for efficiency and
fault-tolerance thresholds.Comment: 46 pages, 10 figures. Additional protocol added which allows
arbitrary circuits to be verified with polynomial securit
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