1,488 research outputs found
Dark states of single NV centers in diamond unraveled by single shot NMR
The nitrogen-vacancy (NV) center in diamond is supposed to be a building
block for quantum computing and nanometer scale metrology at ambient
conditions. Therefore, precise knowledge of its quantum states is crucial.
Here, we experimentally show that under usual operating conditions the NV
exists in an equilibrium of two charge states (70% in the expected negative
(NV-) and 30% in the neutral one (NV0)). Projective quantum non-demolition
measurement of the nitrogen nuclear spin enables the detection even of the
additional, optically inactive state. The nuclear spin can be coherently driven
also in NV0 (T1 ~ 90 ms and T2 ~ 6 micro-s).Comment: 4 pages, 3 figure
Nonmonotonic energy harvesting efficiency in biased exciton chains
We theoretically study the efficiency of energy harvesting in linear exciton
chains with an energy bias, where the initial excitation is taking place at the
high-energy end of the chain and the energy is harvested (trapped) at the other
end. The efficiency is characterized by means of the average time for the
exciton to be trapped after the initial excitation. The exciton transport is
treated as the intraband energy relaxation over the states obtained by
numerically diagonalizing the Frenkel Hamiltonian that corresponds to the
biased chain. The relevant intraband scattering rates are obtained from a
linear exciton-phonon interaction. Numerical solution of the Pauli master
equation that describes the relaxation and trapping processes, reveals a
complicated interplay of factors that determine the overall harvesting
efficiency. Specifically, if the trapping step is slower than or comparable to
the intraband relaxation, this efficiency shows a nonmonotonic dependence on
the bias: it first increases when introducing a bias, reaches a maximum at an
optimal bias value, and then decreases again because of dynamic (Bloch)
localization of the exciton states. Effects of on-site (diagonal) disorder,
leading to Anderson localization, are addressed as well.Comment: 9 pages, 6 figures, to appear in Journal of Chemical Physic
A relaxationless demonstration of the Quantum Zeno Paradox on an individual atom
The driven evolution of the spin of an individual atomic ion on the
ground-state hyperfine resonance is impeded by the observation of the ion in
one of the pertaining eigenstates. Detection of resonantly scattered light
identifies the ion in its upper ``bright'' state. The lower ``dark'' ion state
is free of relaxation and correlated with the detector by a null signal. Null
events represent the straightforward demonstration of the quantum Zeno paradox.
Also, high probability of survival was demonstrated when the ion, driven by a
fractionated pulse, was probed {\em and monitored} during the
intermissions of the drive, such that the ion's evolution is completely
documented.Comment: 7 page
Low-beta cortico-pallidal coherence decreases during movement and correlates with overall reaction time
Beta band oscillations (13-30 Hz) are a hallmark of cortical and subcortical structures that are part of the motor system. In addition to local population activity, oscillations also provide a means for synchronization of activity between regions. Here we examined the role of beta band coherence between the internal globus pallidus (GPi) and (motor) cortex during a simple reaction time task performed by nine patients with idiopathic dystonia. We recorded local field potentials from deep brain stimulation (DBS) electrodes implanted in bilateral GPi in combination with simultaneous whole-head magneto-encephalography (MEG). Patients responded to visually presented go or stop-signal cues by pressing a button with left or right hand. Although coherence between signals from DBS electrodes and MEG sensors was observed throughout the entire beta band, a significant movement-related decrease prevailed in lower beta frequencies (∼13-21 Hz). In addition, patients' absolute coherence values in this frequency range significantly correlated with their median reaction time during the task (p = 0.003, r = 0.89). These findings corroborate the recent idea of two functionally distinct frequency ranges within the beta band, as well as the anti-kinetic character of beta oscillations
Relative "-Numerical Ranges for Applications in Quantum Control and Quantum Information
Motivated by applications in quantum information and quantum control, a new
type of "-numerical range, the relative "-numerical range denoted
, is introduced. It arises upon replacing the unitary group U(N) in
the definition of the classical "-numerical range by any of its compact and
connected subgroups .
The geometric properties of the relative "-numerical range are analysed in
detail. Counterexamples prove its geometry is more intricate than in the
classical case: e.g. is neither star-shaped nor simply-connected.
Yet, a well-known result on the rotational symmetry of the classical
"-numerical range extends to , as shown by a new approach based on
Lie theory. Furthermore, we concentrate on the subgroup , i.e. the -fold tensor product of SU(2),
which is of particular interest in applications. In this case, sufficient
conditions are derived for being a circular disc centered at
origin of the complex plane. Finally, the previous results are illustrated in
detail for .Comment: accompanying paper to math-ph/070103
Coherent control of macroscopic quantum states in a single-Cooper-pair box
A small superconducting electrode (a single-Cooper-pair box) connected to a
reservoir via a Josephson junction constitutes an artificial two-level system,
in which two charge states that differ by 2e are coupled by tunneling of Cooper
pairs. Despite its macroscopic nature involving a large number of electrons,
the two-level system shows coherent superposition of the two charge states, and
has been suggested as a candidate for a qubit, i.e. a basic component of a
quantum computer. Here we report on time-domain observation of the coherent
quantum-state evolution in the two-level system by applying a short voltage
pulse that modifies the energies of the two levels nonadiabatically to control
the coherent evolution. The resulting state was probed by a tunneling current
through an additional probe junction. Our results demonstrate coherent
operation and measurement of a quantum state of a single two-level system, i.e.
a qubit, in a solid-state electronic device.Comment: 4 pages, 4 figures; to be published in Natur
Development of superconducting and cryogenic technology in the Institute for Technical Physics (ITP) of the Research Center Karlsruhe
Cloning and sequence analysis of cDNAs encoding the cytosolic precursors of subunits GapA and GapB of chloroplast glyceraldehyde-3-phosphate dehydrogenase from pea and spinach
Chloroplast glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is composed of two different subunits, GapA and GapB. cDNA clones containing the entire coding sequences of the cytosolic precursors for GapA from pea and for GapB from pea and spinach have been identified, sequenced and the derived amino acid sequences have been compared to the corresponding sequences from tobacco, maize and mustard. These comparisons show that GapB differs from GapA in about 20% of its amino acid residues and by the presence of a flexible and negatively charged C-terminal extension, possibly responsible for the observed association of the enzyme with chloroplast envelopes in vitro. This C-terminal extension (29 or 30 residues) may be susceptible to proteolytic cleavage thereby leading to a conversion of chloroplast GAPDH isoenzyme I into isoenzyme II. Evolutionary rate comparisons at the amino acid sequence level show that chloroplast GapA and GapB evolve roughly two-fold slower than their cytosolic counterpart GapC. GapA and GapB transit peptides evolve about 10 times faster than the corresponding mature subunits. They are relatively long (68 and 83 residues for pea GapA and spinach GapB respectively) and share a similar amino acid framework with other chloroplast transit peptides
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