396 research outputs found
Quantum control without access to the controlling interaction
In our model a fixed Hamiltonian acts on the joint Hilbert space of a quantum
system and its controller. We show under which conditions measurements, state
preparations, and unitary implementations on the system can be performed by
quantum operations on the controller only.
It turns out that a measurement of the observable A and an implementation of
the one-parameter group exp(iAr) can be performed by almost the same sequence
of control operations. Furthermore measurement procedures for A+B, for (AB+BA),
and for i[A,B] can be constructed from measurements of A and B. This shows that
the algebraic structure of the set of observables can be explained by the Lie
group structure of the unitary evolutions on the joint Hilbert space of the
measuring device and the measured system.
A spin chain model with nearest neighborhood coupling shows that the border
line between controller and system can be shifted consistently.Comment: 10 pages, Revte
Time-optimal synthesis of unitary transformations in coupled fast and slow qubit system
In this paper, we study time-optimal control problems related to system of
two coupled qubits where the time scales involved in performing unitary
transformations on each qubit are significantly different. In particular, we
address the case where unitary transformations produced by evolutions of the
coupling take much longer time as compared to the time required to produce
unitary transformations on the first qubit but much shorter time as compared to
the time to produce unitary transformations on the second qubit. We present a
canonical decomposition of SU(4) in terms of the subgroup SU(2)xSU(2)xU(1),
which is natural in understanding the time-optimal control problem of such a
coupled qubit system with significantly different time scales. A typical
setting involves dynamics of a coupled electron-nuclear spin system in pulsed
electron paramagnetic resonance experiments at high fields. Using the proposed
canonical decomposition, we give time-optimal control algorithms to synthesize
various unitary transformations of interest in coherent spectroscopy and
quantum information processing.Comment: 8 pages, 3 figure
A high-precision polarimeter
We have built a polarimeter in order to measure the electron beam
polarization in hall C at JLAB. Using a superconducting solenoid to drive the
pure-iron target foil into saturation, and a symmetrical setup to detect the
Moller electrons in coincidence, we achieve an accuracy of <1%. This sets a new
standard for Moller polarimeters.Comment: 17 pages, 9 figures, submitted to N.I.
Final State Interaction Effects in pol 3He(pol e,e'p)
Asymmetries in quasi-elastic pol 3He(pol e,e'p) have been measured at a
momentum transfer of 0.67 (GeV/c)^2 and are compared to a calculation which
takes into account relativistic kinematics in the final state and a
relativistic one-body current operator. With an exact solution of the Faddeev
equation for the 3He-ground state and an approximate treatment of final state
interactions in the continuum good agreement is found with the experimental
data.Comment: 11 pages, 6 figures, submitted to Phys. Lett. B, revised version,
sensitivity study to relativity and NN-potential adde
Correlated Strength in Nuclear Spectral Function
We have carried out an (e,e'p) experiment at high momentum transfer and in
parallel kinematics to measure the strength of the nuclear spectral function
S(k,E) at high nucleon momenta k and large removal energies E. This strength is
related to the presence of short-range and tensor correlations, and was known
hitherto only indirectly and with considerable uncertainty from the lack of
strength in the independent-particle region. This experiment confirms by direct
measurement the correlated strength predicted by theory.Comment: 4 pages, 2 figures, accepted by Phys. Rev. Let
Precise Neutron Magnetic Form Factors
Precise data on the neutron magnetic form factor G_{mn} have been obtained
with measurements of the ratio of cross sections of D(e,e'n) and D(e,e'p) up to
momentum transfers of Q^2 = 0.9 (GeV/c)^2. Data with typical uncertainties of
1.5% are presented. These data allow for the first time to extract a precise
value of the magnetic radius of the neutron.Comment: 10 pages, 2 figures, submitted to Physics Letters
Vector and Tensor Analyzing Powers of the H(d,gamma)He-3 capture reaction
Precise measurements of the deuteron vector analyzing power Ayd and the
tensor analyzing power Ayy of the H(d,gamma)He-3 capture reaction have been
performed at deuteron energies of 29MeV and 45MeV. The data have been compared
to theoretical state-of-the-art calculations available today. Due to the large
sensitivity of polarization observables and the precision of the data light
could be shed on small effects present in the dynamics of the reaction.Comment: 11 pages, 24 figures, submitted for publication to PRC, revised after
referee proces
A Measurement of the Electric Form Factor of the Neutron through at (GeV/c)
We report the first measurement of the neutron electric form factor
via using a solid polarized target. was
determined from the beam-target asymmetry in the scattering of longitudinally
polarized electrons from polarized deuterated ammonia, ND. The
measurement was performed in Hall C at Thomas Jefferson National Accelerator
Facility (TJNAF) in quasi free kinematics with the target polarization
perpendicular to the momentum transfer. The electrons were detected in a
magnetic spectrometer in coincidence with neutrons in a large solid angle
segmented detector. We find at (GeV/c).Comment: Latex2e 5 pages, 3 figure
Optimal Control for Generating Quantum Gates in Open Dissipative Systems
Optimal control methods for implementing quantum modules with least amount of
relaxative loss are devised to give best approximations to unitary gates under
relaxation. The potential gain by optimal control using relaxation parameters
against time-optimal control is explored and exemplified in numerical and in
algebraic terms: it is the method of choice to govern quantum systems within
subspaces of weak relaxation whenever the drift Hamiltonian would otherwise
drive the system through fast decaying modes. In a standard model system
generalising decoherence-free subspaces to more realistic scenarios,
openGRAPE-derived controls realise a CNOT with fidelities beyond 95% instead of
at most 15% for a standard Trotter expansion. As additional benefit it requires
control fields orders of magnitude lower than the bang-bang decouplings in the
latter.Comment: largely expanded version, superseedes v1: 10 pages, 5 figure
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