J. Bergli

Direct generation of charge carriers in c-Si solar cells due to embedded nanoparticles

It is known that silicon is an indirect band gap material, reducing its efficiency in photovoltaic applications. Using surface plasmons in metallic nanoparticles embedded in a solar cell has recently been proposed as a way to increase the efficiency of thin-film silicon solar cells. The dipole mode that dominates the plasmons in small particles produces an electric field having Fourier components with all wave numbers. In this work, we show that such a field creates electron-hole-pairs without phonon assistance, and discuss the importance of this effect compared to radiation from the particle and losses due to heating.

Non-Gaussian low-frequency noise as a source of qubit decoherence.

With the growing efforts in isolating solid-state qubits from external decoherence sources, the material-inherent sources of noise start to play crucial role. One representative example is electron traps in the device material or substrate. Electrons can tunnel or hop between a charged and an empty trap, or between a trap and a gate electrode. A single trap typically produces telegraph noise and can hence be modeled as a bistable fluctuator. Since the distribution of hopping rates is exponentially broad, many traps produce flicker-noise with spectrum close to 1/f. Here we develop a theory of decoherence of a qubit in the environment consisting of two-state fluctuators, which experience transitions between their states induced by interaction with thermal bath. Due to interaction with the qubit the fluctuators produce 1/f-noise in the qubits eigenfrequency. We calculate the results of qubit manipulations - free induction and echo signals - in such environment. The main problem is that in many important cases the relevant random process is both non-Markovian and non-Gaussian. Consequently the results in general cannot be represented by pair correlation function of the qubit eigenfrequency fluctuations. Our calculations are based on analysis of the density matrix of the qubit using methods developed for stochastic differential equations. The proper generating functional is then averaged over different fluctuators using the so-called Holtsmark procedure. The analytical results are compared with simulations allowing checking accuracy of the averaging procedure and evaluating mesoscopic fluctuations. The results allow understanding some observed features of the echo decay in Josephson qubits.We study decoherence in a qubit with the distance between the two levels affected by random flips of bistable fluctuators. For the case of a single fluctuator we evaluate explicitly an exact expression for the phase-memory decay in the echo experiment with a resonant ac excitation. The echo signal as a function of time shows a sequence of plateaus. The position and the height of the plateaus can be used to extract the fluctuator switching rate gamma and its coupling strength v. At small times the logarithm of the echo signal is proportional to t3. The plateaus disappear when the decoherence is induced by many fluctuators. In this case the echo signal depends on the distribution of the fluctuators parameters. According to our analysis, the results significantly deviate from those obtained in the Gaussian model as soon as v greater than or approximately equal gamma.

Decoherence in qubits due to low-frequency noise

The efficiency of the future devices for quantum information processing will be limited mostly by the finite decoherence rates of the qubits. Recently, substantial progress was achieved in enhancing the time within which a solid-state qubit demonstrates coherent dynamics. This progress is based mostly on a successful isolation of the qubits from external decoherence sources. Under these conditions, the material-inherent sources of noise start to play a crucial role. In most cases, the noise that the quantum device demonstrates has a 1/f spectrum. This suggests that the environment that destroys the phase coherence of the qubit can be thought of as a system of two-state fluctuators, which experience random hops between their states. In this short review, the current state of the theory of the decoherence due to the qubit interaction with the fluctuators is discussed. The effect of such an environment on two different protocols of the qubit manipulations, free induction and echo signal, is described. It turns out that in many important cases the noise produced by the fluctuators is non-Gaussian. Consequently, the results of the interaction of the qubit with the fluctuators are not determined by the pair correlation function alone. We describe the effect of the fluctuators using the so-called spin-fluctuator model. Being quite realistic, this model allows one to exactly evaluate the qubit dynamics in the presence of one fluctuator. This solution is found, and its 5 Author to whom any correspondence should be addressed.

Dephasing and dissipation in qubit thermodynamics

We analyze the stochastic evolution and dephasing of a qubit within the quantum jump approach. It allows one to treat individual realizations of inelastic processes, and in this way it provides solutions, for instance, to problems in quantum thermodynamics and distributions in statistical mechanics. We demonstrate that dephasing and relaxation of the qubit render the Jarzynski and Crooks fluctuation relations (FRs) of nonequilibrium thermodynamics intact. On the contrary, the standard two-measurement protocol, taking into account only the fluctuations of the internal energy U, leads to deviations in FRs under the same conditions. We relate the average 〈e(-βU)〉 (where β is the inverse temperature) with the qubits relaxation and dephasing rates in the weak dissipation limit and discuss this relationship for different mechanisms of decoherence.

