11,089 research outputs found
Programmable networks for quantum algorithms
The implementation of a quantum computer requires the realization of a large
number of N-qubit unitary operations which represent the possible oracles or
which are part of the quantum algorithm. Until now there are no standard ways
to uniformly generate whole classes of N-qubit gates. We have developed a
method to generate arbitrary controlled phase shift operations with a single
network of one-qubit and two-qubit operations. This kind of network can be
adapted to various physical implementations of quantum computing and is
suitable to realize the Deutsch-Jozsa algorithm as well as Grover's search
algorithm.Comment: 4 pages. Accepted version; Journal-ref. adde
Interaction-free quantum computation
In this paper, we study the quantum computation realized by an
interaction-free measurement (IFM). Using Kwiat et al.'s interferometer, we
construct a two-qubit quantum gate that changes one particle's trajectory
according to whether or not the other particle exists in the interferometer. We
propose a method for distinguishing Bell-basis vectors, each of which consists
of a pair of an electron and a positron, by this gate. (This is called the
Bell-basis measurement.) This method succeeds with probability 1 in the limit
of , where N is the number of beam splitters in the
interferometer. Moreover, we can carry out a controlled-NOT gate operation by
the above Bell-basis measurement and the method proposed by Gottesman and
Chuang. Therefore, we can prepare a universal set of quantum gates by the IFM.
This means that we can execute any quantum algorithm by the IFM.Comment: 11 pages, 7 figures, LaTex2
A conditional quantum phase gate between two 3-state atoms
We propose a scheme for conditional quantum logic between two 3-state atoms
that share a quantum data-bus such as a single mode optical field in cavity QED
systems, or a collective vibrational state of trapped ions. Making use of
quantum interference, our scheme achieves successful conditional phase
evolution without any real transitions of atomic internal states or populating
the quantum data-bus. In addition, it only requires common addressing of the
two atoms by external laser fields.Comment: 8 fig
Structure of strongly coupled, multi-component plasmas
We investigate the short-range structure in strongly coupled fluidlike plasmas using the hypernetted chain approach generalized to multicomponent systems. Good agreement with numerical simulations validates this method for the parameters considered. We found a strong mutual impact on the spatial arrangement for systems with multiple ion species which is most clearly pronounced in the static structure factor. Quantum pseudopotentials were used to mimic diffraction and exchange effects in dense electron-ion systems. We demonstrate that the different kinds of pseudopotentials proposed lead to large differences in both the pair distributions and structure factors. Large discrepancies were also found in the predicted ion feature of the x-ray scattering signal, illustrating the need for comparison with full quantum calculations or experimental verification
Another Derivation of a Sum Rule for the Two-Dimensional Two-Component Plasma
In a two-dimensional two-component plasma, the second moment of the number
density correlation function has the simple value , where is the dimensionless coupling
constant. This result is derived directly by using diagrammatic methods.Comment: 10 pages, uses axodraw.sty, elsart.sty, elsart12.sty, subeq.sty;
accepted for publication in Physica A, May 200
Heisenberg chains cannot mirror a state
Faithful exchange of quantum information can in future become a key part of
many computational algorithms. Some Authors suggest to use chains of mutually
coupled spins as channels for quantum communication. One can divide these
proposals into the groups of assisted protocols, which require some additional
action from the users, and natural ones, based on the concept of state
mirroring. We show that mirror is fundamentally not the feature chains of
spins-1/2 coupled by the Heisenberg interaction, but without local magnetic
fields. This fact has certain consequences in terms of the natural state
transfer
Tunable asymmetric reflectance in silver films near the percolation threshold
We report on the optical characterization of semicontinuous nanostructured
silver films exhibiting tunable optical reflectance asymmetries. The films are
obtained using a multi-step process, where a nanocrystalline silver film is
first chemically deposited on a glass substrate and then subsequently coated
with additional silver via thermal vacuum-deposition. The resulting films
exhibit reflectance asymmetries whose dispersions may be tuned both in sign and
in magnitude, as well as a universal, tunable spectral crossover point. We
obtain a correlation between the optical response and charge transport in these
films, with the spectral crossover point indicating the onset of charge
percolation. Such broadband, dispersion-tunable asymmetric reflectors may find
uses in future light-harvesting systems.Comment: 18 pages, 5 figures, accepted by Journal of Applied Physic
The Measurement Calculus
Measurement-based quantum computation has emerged from the physics community
as a new approach to quantum computation where the notion of measurement is the
main driving force of computation. This is in contrast with the more
traditional circuit model which is based on unitary operations. Among
measurement-based quantum computation methods, the recently introduced one-way
quantum computer stands out as fundamental.
We develop a rigorous mathematical model underlying the one-way quantum
computer and present a concrete syntax and operational semantics for programs,
which we call patterns, and an algebra of these patterns derived from a
denotational semantics. More importantly, we present a calculus for reasoning
locally and compositionally about these patterns.
We present a rewrite theory and prove a general standardization theorem which
allows all patterns to be put in a semantically equivalent standard form.
Standardization has far-reaching consequences: a new physical architecture
based on performing all the entanglement in the beginning, parallelization by
exposing the dependency structure of measurements and expressiveness theorems.
Furthermore we formalize several other measurement-based models:
Teleportation, Phase and Pauli models and present compositional embeddings of
them into and from the one-way model. This allows us to transfer all the theory
we develop for the one-way model to these models. This shows that the framework
we have developed has a general impact on measurement-based computation and is
not just particular to the one-way quantum computer.Comment: 46 pages, 2 figures, Replacement of quant-ph/0412135v1, the new
version also include formalization of several other measurement-based models:
Teleportation, Phase and Pauli models and present compositional embeddings of
them into and from the one-way model. To appear in Journal of AC
Efficient Scheme for Initializing a Quantum Register with an Arbitrary Superposed State
Preparation of a quantum register is an important step in quantum computation
and quantum information processing. It is straightforward to build a simple
quantum state such as |i_1 i_2 ... i_n\ket with being either 0 or 1,
but is a non-trivial task to construct an {\it arbitrary} superposed quantum
state. In this Paper, we present a scheme that can most generally initialize a
quantum register with an arbitrary superposition of basis states.
Implementation of this scheme requires standard 1- and 2-bit gate
operations, {\it without introducing additional quantum bits}. Application of
the scheme in some special cases is discussed.Comment: 4 pages, 4 figures, accepted by Phys. Rev.
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