15,566 research outputs found
Microwave spectroscopy of a carbon nanotube charge qubit
Carbon nanotube quantum dots allow accurate control of electron charge, spin
and valley degrees of freedom in a material which is atomically perfect and can
be grown isotopically pure. These properties underlie the unique potential of
carbon nanotubes for quantum information processing, but developing nanotube
charge, spin, or spin-valley qubits requires efficient readout techniques as
well as understanding and extending quantum coherence in these devices. Here,
we report on microwave spectroscopy of a carbon nanotube charge qubit in which
quantum information is encoded in the spatial position of an electron. We
combine radio-frequency reflectometry measurements of the quantum capacitance
of the device with microwave manipulation to drive transitions between the
qubit states. This approach simplifies charge-state readout and allows us to
operate the device at an optimal point where the qubit is first-order
insensitive to charge noise. From these measurements, we are able to quantify
the degree of charge noise experienced by the qubit and obtain an inhomogeneous
charge coherence of 5 ns. We use a chopped microwave signal whose duty-cycle
period is varied to measure the decay of the qubit states, yielding a charge
relaxation time of 48 ns
Field-driven dynamics of nematic microcapillaries
Polymer-dispersed liquid crystal (PDLC) composites have long been a focus of
study for their unique electro-optical properties which have resulted in
various applications such as switchable (transparent/translucent) windows.
These composites are manufactured using desirable "bottom-up" techniques, such
as phase separation of a liquid crystal/polymer mixture, which enable
production of PDLC films at very large scales. LC domains within PDLCs are
typically spheroidal, as opposed to rectangular for an LCD panel, and thus
exhibit substantially different behaviour in the presence of an external field.
The fundamental difference between spheroidal and rectangular nematic domains
is that the former results in the presence of nanoscale orientational defects
in LC order while the latter does not. Progress in the development and
optimization of PDLC electro-optical properties has progressed at a relatively
slow pace due to this increased complexity. In this work, continuum simulations
are performed in order to capture the complex formation and electric
field-driven switching dynamics of approximations of PDLC domains. Using a
simplified elliptic cylinder (microcapillary) geometry as an approximation of
spheroidal PDLC domains, the effects of geometry (aspect ratio), surface
anchoring, and external field strength are studied through the use of the
Landau--de Gennes model of the nematic LC phase.Comment: 22 pages, 9 figures, Physical Review
Smooth optimal control with Floquet theory
This paper describes an approach to construct temporally shaped control
pulses that drive a quantum system towards desired properties. A
parametrization in terms of periodic functions with pre-defined frequencies
permits to realize a smooth, typically simple shape of the pulses; their
optimization can be performed based on a variational analysis with Floquet
theory. As we show with selected specific examples, this approach permits to
control the dynamics of interacting spins, such that gate operations and
entanglement dynamics can be implemented with very high accuracy
The Expansion in Width for Domain Walls in Nematic Liquid Crystals in External Magnetic Field
The improved expansion in width is applied to curved domain walls in uniaxial
nematic liquid crystals in external magnetic field. In the present paper we
concentrate on the case of equal elastic constants. We obtain approximate form
of the director field up to second order in magnetic coherence length.Comment: 18 pages, Latex 2.09, no figure
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