8,264 research outputs found
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
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
Quantum algorithms know in advance 50% of the solution they will find in the future
Quantum algorithms require less operations than classical algorithms. The
exact reason of this has not been pinpointed until now. Our explanation is that
quantum algorithms know in advance 50% of the solution of the problem they will
find in the future. In fact they can be represented as the sum of all the
possible histories of a respective "advanced information classical algorithm".
This algorithm, given the advanced information (50% of the bits encoding the
problem solution), performs the operations (oracle's queries) still required to
identify the solution. Each history corresponds to a possible way of getting
the advanced information and a possible result of computing the missing
information. This explanation of the quantum speed up has an immediate
practical consequence: the speed up comes from comparing two classical
algorithms, with and without advanced information, with no physics involved.
This simplification could open the way to a systematic exploration of the
possibilities of speed up.Comment: The example of new quantum speed up that was just outlined in the
previous version (finding the character of a permutation) is fully deployed
in the present version. There are minor distributed changes to the writin
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
Effect of electrical bias on spin transport across a magnetic domain wall
We present a theory of the current-voltage characteristics of a magnetic
domain wall between two highly spin-polarized materials, which takes into
account the effect of the electrical bias on the spin-flip probability of an
electron crossing the wall. We show that increasing the voltage reduces the
spin-flip rate, and is therefore equivalent to reducing the width of the domain
wall. As an application, we show that this effect widens the temperature window
in which the operation of a unipolar spin diode is nearly ideal.Comment: 11 pages, 3 figure
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.
Non-locality and gauge freedom in Deutsch and Hayden's formulation of quantum mechanics
Deutsch and Hayden have proposed an alternative formulation of quantum
mechanics which is completely local. We argue that their proposal must be
understood as having a form of `gauge freedom' according to which
mathematically distinct states are physically equivalent. Once this gauge
freedom is taken into account, their formulation is no longer local.Comment: 3 page
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
Sequential Quantum Cloning
Not all unitary operations upon a set of qubits can be implemented by
sequential interactions between each qubit and an ancillary system. We analyze
the specific case of sequential quantum cloning 1->M and prove that the minimal
dimension D of the ancilla grows linearly with the number of clones M. In
particular, we obtain D = 2M for symmetric universal quantum cloning and D =
M+1 for symmetric phase-covariant cloning. Furthermore, we provide a recipe for
the required ancilla-qubit interactions in each step of the sequential
procedure for both cases.Comment: 4 pages, no figures. New version with changes. Accepted in Physical
Review Letter
Recognizing Small-Circuit Structure in Two-Qubit Operators and Timing Hamiltonians to Compute Controlled-Not Gates
This work proposes numerical tests which determine whether a two-qubit
operator has an atypically simple quantum circuit. Specifically, we describe
formulae, written in terms of matrix coefficients, characterizing operators
implementable with exactly zero, one, or two controlled-not (CNOT) gates and
all other gates being one-qubit. We give an algorithm for synthesizing
two-qubit circuits with optimal number of CNOT gates, and illustrate it on
operators appearing in quantum algorithms by Deutsch-Josza, Shor and Grover. In
another application, our explicit numerical tests allow timing a given
Hamiltonian to compute a CNOT modulo one-qubit gates, when this is possible.Comment: 4 pages, circuit examples, an algorithm and a new application (v3
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