6,107 research outputs found
Structural design options for the new 34 meter beam waveguide antenna
In addition to the successful network of 34 m High Efficiency antennas recently built by JPL, the Deep Space Network (DSN) is embarking on the construction of a 34 m high performance, research and development antenna with beam waveguide optics at the Venus site. The construction of this antenna presents many engineering challenges in the area of structural, mechanical, RF, and pointing system design. A set of functional and structural design requirements is outlined to guide analysts in the final configuration selection. Five design concepts are presented covering both the conventional center-fed beam optics as well as the nonconventional, by-pass beam configuration. The merits of each concept are discussed with an emphasis on obtaining a homologous design. The preliminary results of structural optimization efforts, currently in progress, are promising, indicating the feasibility of meeting, as a minimum, all X-band (8.4 GHz) requirements, with a goal towards meeting Ka-band (32 GHz) quality performance, at the present budget constraints
Reconstruction of Liouvillian Superoperators
We show how to determine (reconstruct) a master equation governing the time
evolution of an open quantum system.
We present a general algorithm for the reconstruction of the corresponding
Liouvillian superoperators. Dynamics of a two-level atom in various
environments is discussed in detail.Comment: 4 pages, revtex, 1 eps figure, accepted for publication in Phys. Rev.
Optimal estimation of group transformations using entanglement
We derive the optimal input states and the optimal quantum measurements for
estimating the unitary action of a given symmetry group, showing how the
optimal performance is obtained with a suitable use of entanglement. Optimality
is defined in a Bayesian sense, as minimization of the average value of a given
cost function. We introduce a class of cost functions that generalizes the
Holevo class for phase estimation, and show that for states of the optimal form
all functions in such a class lead to the same optimal measurement. A first
application of the main result is the complete proof of the optimal efficiency
in the transmission of a Cartesian reference frame. As a second application, we
derive the optimal estimation of a completely unknown two-qubit maximally
entangled state, provided that N copies of the state are available. In the
limit of large N, the fidelity of the optimal estimation is shown to be
1-3/(4N).Comment: 11 pages, no figure
Local unitary versus local Clifford equivalence of stabilizer and graph states
The equivalence of stabilizer states under local transformations is of
fundamental interest in understanding properties and uses of entanglement. Two
stabilizer states are equivalent under the usual stochastic local operations
and classical communication criterion if and only if they are equivalent under
local unitary (LU) operations. More surprisingly, under certain conditions, two
LU equivalent stabilizer states are also equivalent under local Clifford (LC)
operations, as was shown by Van den Nest et al. [Phys. Rev. \textbf{A71},
062323]. Here, we broaden the class of stabilizer states for which LU
equivalence implies LC equivalence () to include all
stabilizer states represented by graphs with neither cycles of length 3 nor 4.
To compare our result with Van den Nest et al.'s, we show that any stabilizer
state of distance is beyond their criterion. We then further prove
that holds for a more general class of stabilizer states
of . We also explicitly construct graphs representing
stabilizer states which are beyond their criterion: we identify all 58 graphs
with up to 11 vertices and construct graphs with () vertices
using quantum error correcting codes which have non-Clifford transversal gates.Comment: Revised version according to referee's comments. To appear in
Physical Review
Clean Positive Operator Valued Measures
In quantum mechanics the statistics of the outcomes of a measuring apparatus
is described by a positive operator valued measure (POVM). A quantum channel
transforms POVM's into POVM's, generally irreversibly, thus loosing some of the
information retrieved from the measurement. This poses the problem of which
POVM's are "undisturbed", namely they are not irreversibly connected to another
POVM. We will call such POVM clean. In a sense, the clean POVM's would be
"perfect", since they would not have any additional "extrinsical" noise. Quite
unexpectedly, it turns out that such cleanness property is largely unrelated to
the convex structure of POVM's, and there are clean POVM's that are not
extremal and vice-versa. In this paper we solve the cleannes classification
problem for number n of outcomes n<=d (d dimension of the Hilbert space), and
we provide a a set of either necessary or sufficient conditions for n>d, along
with an iff condition for the case of informationally complete POVM's for
n=d^2.Comment: Minor changes. amsart 21 pages. Accepted for publication on J. Math.
Phy
Decoherence Free Subspaces for Quantum Computation
Decoherence in quantum computers is formulated within the Semigroup approach.
The error generators are identified with the generators of a Lie algebra. This
allows for a comprehensive description which includes as a special case the
frequently assumed spin-boson model. A generic condition is presented for
error-less quantum computation: decoherence-free subspaces are spanned by those
states which are annihilated by all the generators. It is shown that these
subspaces are stable to perturbations and moreover, that universal quantum
computation is possible within them.Comment: 4 pages, no figures. Conditions for decoherence-free subspaces made
more explicit, updated references. To appear in PR
Experimental implementation of an adiabatic quantum optimization algorithm
We report the realization of a nuclear magnetic resonance computer with three
quantum bits that simulates an adiabatic quantum optimization algorithm.
Adiabatic quantum algorithms offer new insight into how quantum resources can
be used to solve hard problems. This experiment uses a particularly well suited
three quantum bit molecule and was made possible by introducing a technique
that encodes general instances of the given optimization problem into an easily
applicable Hamiltonian. Our results indicate an optimal run time of the
adiabatic algorithm that agrees well with the prediction of a simple
decoherence model.Comment: REVTeX, 5 pages, 4 figures, improved lay-out; accepted for
publication in Physical Review Letter
Efficient implementation of selective recoupling in heteronuclear spin systems using Hadamard matrices
We present an efficient scheme which couples any designated pair of spins in
heteronuclear spin systems. The scheme is based on the existence of Hadamard
matrices. For a system of spins with pairwise coupling, the scheme
concatenates intervals of system evolution and uses at most pulses
where . Our results demonstrate that, in many systems, selective
recoupling is possible with linear overhead, contrary to common speculation
that exponential effort is always required.Comment: 7 pages, 4 figures, mypsfig2, revtex, submitted April 27, 199
Preserving coherence in quantum computation by pairing quantum bits
A scheme is proposed for protecting quantum states from both independent
decoherence and cooperative decoherence. The scheme operates by pairing each
qubit (two-state quantum system) with an ancilla qubit and by encoding the
states of the qubits into the corresponding coherence-preserving states of the
qubit-pairs. In this scheme, the amplitude damping (loss of energy) is
prevented as well as the phase damping (dephasing) by a strategy called the
free-Hamiltonian-elimination We further extend the scheme to include quantum
gate operations and show that loss and decoherence during the gate operations
can also be prevented.Comment: 12 pages, Latex, some correction in the reference and introduction.
Jour-ref: Phys. Rev. Lett. 79, 1953, 199
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