2,122 research outputs found
On Quantum Control via Encoded Dynamical Decoupling
I revisit the ideas underlying dynamical decoupling methods within the
framework of quantum information processing, and examine their potential for
direct implementations in terms of encoded rather than physical degrees of
freedom. The usefulness of encoded decoupling schemes as a tool for engineering
both closed- and open-system encoded evolutions is investigated based on simple
examples.Comment: 12 pages, no figures; REVTeX style. This note collects various
theoretical considerations complementing/motivated by the experimental
demonstration of encoded control by Fortunato et a
Simulating Hamiltonians in Quantum Networks: Efficient Schemes and Complexity Bounds
We address the problem of simulating pair-interaction Hamiltonians in n node
quantum networks where the subsystems have arbitrary, possibly different,
dimensions. We show that any pair-interaction can be used to simulate any other
by applying sequences of appropriate local control sequences. Efficient schemes
for decoupling and time reversal can be constructed from orthogonal arrays.
Conditions on time optimal simulation are formulated in terms of spectral
majorization of matrices characterizing the coupling parameters. Moreover, we
consider a specific system of n harmonic oscillators with bilinear interaction.
In this case, decoupling can efficiently be achieved using the combinatorial
concept of difference schemes. For this type of interactions we present optimal
schemes for inversion.Comment: 19 pages, LaTeX2
Decoherence-Free Subspaces for Multiple-Qubit Errors: (I) Characterization
Coherence in an open quantum system is degraded through its interaction with
a bath. This decoherence can be avoided by restricting the dynamics of the
system to special decoherence-free subspaces. These subspaces are usually
constructed under the assumption of spatially symmetric system-bath coupling.
Here we show that decoherence-free subspaces may appear without spatial
symmetry. Instead, we consider a model of system-bath interactions in which to
first order only multiple-qubit coupling to the bath is present, with
single-qubit system-bath coupling absent. We derive necessary and sufficient
conditions for the appearance of decoherence-free states in this model, and
give a number of examples. In a sequel paper we show how to perform universal
and fault tolerant quantum computation on the decoherence-free subspaces
considered in this paper.Comment: 18 pages, no figures. Major changes. Section on universal fault
tolerant computation removed. This section contained a crucial error. A new
paper [quant-ph/0007013] presents the correct analysi
Semiconductor-based Geometrical Quantum Gates
We propose an implementation scheme for holonomic, i.e., geometrical, quantum
information processing based on semiconductor nanostructures. Our quantum
hardware consists of coupled semiconductor macroatoms addressed/controlled by
ultrafast multicolor laser-pulse sequences. More specifically, logical qubits
are encoded in excitonic states with different spin polarizations and
manipulated by adiabatic time-control of the laser amplitudes . The two-qubit
gate is realized in a geometric fashion by exploiting dipole-dipole coupling
between excitons in neighboring quantum dots.Comment: 4 Pages LaTeX, 3 Figures included. To appear in PRB (Rapid Comm.
NMR Techniques for Quantum Control and Computation
Fifty years of developments in nuclear magnetic resonance (NMR) have resulted
in an unrivaled degree of control of the dynamics of coupled two-level quantum
systems. This coherent control of nuclear spin dynamics has recently been taken
to a new level, motivated by the interest in quantum information processing.
NMR has been the workhorse for the experimental implementation of quantum
protocols, allowing exquisite control of systems up to seven qubits in size.
Here, we survey and summarize a broad variety of pulse control and tomographic
techniques which have been developed for and used in NMR quantum computation.
