The method of constrained dynamical systems on the quantum-classical phase space is
utilized to develop a theory of quantum-classical hybrid systems. Effects of the classical
degrees of freedom on the quantum part are modeled using an appropriate constraint,
and the interaction also includes the effects of neglected degrees of freedom. Dynamical
law of the theory is given in terms of nonlinear stochastic differential equations
with Hamiltonian and gradient terms. The theory provides a successful dynamical description
of the collapse during quantum measurement. (paper)$$$$
Efficient quantum circuits for diagonal unitaries without ancillas http://dx.doi.org/10.1088/1367-2630/16/3/033040 by Welch, Jonathan; Mostame, Sarah; Aspuru-Guzik, Alan (Department of Chemistry and Chemical
Biology, Harvard University, Cambridge, MA 02138 (United States)); Greenbaum, Daniel
(MIT Lincoln Laboratory, 244 Wood Street, Lexington, MA 02420 (United States)), E-mail:
jwelch@fas.harvard.edu Read MoreCollapse
[en]
The accurate evaluation of diagonal unitary operators is often the most resource-intensive
element of quantum algorithms such as real-space quantum simulation and Grover search.
Efficient circuits have been demonstrated in some cases but generally require ancilla
registers, which can dominate the qubit resources. In this paper, we give a simple
way to construct efficient circuits for diagonal unitaries without ancillas, using
a correspondence between Walsh functions and a basis for diagonal operators. This
correspondence reduces the problem of constructing the minimal-depth circuit within
a given error tolerance, for an arbitrary diagonal unitary e^{if(x-^}) in
the |x〉 basis, to that of finding the minimal-length Walsh-series approximation
to the function f(x). We apply this approach to the quantum simulation of the classical
Eckart barrier problem of quantum chemistry, demonstrating that high-fidelity quantum
simulations can be achieved with few qubits and low depth$$$$
Recent developments in the theory of plasma-based collisionally excited x-ray lasers
(XRL) have shown an optimization potential based on the dependence of the absorption
region of the pumping laser on its angle of incidence on the plasma. For the experimental
proof of this idea, a number of diagnostic schemes were developed, tested, qualified
and applied. A high-resolution imaging system, yielding the keV emission profile perpendicular
to the target surface, provided positions of the hottest plasma regions, interesting
for the benchmarking of plasma simulation codes. The implementation of a highly efficient
spectrometer for the plasma emission made it possible to gain information about the
abundance of the ionization states necessary for the laser action in the plasma. The
intensity distribution and deflection angle of the pump laser beam could be imaged
for single XRL shots, giving access to its refraction process within the plasma. During
a European collaboration campaign at the Lund Laser Center, Sweden, the optimization
of the pumping laser incidence angle resulted in a reduction of the required pumping
energy for a Ni-like Mo XRL, which enabled the operation at a repetition rate of 10
Hz. Using the experiences gained there, the XRL performance at the PHELIX facility,
GSI Darmstadt with respect to achievable repetition rate and at wavelengths below
20 nm was significantly improved, and also important information for the development
towards multi-100 eV plasma XRLs was acquired. Due to the setup improvements achieved
during the work for this thesis, the PHELIX XRL system now has reached a degree of
reproducibility and versatility which is sufficient for demanding applications like
the XRL spectroscopy of heavy ions. In addition, a European research campaign, aiming
towards plasma XRLs approaching the water-window (wavelengths below 5 nm) was initiated.
(orig.)$$$$
The aim of the presented experiment was to investigate the feasibility of satellite-based
global quantum key distribution. In this context, a free-space quantum key distribution
experiment over a real distance of 144 km was performed. The transmitter and the receiver
were situated in 2500 m altitude on the Canary Islands of La Palma and Tenerife, respectively.
The small and compact transmitter unit generated attenuated laser pulses, that were
sent to the receiver via a 15-cm optical telescope. The receiver unit for polarisation
analysis and detection of the sent pulses was integrated into an existing mirror telescope
designed for classical optical satellite communications. To ensure the required stability
and efficiency of the optical link in the presence of atmospheric turbulence, the
two telescopes were equipped with a bi-directional automatic tracking system. Still,
due to stray light and high optical attenuation, secure key exchange would not be
possible using attenuated pulses in connection with the standard BB84 protocol. The
photon number statistics of attenuated pulses follows a Poissonian distribution. Hence,
by removing a photon from all pulses containing two or more photons, an eavesdropper
could measure its polarisation without disturbing the polarisation state of the remaining
pulse. In this way, he can gain information about the key without introducing detectable
errors. To protect against such attacks, the presented experiment employed the recently
developed method of using additional ''decoy'' states, i.e., the the intensity of
the pulses created by the transmitter were varied in a random manner. By analysing
the detection probabilities of the different pulses individually, a photon-number-splitting
attack can be detected. Thanks to the decoy-state analysis, the secrecy of the resulting
quantum key could be ensured despite the Poissonian nature of the emitted pulses.