Decoherence of a qubit due to either a quantum fluctuator, or classical telegraph noise

We investigate the decoherence of a qubit coupled to either a quantum two-level system (TLS) again coupled to an environment, or a classical fluctuator modeled by random telegraph noise. In order to do this we construct a model for the quantum TLS where we can adjust the temperature of its environment, and the decoherence rate independently. The model has a well-defined classical limit at any temperature and this corresponds to the appropriate random telegraph process, which is symmetric at high temperatures and becomes asymmetric at low temperatures. We find that the difference in the qubit decoherence rates predicted by the two models depends on the ratio between the qubit-TLS coupling and the decoherence rate in the pointer basis of the TLS. This is then the relevant parameter which determines whether the TLS has to be treated quantum mechanically or can be replaced by a classical telegraph process. We also compare the mutual information between the qubit and the TLS in the classical and quantum cases.

Decoherence of a qubit by non-Gaussian noise at an arbitrary working point

The decoherence of a qubit due to a classical non-Gaussian noise with correlation time longer than the decoherence time is discussed for arbitrary working points of the qubit. A method is developed that allows an exact formula for the phase-memory functional in the presence of independent random telegraph noise sources to be derived.

Exact solution for the dynamical decoupling of a qubit with telegraph noise

We study the dissipative dynamics of a qubit that is afflicted by classical random telegraph noise and is subject to dynamical decoupling. We derive exact formulas for the qubit dynamics at arbitrary working points in the limit of infinitely strong control pulses (bang-bang control), and we investigate in great detail the efficiency of the dynamical decoupling techniques for both Gaussian and non-Gaussian (slow) noise at qubit pure dephasing and at optimal point. We demonstrate that control sequences can be successfully implemented as diagnostic tools to infer spectral properties of a few fluctuators interacting with the qubit. The analysis is extended in order to include the effect of noise in the pulses, and we give upper bounds on the noise levels that can be tolerated in the pulses while still achieving an efficient dynamical decoupling performance.

Non-Gaussian dephasing in flux qubits due to 1/f noise

A remarkable work 1 has appeared recently, reporting on measurements of the two-pulse echo signal in Josephsonjunction flux qubits revealing its dependence of the time interval between the pulses. The authors interpreted their results in terms of the recent theory 2 of the decoherence in qubits caused by 1/f noise and obtained the qubit dephasing rate, , based on the postulation of the Gaussian statistics of the noise. Although this looks like a natural starting point, the importance of the echo signal data for understanding the underlying mechanisms of the qubits, decoherence calls for careful examination of the assumptions built into the theoretical description. In this paper we develop a theory of the time dependence of the echo signal making use of an exactly solvable but experimentally realistic model. We demonstrate that in many practical realizations of 1/f noise the results based on the Gaussian assumption need to be significantly corrected. We show that deviation of noise statistics from the Gaussian changes significantly the time dependence of the echo signal. Using the exactly solvable non-Gaussian spin-fluctuator model for the 1/f noise we analyze the dependence of the echo signal on both time and the qubit working point, and compare the obtained results with those derived within the Gaussian approximation. Let us start with a brief review of the procedure used in Ref. 1 and a similar study reported in Ref. 3. In order to separate the relaxation due to direct transitions between the energy levels of the qubit, characterized by the time T1, the

Is weak temperature dependence of electron dephasing possible

The first-principle theory of electron dephasing by disorder-induced two state fluctuators is developed. There exist two mechanisms of dephasing. First, dephasing occurs due to direct transitions between the defect levels caused by inelastic electron-defect scattering. The second mechanism is due to violation of the time reversal symmetry caused by time-dependent fluctuations of the scattering potential. These fluctuations originate from an interaction between the dynamic defects and conduction electrons forming a thermal bath. The first contribution to the dephasing rate saturates as temperature decreases. The second contribution does not saturate, although its temperature dependence is rather weak,

Possible weak temperature dependence of electron dephasing

\propto T^{1/3}