1,574 research outputs found
Automated operation of a home made torque magnetometer using LabVIEW
In order to simplify and optimize the operation of our home made torque
magnetometer we created a new software system. The architecture is based on
parallel, independently running instrument handlers communicating with a main
control program. All programs are designed as command driven state machines
which greatly simplifies their maintenance and expansion. Moreover, as the main
program may receive commands not only from the user interface, but also from
other parallel running programs, an easy way of automation is achieved. A
program working through a text file containing a sequence of commands and
sending them to the main program suffices to automatically have the system
conduct a complex set of measurements. In this paper we describe the system's
architecture and its implementation in LabVIEW.Comment: 6 pages, 7 figures, submitted to Rev. Sci. Inst
Practical learning method for multi-scale entangled states
We describe a method for reconstructing multi-scale entangled states from a
small number of efficiently-implementable measurements and fast
post-processing. The method only requires single particle measurements and the
total number of measurements is polynomial in the number of particles. Data
post-processing for state reconstruction uses standard tools, namely matrix
diagonalisation and conjugate gradient method, and scales polynomially with the
number of particles. Our method prevents the build-up of errors from both
numerical and experimental imperfections
Optimal, reliable estimation of quantum states
Accurately inferring the state of a quantum device from the results of
measurements is a crucial task in building quantum information processing
hardware. The predominant state estimation procedure, maximum likelihood
estimation (MLE), generally reports an estimate with zero eigenvalues. These
cannot be justified. Furthermore, the MLE estimate is incompatible with error
bars, so conclusions drawn from it are suspect. I propose an alternative
procedure, Bayesian mean estimation (BME). BME never yields zero eigenvalues,
its eigenvalues provide a bound on their own uncertainties, and it is the most
accurate procedure possible. I show how to implement BME numerically, and how
to obtain natural error bars that are compatible with the estimate. Finally, I
briefly discuss the differences between Bayesian and frequentist estimation
techniques.Comment: RevTeX; 14 pages, 2 embedded figures. Comments enthusiastically
welcomed
Synthesis and Bulk Properties of Oxychloride Superconductor Ca2-xNaxCuO2Cl2
Polycrystalline samples and submillimeter size single crystals of Na-doped
Ca2CuO2Cl2 have been synthesized under high pressure. A series of experiments
showed that the Na content depends not only on the pressure during the
synthesis but also on the synthesis temperature and time. From a comparison of
the Na-CCOC data with those of structurally related La214 cuprate
superconductors we concluded that chlorine at the apical site is less effective
that oxygen in supplying charge carriers to the CuO2 plans. As a result, the
coupling between the CuO2 planes is weakened, the transition temperature Tc is
reduced and the anisotropic nature is enhanced.Comment: 7 pages, 7 figures, 1 table, presenthed at the Eucas 2007 conference.
Accepted for "Journal of Physics: Conference Series (JPCS)" 2008 and European
News Forum, Issue 3 (2008
Scalable Noise Estimation with Random Unitary Operators
We describe a scalable stochastic method for the experimental measurement of
generalized fidelities characterizing the accuracy of the implementation of a
coherent quantum transformation. The method is based on the motion reversal of
random unitary operators. In the simplest case our method enables direct
estimation of the average gate fidelity. The more general fidelities are
characterized by a universal exponential rate of fidelity loss. In all cases
the measurable fidelity decrease is directly related to the strength of the
noise affecting the implementation -- quantified by the trace of the
superoperator describing the non--unitary dynamics. While the scalability of
our stochastic protocol makes it most relevant in large Hilbert spaces (when
quantum process tomography is infeasible), our method should be immediately
useful for evaluating the degree of control that is achievable in any prototype
quantum processing device. By varying over different experimental arrangements
and error-correction strategies additional information about the noise can be
determined.Comment: 8 pages; v2: published version (typos corrected; reference added
Exponential speed-up with a single bit of quantum information: Testing the quantum butterfly effect
We present an efficient quantum algorithm to measure the average fidelity
decay of a quantum map under perturbation using a single bit of quantum
information. Our algorithm scales only as the complexity of the map under
investigation, so for those maps admitting an efficient gate decomposition, it
provides an exponential speed up over known classical procedures. Fidelity
decay is important in the study of complex dynamical systems, where it is
conjectured to be a signature of quantum chaos. Our result also illustrates the
role of chaos in the process of decoherence.Comment: 4 pages, 2 eps figure
Verifying multi-partite mode entanglement of W states
We construct a method for verifying mode entanglement of N-mode W states. The
ideal W state contains exactly one excitation symmetrically shared between N
modes, but our method takes the existence of higher numbers of excitations into
account, as well as the vacuum state and other deviations from the ideal state.
Moreover, our method distinguishes between full N-party entanglement and states
with M-party entanglement with M<N, including mixtures of the latter. We
specialize to the case N=4 for illustrative purposes. In the optical case,
where excitations are photons, our method can be implemented using linear
optics.Comment: 11 pages, 12 figure
Minimax mean estimator for the trine
We explore the question of state estimation for a qubit restricted to the
- plane of the Bloch sphere, with the trine measurement. In our earlier
work [H. K. Ng and B.-G. Englert, eprint arXiv:1202.5136[quant-ph] (2012)],
similarities between quantum tomography and the tomography of a classical die
motivated us to apply a simple modification of the classical estimator for use
in the quantum problem. This worked very well. In this article, we adapt a
different aspect of the classical estimator to the quantum problem. In
particular, we investigate the mean estimator, where the mean is taken with a
weight function identical to that in the classical estimator but now with
quantum constraints imposed. Among such mean estimators, we choose an optimal
one with the smallest worst-case error-the minimax mean estimator-and compare
its performance with that of other estimators. Despite the natural
generalization of the classical approach, this minimax mean estimator does not
work as well as one might expect from the analogous performance in the
classical problem. While it outperforms the often-used maximum-likelihood
estimator in having a smaller worst-case error, the advantage is not
significant enough to justify the more complicated procedure required to
construct it. The much simpler adapted estimator introduced in our earlier work
is still more effective. Our previous work emphasized the similarities between
classical and quantum state estimation; in contrast, this paper highlights how
intuition gained from classical problems can sometimes fail in the quantum
arena.Comment: 18 pages, 3 figure
Anisotropy and internal field distribution of MgB2 in the mixed state at low temperatures
Magnetization and muon spin relaxation on MgB2 were measured as a function of
field at 2 K. Both indicate an inverse-squared penetration depth strongly
decreasing with increasing field H below about 1 T. Magnetization also suggests
the anisotropy of the penetration depth to increase with increasing H,
interpolating between a low Hc1 and a high Hc2 anisotropy. Torque vs angle
measurements are in agreement with this finding, while also ruling out drastic
differences between the mixed state anisotropies of the two basic length scales
penetration depth and coherence length.Comment: 4 pages, 4 figure
Anisotropic properties of MgB2 by torque magnetometry
Anisotropic properties of superconducting MgB2 obtained by torque
magnetometry are compared to theoretical predictions, concentrating on two
issues. Firstly, the angular dependence of Hc2 is shown to deviate close to Tc
from the dependence assumed by anisotropic Ginzburg-Landau theory. Secondly,
from the evaluation of torque vs angle curves it is concluded that the
anisotropy of the penetration depth gamma_lambda has to be substantially higher
at low temperature than theoretical estimates, at least in fields higher than
0.2 T.Comment: 2 p.,2 Fig., submitted to Physica C (M2S-Rio proceedings); v2: 1 ref
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