1,254 research outputs found
Perfect state transfer in quantum spin networks
We propose a class of qubit networks that admit perfect transfer of any
quantum state in a fixed period of time. Unlike many other schemes for quantum
computation and communication, these networks do not require qubit couplings to
be switched on and off. When restricted to N-qubit spin networks of identical
qubit couplings, we show that 2 log_3 N is the maximal perfect communication
distance for hypercube geometries. Moreover, if one allows fixed but different
couplings between the qubits then perfect state transfer can be achieved over
arbitrarily long distances in a linear chain.Comment: 4 pages, 1 figur
Geometric phases induced in auxiliary qubits by many-body systems near its critical points
The geometric phase induced in an auxiliary qubit by a many-body system is
calculated and discussed. Two kinds of coupling between the auxiliary qubit and
the many-body system are considered, which lead to dephasing and dissipation in
the qubit, respectively. As an example, we consider the XY spin-chain
dephasingly couple to a qubit, the geometric phase induced in the qubit is
presented and discussed. The results show that the geometric phase might be
used to signal the critical points of the many-body system, and it tends to
zero with the parameters of the many-body system going away from the critical
points
Adiabatic geometric phases in hydrogenlike atoms
We examine the effect of spin-orbit coupling on geometric phases in
hydrogenlike atoms exposed to a slowly varying magnetic field. The marginal
geometric phases associated with the orbital angular momentum and the intrinsic
spin fulfill a sum rule that explicitly relates them to the corresponding
geometric phase of the whole system. The marginal geometric phases in the
Zeeman and Paschen-Back limit are analyzed. We point out the existence of nodal
points in the marginal phases that may be detected by topological means.Comment: Clarifying material added, one figure removed, journal reference
adde
Staying adiabatic with unknown energy gap
We introduce an algorithm to perform an optimal adiabatic evolution that
operates without an apriori knowledge of the system spectrum. By probing the
system gap locally, the algorithm maximizes the evolution speed, thus
minimizing the total evolution time. We test the algorithm on the Landau-Zener
transition and then apply it on the quantum adiabatic computation of 3-SAT: The
result is compatible with an exponential speed-up for up to twenty qubits with
respect to classical algorithms. We finally study a possible algorithm
improvement by combining it with the quantum Zeno effect.Comment: 4 pages, 4 figure
Quantal interferometry with dissipative internal motion
In presence of dissipation, quantal states may acquire complex-valued phase
effects. We suggest a notion of dissipative interferometry that accommodates
this complex-valued structure and that may serve as a tool for analyzing the
effect of certain kinds of external influences on quantal interference. The
concept of mixed-state phase and concomitant gauge invariance is extended to
dissipative internal motion. The resulting complex-valued mixed-state
interference effects lead to well-known results in the unitary limit and in the
case of dissipative motion of pure quantal states. Dissipative interferometry
is applied to fault-tolerant geometric quantum computation.Comment: Slight revision, journal reference adde
Classical Concepts in Quantum Programming
The rapid progress of computer technology has been accompanied by a
corresponding evolution of software development, from hardwired components and
binary machine code to high level programming languages, which allowed to
master the increasing hardware complexity and fully exploit its potential.
This paper investigates, how classical concepts like hardware abstraction,
hierarchical programs, data types, memory management, flow of control and
structured programming can be used in quantum computing. The experimental
language QCL will be introduced as an example, how elements like irreversible
functions, local variables and conditional branching, which have no direct
quantum counterparts, can be implemented, and how non-classical features like
the reversibility of unitary transformation or the non-observability of quantum
states can be accounted for within the framework of a procedural programming
language.Comment: 11 pages, 4 figures, software available from
http://tph.tuwien.ac.at/~oemer/qcl.html, submitted for QS2002 proceeding
Visualizing quantum entanglement and the EPR paradox during the photodissociation of a diatomic molecule using two ultrashort laser pulses
We investigate theoretically the dissociative ionization of a H2+ molecule
using two ultrashort laser (pump-probe) pulses. The pump pulse prepares a
dissociating nuclear wave packet on an ungerade surface of H2+. Next, an UV (or
XUV) probe pulse ionizes this dissociating state at large (R = 20 - 100 bohr)
internuclear distance. We calculate the momenta distributions of protons and
photoelectrons which show a (two-slit-like) interference structure. A general,
simple interference formula is obtained which depends on the electron and
protons momenta, as well as on the pump-probe delay on the pulses durations and
polarizations. This interference can be interpreted as visualization of an
electron state delocalized over the two-centres. This state is an entangled
state of a hydrogen atom with a momentum p and a proton with an opposite
momentum. -p dissociating on the ungerade surface of H2+. This pump-probe
scheme can be used to reveal the nonlocality of the electron which intuitively
should be localized on just one of the protons separated by the distance R much
larger than the atomic Bohr orbit
Optimal purification of single qubits
We introduce a new decomposition of the multiqubit states of the form
and employ it to construct the optimal single qubit
purification procedure. The same decomposition allows us to study optimal
quantum cloning and state estimation of mixed states.Comment: 4 pages, 1 figur
Geometric phase in dephasing systems
Beyond the quantum Markov approximation, we calculate the geometric phase of
a two-level system driven by a quantized magnetic field subject to phase
dephasing. The phase reduces to the standard geometric phase in the weak
coupling limit and it involves the phase information of the environment in
general. In contrast with the geometric phase in dissipative systems, the
geometric phase acquired by the system can be observed on a long time scale. We
also show that with the system decohering to its pointer states, the geometric
phase factor tends to a sum over the phase factors pertaining to the pointer
states.Comment: 4 page
Schmidt Analysis of Pure-State Entanglement
We examine the application of Schmidt-mode analysis to pure state
entanglement. Several examples permitting exact analytic calculation of Schmidt
eigenvalues and eigenfunctions are included, as well as evaluation of the
associated degree of entanglement.Comment: 5 pages, 3 figures, for C.M. Bowden memoria
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