39 research outputs found
Adiabatic information transport in the presence of decoherence
We study adiabatic population transfer between discrete positions. Being
closely related to STIRAP in optical systems, this transport is coherent and
robust against variations of experimental parameters. Thanks to these
properties the scheme is a promising candidate for transport of quantum
information in quantum computing. We study the effects of spatially registered
noise sources on the quantum transport and in particular model Markovian
decoherence via non-local coupling to nearby quantum point contacts which serve
as information readouts. We find that the rate of decoherence experienced by a
spatial superposition initially grows with spatial separation but surprisingly
then plateaus. In addition we include non-Markovian effects due to couplings to
nearby two level systems and we find that although the population transport
exhibits robustness in the presence of both types of noise sources, the
transport of a spatial superposition exhibits severe fragility.Comment: 11page
Geometric quantum gates with superconducting qubits
We suggest a scheme to implement a universal set of non-Abelian geometric
transformations for a single logical qubit composed of three superconducting
transmon qubits coupled to a single cavity. The scheme utilizes an adiabatic
evolution in a rotating frame induced by the effective tripod Hamiltonian which
is achieved by longitudinal driving of the transmons. The proposal is
experimentally feasible with the current state of the art and could serve as a
first proof of principle for geometric quantum computing.Comment: 7 pages, 5 figure
Collisional Quantum Brownian Motion
We derive a quantum master equation from first principles to describe
friction in one dimensional, collisional Brownian motion. We are the first to
avoid an ill-defined square of the Dirac delta function by using localized wave
packets rather than plane waves. Solving the Schr\"odinger equation for two
colliding particles, we discover that the previously found position diffusion
is not a physical process, but an artifact of the approximation of a coarse
grained time scale, which in turn is needed to find Markkovian dynamics.Comment: 5 pages, 1 figur
On the rotating wave approximation in the adiabatic limit
I revisit a longstanding question in quantum optics; When is the rotating
wave approximation justified? In terms of the Jaynes-Cummings and Rabi models I
demonstrate that the approximation in general breaks down in the adiabatic
limit regardless of system parameters. This is explicitly shown by comparing
Berry phases of the two models, where it is found that this geometrical phase
is strictly zero in the Rabi model contrary to the non-trivial Berry phase of
the Jaynes-Cummings model. The source of this surprising result is traced back
to different topologies in the two models.Comment: 8 pages, 3 figure
Langmuir probe electronics upgrade on the tokamak a configuration variable
A detailed description of the Langmuir probe electronics upgrade for TCV (Tokamak a Configuration Variable) is presented. The number of amplifiers and corresponding electronics has been increased from 48 to 120 in order to simultaneously connect all of the 114 Langmuir probes currently mounted in the TCV divertor and main-wall tiles. Another set of 108 amplifiers is ready to be installed in order to connect 80 new probes, built in the frame of the TCV divertor upgrade. Technical details of the amplifier circuitry are discussed as well as improvements over the first generation of amplifiers developed at SPC (formerly CRPP) in 1993/1994 and over the second generation developed in 2012/2013. While the new amplifiers have been operated successfully for over a year, it was found that their silicon power transistors can be damaged during some off-normal plasma events. Possible solutions are discussed. (C) 2019 Author(s)
Real-time plasma state monitoring and supervisory control on TCV
In ITER and DEMO, various control objectives related to plasma control must be simultaneously achieved by the plasma control system (PCS), in both normal operation as well as off-normal conditions. The PCS must act on off-normal events and deviations from the target scenario, since certain sequences (chains) of events can precede disruptions. It is important that these decisions are made while maintaining a coherent prioritization between the real-time control tasks to ensure high-performance operation. In this paper, a generic architecture for task-based integrated plasma control is proposed. The architecture is characterized by the separation of state estimation, event detection, decisions and task execution among different algorithms, with standardized signal interfaces. Central to the architecture are a plasma state monitor and supervisory controller. In the plasma state monitor, discrete events in the continuous-valued plasma state are modeled using finite state machines. This provides a high-level representation of the plasma state. The supervisory controller coordinates the execution of multiple plasma control tasks by assigning task priorities, based on the finite states of the plasma and the pulse schedule. These algorithms were implemented on the TCV digital control system and integrated with actuator resource management and existing state estimation algorithms and controllers. The plasma state monitor on TCV can track a multitude of plasma events, related to plasma current, rotating and locked neoclassical tearing modes, and position displacements. In TCV experiments on simultaneous control of plasma pressure, safety factor profile and NTMs using electron cyclotron heating (ECH) and current drive (ECCD), the supervisory controller assigns priorities to the relevant control tasks. The tasks are then executed by feedback controllers and actuator allocation management. This work forms a significant step forward in the ongoing integration of control capabilities in experiments on TCV, in support of tokamak reactor operation