Search results for "Phase qubit"

Quantifying, characterizing, and controlling information flow in ultracold atomic gases

2011

We study quantum information flow in a model comprising of an impurity qubit immersed in a Bose-Einstein condensed reservoir. We demonstrate how information flux between the qubit and the condensate can be manipulated by engineering the ultracold reservoir within experimentally realistic limits. We place a particular emphasis on non-Markovian dynamics, characterized by a reversed flow of information from the background gas to the qubit and identify a controllable crossover between Markovian and non-Markovian dynamics in the parameter space of the model.

Efficient protocol for qubit initialization with a tunable environment

2017

We propose an efficient qubit initialization protocol based on a dissipative environment that can be dynamically adjusted. Here the qubit is coupled to a thermal bath through a tunable harmonic oscillator. On-demand initialization is achieved by sweeping the oscillator rapidly into resonance with the qubit. This resonant coupling with the engineered environment induces fast relaxation to the ground state of the system, and a consecutive rapid sweep back to off resonance guarantees weak excess dissipation during quantum computations. We solve the corresponding quantum dynamics using a Markovian master equation for the reduced density operator of the qubit-bath system. This allows us to optim…

Interaction-free measurements with superconducting qubits

2008

An interaction-free measurement protocol is described for a quantum circuit consisting of a superconducting qubit and a read-out Josephson junction. By measuring the state of the qubit one can ascertain the presence of a current pulse through the circuit at a previous time without any energy exchange between the qubit and the pulse.

Electromagnetically induced tunnelling suppression in a flux qubit

2003

Motivated by recent experiments wherein Josephson devices are irradiated by microwaves fields or are coupled to LC resonators, we theoretically investigate the dynamics of a flux qubit coupled to a monochromatic bosonic mode. We define strong coupling conditions under which the qubit tunnelling frequency between the localized flux states can be controlled and even suppressed. The practical realization of such a regime leading to this hindered dynamics is discussed.

Quantum information processing on spin degrees of freedom in QDs placed in diluted magnetic semiconductor

2006

The spin degrees of freedom in quantum dot (QD) embedded in a diluted magnetic semiconductor (DMS) medium are considered in a model of a qubit and a gate for quantum information processing (QIP). The qubit is defined as a singlet and triplet pair of states of two electrons in a He-type QD in the DMS medium with strongly enhanced gyromagnetic factor. Methods of qubit rotation (Rabi oscillations) as well as two-qubit operations are suggested and analyzed. Moreover, decoherence related to spin waves (magnon-induced dephasing) in this new system (QD in DMS) is studied, and the relevant time-scale is estimated in accordance with preliminary experimental results. (© 2006 WILEY-VCH Verlag GmbH & C…

Measuring charge based quantum bits by a superconducting single-electron transistor

2002

Single-electron transistors have been proposed to be used as a read-out device for Cooper pair charge qubits. Here we show that a coupled superconducting transistor at a threshold voltage is much more effective in measuring the state of a qubit than a normal-metal transistor at the same voltage range. The effect of the superconducting gap is to completely block the current through the transistor when the qubit is in the logical state 1, compared to the mere diminishment of the current in the normal-metal case. The time evolution of the system is solved when the measuring device is driven out of equilibrium and the setting is analysed numerically for parameters accessible by lithographic alu…

Simultaneous readout of two charge qubits

2006

We consider a system of two solid state charge qubits, coupled to a single read-out device, consisting of a single-electron transistor (SET). The conductance of each tunnel junction is influenced by its neighboring qubit, and thus the current through the transistor is determined by the qubits' state. The full counting statistics of the electrons passing the transistor is calculated, and we discuss qubit dephasing, as well as the quantum efficiency of the readout. The current measurement is then compared to readout using real-time detection of the SET island's charge state. For the latter method we show that the quantum efficiency is always unity. Comparing the two methods a simple geometric…

Freezing the dynamics of a rf SQUID qubit via its strong coupling to a quantized microwave field

2004

In this paper we show the results concerning the study of the dynamics of a rf SQUID qubit exposed to a quantized monochromatic microwave source in the strong coupling limit. We bring out more details of the possibility both of controlling and hindering the oscillations between the two qubit flux states when we consider opportunely prepared initial field states. The importance of conceiving of such kinds of theoretical schemes in view of possible applications in the context of quantum computing is briefly discussed.

Model of Qubit in Multi-Electron Quantum Dot

2001

Resonant effects in a SQUID qubit subjected to nonadiabatic changes

2013

By quickly modifying the shape of the effective potential of a double SQUID flux qubit from a single-well to a double-well condition, we experimentally observe an anomalous behavior, namely an alternance of resonance peaks, in the probability to find the qubit in a given flux state. The occurrence of Landau-Zener transitions as well as resonant tunneling between degenerate levels in the two wells may be invoked to partially justify the experimental results. A quantum simulation of the time evolution of the system indeed suggests that the observed anomalous behavior can be imputable to quantum coherence effects. The interplay among all these mechanisms has a practical implication for quantum…