For a channel attenuation as high as 35 dB, a secret key rate of up to 250 bit/s was
achieved. Our outdoor experiment was carried out under real atmospheric conditions
and with a channel attenuation comparable to an optical link from ground to a satellite
in low earth orbit. Hence, it definitely shows the feasibility of satellite-based
quantum key distribution using a technologically comparatively simple system. (orig.)$$$$
The present doctoral thesis describes experimentally measured properties of the resonance
spectra of flat microwave billiards with partially broken timereversal invariance
induced by an embedded magnetized ferrite. A vector network analyzer determines the
complex scattering matrix elements. The data is interpreted in terms of the scattering
formalism developed in nuclear physics. At low excitation frequencies the scattering
matrix displays isolated resonances. At these the effect of the ferrite on isolated
resonances (singlets) and pairs of nearly degenerate resonances (doublets) is investigated.
The hallmark of time-reversal symmetry breaking is the violation of reciprocity, i.e.
of the symmetry of the scattering matrix. One finds that reciprocity holds in singlets;
it is violated in doublets. This is modeled by an effective Hamiltonian of the resonator.
A comparison of the model to the data yields time-reversal symmetry breaking matrix
elements in the order of the level spacing. Their dependence on the magnetization
of the ferrite is understood in terms of its magnetic properties. At higher excitation
frequencies the resonances overlap and the scattering matrix elements fluctuate irregularly
(Ericson fluctuations). They are analyzed in terms of correlation functions. The data
are compared to three models based on random matrix theory. The model by Verbaarschot,
Weidenmueller and Zirnbauer describes time-reversal invariant scattering processes.
The one by Fyodorov, Savin and Sommers achieves the same for systems with complete
time-reversal symmetry breaking. An extended model has been developed that accounts
for partial breaking of time-reversal invariance. This extended model is in general
agreement with the data, while the applicability of the other two models is limited.
The cross-correlation function between forward and backward reactions determines the
time-reversal symmetry breaking matrix elements of the Hamiltonian to up to 0.3 mean
level spacings. Finally the sensitivity of the elastic enhancement factor to time-reversal
symmetry breaking is studied. Based on the data elastic enhancement factors below
2 are found which is consistent with breaking of time-reversal invariance in the regime
of overlapping resonances. The present work provides the framework to probe for broken
time-reversal invariance in any scattering data by a multitude of methods in the whole
range between isolated and overlapping resonances. (orig.)$$$$
A remarkable variety of particle acceleration occurs in the solar system, from lightning-related
acceleration of electrons to tens of MeV energy in less than a millisecond in planetary
atmospheres; to acceleration of auroral and radiation belt particles in planetary
magnetospheres; to acceleration at planetary bow shocks, co-rotating interplanetary
region shocks, shocks driven by fast coronal mass ejections, and at the heliospheric
termination shock; to acceleration in magnetic reconnection regions in solar flares
and at planetary magnetopause and magnetotail current sheets. These acceleration processes
often occur in conjunction with transient energy releases, and some are very efficient,
with the accelerated particles containing ∼ 10-50% of the total energy released. Others
are highly selective; for example, the acceleration in ^{3}He-rich solar particle
events enriches ^{3}He by a factor of up to 10,000 or more relative to ^{4}He.
Unlike acceleration processes outside the solar system, the accelerated particles
and the physical conditions in the acceleration region can be studied through direct
in situ measurements, and/or through detailed imaging and spectroscopy. Here I review
recent observations of these acceleration phenomena, our current understanding of
the physics involved, and the applicability to particle acceleration elsewhere in
the universe. (author)$$$$
The equations of fluid mechanics, coupled with those that describe matter transportation
at the molecular level must be handled effectively. Putting the fluid into equations,
we model the Bloch NMR flow equations into the harmonic wave equation for the analysis
of general fluid flow. We derived the solution of the modelled harmonic equation in
non relativistic quantum mechanics and discuss its semi classical application to illustrate
its potential wide-ranging usefulness in the search for the best possible data obtainable
for general fluid flow analysis. Representing the solution of the derived harmonic
wave equation by a normalized state function is quite useful in generating the properly
normalized wave functions and in the efficient evaluation of expectation values of
many operators that can be fundamental to the analysis of fluid flow especially at
the microscopic level. (author)$$$$
The time evolution of plasma potential has been measured with a retarding field analyzer
in pulsed operation mode with electron cyclotron resonance ion sources at JYFL and
RIKEN. Three different ion sources with microwave frequencies ranging from 6.4 to
18 GHz were employed for the experiments. The plasma potential was observed to increase
10-75 % during the Pre-glow and 10-30 % during the afterglow compared to steady state.
The paper is followed by the slides of the presentation. (authors)$$$$
Was Newton right in thinking that the pull of gravity on a body is governed by its
inertial mass? If he was, then all bodies have the same free fall acceleration, as
Galileo tried to check in his classic but not very accurate experiments at the leaning
tower of Pisa. This equivalence of inertial and gravitational mass went on to become
a basic principle in Einstein's general theory of relativity, and so is a cornerstone
in our understanding of the Universe$$$$