1,734 research outputs found
Limits of control for quantum systems: kinematical bounds on the optimization of observables and the question of dynamical realizability
In this paper we investigate the limits of control for mixed-state quantum
systems. The constraint of unitary evolution for non-dissipative quantum
systems imposes kinematical bounds on the optimization of arbitrary
observables. We summarize our previous results on kinematical bounds and show
that these bounds are dynamically realizable for completely controllable
systems. Moreover, we establish improved bounds for certain partially
controllable systems. Finally, the question of dynamical realizability of the
bounds for arbitary partially controllable systems is shown to depend on the
accessible sets of the associated control system on the unitary group U(N) and
the results of a few control computations are discussed briefly.Comment: 5 pages, orginal June 30, 2000, revised September 28, 200
Fundamental Speed Limits on Quantum Coherence and Correlation Decay
The study and control of coherence in quantum systems is one of the most
exciting recent developments in physics. Quantum coherence plays a crucial role
in emerging quantum technologies as well as fundamental experiments. A major
obstacle to the utilization of quantum effects is decoherence, primarily in the
form of dephasing that destroys quantum coherence, and leads to effective
classical behaviour. We show that there are universal relationships governing
dephasing, which constrain the relative rates at which quantum correlations can
disappear. These effectively lead to speed limits which become especially
important in multi-partite systems
First Order Static Excitation Potential: Scheme for Excitation Energies and Transition Moments
We present an approximation scheme for the calculation of the principal
excitation energies and transition moments of finite many-body systems. The
scheme is derived from a first order approximation to the self energy of a
recently proposed extended particle-hole Green's function. A hermitian
eigenvalue problem is encountered of the same size as the well-known Random
Phase Approximation (RPA). We find that it yields a size consistent description
of the excitation properties and removes an inconsistent treatment of the
ground state correlation by the RPA. By presenting a hermitian eigenvalue
problem the new scheme avoids the instabilities of the RPA and should be well
suited for large scale numerical calculations. These and additional properties
of the new approximation scheme are illuminated by a very simple exactly
solvable model.Comment: 15 pages revtex, 1 eps figure included, corrections in Eq. (A1) and
Sec. II
Complete controllability of quantum systems
Sufficient conditions for complete controllability of -level quantum
systems subject to a single control pulse that addresses multiple allowed
transitions concurrently are established. The results are applied in particular
to Morse and harmonic-oscillator systems, as well as some systems with
degenerate energy levels. Morse and harmonic oscillators serve as models for
molecular bonds, and the standard control approach of using a sequence of
frequency-selective pulses to address a single transition at a time is either
not applicable or only of limited utility for such systems.Comment: 8 pages, expanded and revised versio
An optical linewidth study of a chromoprotein-C-phycocyanin in a low-temperature glass
The temperature dependence of spectral holes burnt into a phycocyanin-doped ethylene glycol/water glass is investigated in the temperature range between 1.5 and 15 K. The data are well described by a power law with an exponent of 1.16 ± 0.1. Chromoproteins thus behave very much the same as glasses doped with small impurity molecules
Review of biorthogonal coupled cluster representations for electronic excitation
Single reference coupled-cluster (CC) methods for electronic excitation are
based on a biorthogonal representation (bCC) of the (shifted) Hamiltonian in
terms of excited CC states, also referred to as correlated excited (CE) states,
and an associated set of states biorthogonal to the CE states, the latter being
essentially configuration interaction (CI) configurations. The bCC
representation generates a non-hermitian secular matrix, the eigenvalues
representing excitation energies, while the corresponding spectral intensities
are to be derived from both the left and right eigenvectors. Using the
perspective of the bCC representation, a systematic and comprehensive analysis
of the excited-state CC methods is given, extending and generalizing previous
such studies. Here, the essential topics are the truncation error
characteristics and the separability properties, the latter being crucial for
designing size-consistent approximation schemes. Based on the general order
relations for the bCC secular matrix and the (left and right) eigenvector
matrices, formulas for the perturbation-theoretical (PT) order of the
truncation errors (TEO) are derived for energies, transition moments, and
property matrix elements of arbitrary excitation classes and truncation levels.
In the analysis of the separability properties of the transition moments, the
decisive role of the so-called dual ground state is revealed. Due to the use of
CE states the bCC approach can be compared to so-called intermediate state
representation (ISR) methods based exclusively on suitably orthonormalized CE
states. As the present analysis shows, the bCC approach has decisive advantages
over the conventional CI treatment, but also distinctly weaker TEO and
separability properties in comparison with a full (and hermitian) ISR method
Mesoscopic Effects in Quantum Phases of Ultracold Quantum Gases in Optical Lattices
We present a wide array of quantum measures on numerical solutions of 1D
Bose- and Fermi-Hubbard Hamiltonians for finite-size systems with open boundary
conditions. Finite size effects are highly relevant to ultracold quantum gases
in optical lattices, where an external trap creates smaller effective regions
in the form of the celebrated "wedding cake" structure and the local density
approximation is often not applicable. Specifically, for the Bose-Hubbard
Hamiltonian we calculate number, quantum depletion, local von-Neumann entropy,
generalized entanglement or Q-measure, fidelity, and fidelity susceptibility;
for the Fermi-Hubbard Hamiltonian we also calculate the pairing correlations,
magnetization, charge-density correlations, and antiferromagnetic structure
factor. Our numerical method is imaginary time propagation via time-evolving
block decimation. As part of our study we provide a careful comparison of
canonical vs. grand canonical ensembles and Gutzwiller vs. entangled
simulations. The most striking effect of finite size occurs for bosons: we
observe a strong blurring of the tips of the Mott lobes accompanied by higher
depletion, and show how the location of the first Mott lobe tip approaches the
thermodynamic value as a function of system size.Comment: 13 pages, 10 figure
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