4,552 research outputs found
Adiabatic optical entanglement between electron spins in separate quantum dots
We present an adiabatic approach to the design of entangling quantum
operations with two electron spins localized in separate InAs/GaAs quantum dots
via the Coulomb interaction between optically-excited localized states.
Slowly-varying optical pulses minimize the pulse noise and the relaxation of
the excited states. An analytic "dressed state" solution gives a clear physical
picture of the entangling process, and a numerical solution is used to
investigate the error dynamics. For two vertically-stacked quantum dots we show
that, for a broad range of dot parameters, a two-spin state with concurrence
can be obtained by four optical pulses with durations
ns.Comment: 7 pages, 5 figure
Fast initialization of the spin state of an electron in a quantum dot in the Voigt configuration
We consider the initialization of the spin-state of a single electron trapped
in a self-assembled quantum dot via optical pumping of a trion level. We show
that with a magnetic field applied perpendicular to the growth direction of the
dot, a near-unity fidelity can be obtained in a time equal to a few times the
inverse of the spin-conserving trion relaxation rate. This method is several
orders-of-magnitude faster than with the field aligned parallel, since this
configuration must rely on a slow hole spin-flip mechanism. This increase in
speed does result in a limit on the maximum obtainable fidelity, but we show
that for InAs dots, the error is very small.Comment: 4 pages, 4 figure
Attacking Group Protocols by Refuting Incorrect Inductive Conjectures
Automated tools for finding attacks on flawed security protocols often fail to deal adequately with group protocols. This is because the abstractions made to improve performance on fixed 2 or 3 party protocols either preclude the modelling of group protocols all together, or permit modelling only in a fixed scenario, which can prevent attacks from being discovered. This paper describes Coral, a tool for finding counterexamples to incorrect inductive conjectures, which we have used to model protocols for both group key agreement and group key management, without any restrictions on the scenario. We will show how we used Coral to discover 6 previously unknown attacks on 3 group protocols
Fast spin rotations by optically controlled geometric phases in a quantum dot
We demonstrate optical control of the geometric phase acquired by one of the
spin states of an electron confined in a charge-tunable InAs quantum dot via
cyclic 2pi excitations of an optical transition in the dot. In the presence of
a constant in-plane magnetic field, these optically induced geometric phases
result in the effective rotation of the spin about the magnetic field axis and
manifest as phase shifts in the spin quantum beat signal generated by two
time-delayed circularly polarized optical pulses. The geometric phases
generated in this manner more generally perform the role of a spin phase gate,
proving potentially useful for quantum information applications.Comment: 4 pages, 3 figures, resubmitted to Physical Review Letter
Improved immune-suppression techniques for the exongrafting of human tumours.
The transplantability of a xenografted human adenocarcinoma has been examined in mice that had been immune-suppressed by thymectomy and whole-body irradiation and the results have been compared with transplantation into athymic (nude) mice. Two alternative techniques were used to prevent marrow failure following whole-body irradiation: reconstituting the animals with a marrow graft, or protecting them by an injection of cytosine arabinoside (Ara-C) 2 days before the irradiation. The results show that the Ara-C-prepared mice were more receptive to transplantation than marrow-grafted or nude mice, and they were the only animals that developed regional metastases from implanted xenografts. Some recovery of immunity occurred in both types of immune-suppressed mice, which was evident more than 5 weeks after immune-suppression and which was more marked in females than in males. It was concluded that the immune-suppressed mice were superior to nude mice for short-term experiments but they may be less satisfactory for long-term experiments
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