1,867 research outputs found
Feedback control of quantum state reduction
Feedback control of quantum mechanical systems must take into account the probabilistic nature of quantum measurement. We formulate quantum feedback control as a problem of stochastic nonlinear control by considering separately a quantum filtering problem and a state feedback control problem for the filter. We explore the use of stochastic Lyapunov techniques for the design of feedback controllers for quantum spin systems and demonstrate the possibility of stabilizing one outcome of a quantum measurement with unit probability
Modelling and feedback control design for quantum state preparation
The goal of this article is to provide a largely self-contained introduction to the modelling of controlled quantum systems under continuous observation, and to the design of feedback controls that prepare particular quantum states. We describe a bottom-up approach, where a field-theoretic model is subjected to statistical inference and is ultimately controlled. As an example, the formalism is applied to a highly idealized interaction of an atomic ensemble with an optical field. Our aim is to provide a unified outline for the modelling, from first principles, of realistic experiments in quantum control
Large orbital magnetic moments in carbon nanotubes generated by resonant transport
The nonequilibrium Green's function method is used to study the ballistic
transport in metallic carbon nanotubes when a current is injected from the
electrodes with finite bias voltages. We reveal, both analytically and
numerically, that large loop currents circulating around the tube are induced,
which come from a quantum mechanical interference and are much larger than the
current along the tube axis when the injected electron is resonant with a
time-reversed pair of degenerate states, which are, in fact, inherent in the
zigzag and chiral nanotubes. This results in large orbital magnetic moments,
making the nanotube a molecular solenoid.Comment: 5 pages, 4 figures; typos correcte
Coherent controllers for optical-feedback cooling of quantum oscillators
We study the cooling performance of optical-feedback controllers for open
optical and mechanical resonators in the Linear Quadratic Gaussian setting of
stochastic control theory. We utilize analysis and numerical optimization of
closed-loop models based on quantum stochastic differential equations to show
that coherent control schemes, where we embed the resonator in an
interferometer to achieve all-optical feedback, can outperform optimal
measurement-based feedback control schemes in the quantum regime of low
steady-state excitation number. These performance gains are attributed to the
coherent controller's ability to simultaneously process both quadratures of an
optical probe field without measurement or loss of fidelity, and may guide the
design of coherent feedback schemes for more general problems of robust
nonlinear and robust control.Comment: 15 pages, 20 figures. Submitted to Physical Review X. Follow-up paper
to arXiv:1206.082
Adaptive homodyne measurement of optical phase
We present an experimental demonstration of the power of real-time feedback
in quantum metrology, confirming a theoretical prediction by Wiseman regarding
the superior performance of an adaptive homodyne technique for single-shot
measurement of optical phase. For phase measurements performed on weak coherent
states with no prior knowledge of the signal phase, we show that the variance
of adaptive homodyne estimation approaches closer to the fundamental quantum
uncertainty limit than any previously demonstrated technique. Our results
underscore the importance of real-time feedback for reaching quantum
performance limits in coherent telecommunication, precision measurement and
information processing.Comment: RevTex4, color PDF figures (separate files), submitted to PR
Deterministic Dicke state preparation with continuous measurement and control
We characterize the long-time projective behavior of the stochastic master
equation describing a continuous, collective spin measurement of an atomic
ensemble both analytically and numerically. By adding state based feedback, we
show that it is possible to prepare highly entangled Dicke states
deterministically.Comment: Additional information is available at
http://minty.caltech.edu/Ensemble
Mini-Intein Structures from Extremophiles Suggest a Strategy for Finding Novel Robust Inteins
Inteins are prevalent among extremophiles. Mini-inteins with robust splicing properties are of particular interest for biotechnological applications due to their small size. However, biochemical and structural characterization has still been limited to a small number of inteins, and only a few serve as widely used tools in protein engineering. We determined the crystal structure of a naturally occurring Pol-II mini-intein from Pyrococcus horikoshii and compared all three mini-inteins found in the genome of P. horikoshii. Despite their similar sizes, the comparison revealed distinct differences in the insertions and deletions, implying specific evolutionary pathways from distinct ancestral origins. Our studies suggest that sporadically distributed mini-inteins might be more promising for further protein engineering applications than highly conserved mini-inteins. Structural investigations of additional inteins could guide the shortest path to finding novel robust mini-inteins suitable for various protein engineering purposes
Mini-Intein Structures from Extremophiles Suggest a Strategy for Finding Novel Robust Inteins
Inteins are prevalent among extremophiles. Mini-inteins with robust splicing properties are of particular interest for biotechnological applications due to their small size. However, biochemical and structural characterization has still been limited to a small number of inteins, and only a few serve as widely used tools in protein engineering. We determined the crystal structure of a naturally occurring Pol-II mini-intein from Pyrococcus horikoshii and compared all three mini-inteins found in the genome of P. horikoshii. Despite their similar sizes, the comparison revealed distinct differences in the insertions and deletions, implying specific evolutionary pathways from distinct ancestral origins. Our studies suggest that sporadically distributed mini-inteins might be more promising for further protein engineering applications than highly conserved mini-inteins. Structural investigations of additional inteins could guide the shortest path to finding novel robust mini-inteins suitable for various protein engineering purposes
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