Many of these will be useful in other quantum systems now being considered for
implementation of quantum information processing tasks.Comment: 33 pages, accepted for publication in Rev. Mod. Phys., added
subsection on T_{1,\rho} (V.A.6) and on time-optimal pulse sequences
(III.A.6), redid some figures, made many small changes, expanded reference
Effect of noise on geometric logic gates for quantum computation
We introduce the non-adiabatic, or Aharonov-Anandan, geometric phase as a
tool for quantum computation and show how it could be implemented with
superconducting charge qubits. While it may circumvent many of the drawbacks
related to the adiabatic (Berry) version of geometric gates, we show that the
effect of fluctuations of the control parameters on non-adiabatic phase gates
is more severe than for the standard dynamic gates. Similarly, fluctuations
also affect to a greater extent quantum gates that use the Berry phase instead
of the dynamic phase.Comment: 8 pages, 4 figures; published versio
Decoherence of quantum registers
The dynamical evolution of a quantum register of arbitrary length coupled to
an environment of arbitrary coherence length is predicted within a relevant
model of decoherence. The results are reported for quantum bits (qubits)
coupling individually to different environments (`independent decoherence') and
qubits interacting collectively with the same reservoir (`collective
decoherence'). In both cases, explicit decoherence functions are derived for
any number of qubits. The decay of the coherences of the register is shown to
strongly depend on the input states: we show that this sensitivity is a
characteristic of types of coupling (collective and independent) and not
only of the collective coupling, as has been reported previously. A non-trivial
behaviour ("recoherence") is found in the decay of the off-diagonal elements of
the reduced density matrix in the specific situation of independent
decoherence. Our results lead to the identification of decoherence-free states
in the collective decoherence limit. These states belong to subspaces of the
system's Hilbert space that do not get entangled with the environment, making
them ideal elements for the engineering of ``noiseless'' quantum codes. We also
discuss the relations between decoherence of the quantum register and
computational complexity based on the new dynamical results obtained for the
register density matrix.Comment: Typos corrected. Discussion and references added. 1 figure + 3 tables
added. This updated version contains 13 (double column) pages + 8 figures.
PRA in pres
Bang-bang control of fullerene qubits using ultra-fast phase gates
Quantum mechanics permits an entity, such as an atom, to exist in a
superposition of multiple states simultaneously. Quantum information processing
(QIP) harnesses this profound phenomenon to manipulate information in radically
new ways. A fundamental challenge in all QIP technologies is the corruption of
superposition in a quantum bit (qubit) through interaction with its
environment. Quantum bang-bang control provides a solution by repeatedly
applying `kicks' to a qubit, thus disrupting an environmental interaction.
However, the speed and precision required for the kick operations has presented
an obstacle to experimental realization. Here we demonstrate a phase gate of
unprecedented speed on a nuclear spin qubit in a fullerene molecule (N@C60),
and use it to bang-bang decouple the qubit from a strong environmental
interaction. We can thus trap the qubit in closed cycles on the Bloch sphere,
or lock it in a given state for an arbitrary period. Our procedure uses
operations on a second qubit, an electron spin, in order to generate an
arbitrary phase on the nuclear qubit. We anticipate the approach will be vital
for QIP technologies, especially at the molecular scale where other strategies,
such as electrode switching, are unfeasible
Holonomic quantum gates: A semiconductor-based implementation
We propose an implementation of holonomic (geometrical) quantum gates by
means of semiconductor nanostructures. Our quantum hardware consists of
semiconductor macroatoms driven by sequences of ultrafast laser pulses ({\it
all optical control}). Our logical bits are Coulomb-correlated electron-hole
pairs (excitons) in a four-level scheme selectively addressed by laser pulses
with different polarization. A universal set of single and two-qubit gates is
generated by adiabatic change of the Rabi frequencies of the lasers and by
exploiting the dipole coupling between excitons.Comment: 10 Pages LaTeX, 10 Figures include
Video face replacement
We present a method for replacing facial performances in video. Our approach accounts for differences in identity, visual appearance, speech, and timing between source and target videos. Unlike prior work, it does not require substantial manual operation or complex acquisition hardware, only single-camera video. We use a 3D multilinear model to track the facial performance in both videos. Using the corresponding 3D geometry, we warp the source to the target face and retime the source to match the target performance. We then compute an optimal seam through the video volume that maintains temporal consistency in the final composite. We showcase the use of our method on a variety of examples and present the result of a user study that suggests our results are difficult to distinguish from real video footage.National Science Foundation (U.S.) (Grant PHY-0835713)National Science Foundation (U.S.) (Grant DMS-0739255
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