K. Harrabi

Decoherence of Flux Qubits due to 1 / f Flux Noise

We have investigated decoherence in Josephson-junction flux qubits. Based on the measurements of decoherence at various bias conditions, we discriminate contributions of different noise sources. We present a Gaussian decay function extracted from the echo signal as evidence of dephasing due to 1/f flux noise whose spectral density is evaluated to be about (10(-6)Phi0)2/Hz at 1 Hz. We also demonstrate that, at an optimal bias condition where the noise sources are well decoupled, the coherence observed in the echo measurement is limited mainly by energy relaxation of the qubit.

Recent Advances in Dye Sensitized Solar Cells

Solar energy is an abundant and accessible source of renewable energy available on earth, and many types of photovoltaic (PV) devices like organic, inorganic, and hybrid cells have been developed to harness the energy. PV cells directly convert solar radiation into electricity without affecting the environment. Although silicon based solar cells (inorganic cells) are widely used because of their high efficiency, they are rigid and manufacturing costs are high. Researchers have focused on organic solar cells to overcome these disadvantages. DSSCs comprise a sensitized semiconductor (photoelectrode) and a catalytic electrode (counter electrode) with an electrolyte sandwiched between them and their efficiency depends on many factors. The maximum electrical conversion efficiency of DSSCs attained so far is 11.1%, which is still low for commercial applications. This review examines the working principle, factors affecting the efficiency, and key challenges facing DSSCs.

Counting Individual Trapped Electrons on Liquid Helium

We show that small numbers of electrons, including a single isolated electron, can be held in an electrostatic trap above the surface of superfluid helium. A potential well is created using microfabricated electrodes in a 5 μm diameter pool of helium. Electrons are injected into the trap from an electron reservoir on a helium microchannel. They are individually detected using a superconducting single-electron transistor as an electrometer. A Coulomb staircase is observed as electrons leave the trap one–by–one until the trap is empty. A design for a scalable quantum information processor using an array of electron traps is presented.

Microwave saturation of the Rydberg States of electrons on helium.

We present measurements of the resonant microwave excitation of Rydberg energy levels for surface-state electrons on superfluid helium. The temperature-dependent contribution to the linewidth gamma(T) agrees with theoretical predictions and is very small below 700 mK, in the ripplon scattering regime. Absorption saturation and power broadening were observed as the fraction of electrons in the first excited state was increased to 0.49, close to the thermal excitation limit of 0.5. The Rabi frequency Omega was determined as a function of microwave power. High values of the ratio Omega/gamma confirm this system as an excellent candidate for creating qubits.

Interqubit coupling mediated by a high-excitation-energy quantum object

We consider a system composed of two qubits and a high excitation energy quantum object used to mediate coupling between the qubits. We treat the entire system quantum mechanically and analyze the properties of the eigenvalues and eigenstates of the total Hamiltonian. After reproducing well known results concerning the leading term in the mediated coupling, we obtain an expression for the residual coupling between the qubits in the off state. We also analyze the entanglement between the three objects, i.e., the two qubits and the coupler, in the eigenstates of the total Hamiltonian. Although we focus on the application of our results to the recently realized parametric-coupling scheme with two qubits, we also discuss extensions of our results to harmonicoscillator couplers, couplers that are near resonance with the qubits and multiqubit systems. In particular, we find that certain errors that are absent for a two-qubit system arise when dealing with multiqubit systems.

Density functional theory study on dye-sensitized solar cells using oxadiazole-based dyes

Abstract. Density functional theory (DFT) and time-dependent DFT(TD-DFT) modeling techniques are used to conduct a computational study of the geometry and electronic structure of oxadiazole-based organic sensitizers. A DFT study on the thermodynamic aspects of the charge transport processes associated with dye-sensitized solar cells (DSSCs) suggests that the system with 1,2,4-oxadiazole has a balance among the different crucial factors and may result in the highest incident photon to charge carrier efficiency. The dye/(TiO2)8 anatase clusters were also simulated to illustrate the electron injection efficiency at the interface. This study provides basic understanding of the impact of molecular design on the performance of oxadiazole dyes in DSSCs.

Enhanced Photovoltaic Performance of Dye-Sensitized Solar Cells Using TiO 2 -Graphene Microplatelets Hybrid Photoanode

Hybrid photoanodes for dye-sensitized solar cells (DSSCs) were prepared by simple addition of graphene (GR) microplatelets to TiO2 nanoparticulate paste. Transmission electron microscopy was used to confirm the presence of GR in composite films after heating at 450°C for 30 min. TiO2/GR-based DSSCs were fabricated using an N749 photosensitizer. The UV-Visible absorption spectroscopy, photocurrent-voltage (I-V) characteristic, and electrochemical impedance spectroscopy measurements were carried out to characterize the cells. The results indicate that the GR/TiO2 photoanode improves the performance of the solar cell. This is because the GR/titania electrode accelerates electronic transportation and suppresses the charge recombination. Under optimal conditions, the solar cell based on GR/TiO2 shows power conversion efficiency (PCE) of 4.1%, which is about 30% greater than the cell based on the pristine TiO2 electrode (3.16%). The objective of this study is to develop a fast, cheap, and an effective means to increase the PCE of DSSCs. Density functional theory was used to calculate the bandgap of TiO2 and graphnene-TiO2.

Density Functional Theory Study on the Electronic Structures of Oxadiazole Based Dyes as Photosensitizer for Dye Sensitized Solar Cells

The molecular structures and UV-visible absorption spectra of complex photosensitizers comprising oxadiazole isomers as the π-bridges were analyzed by density functional theory (DFT) and time-dependent DFT. The ground state and excited state oxidation potentials, HOMOs and LUMOs energy levels, and electron injection from the dyes to semiconductor TiO2 have been computed in vacuum here. The results show that all of the dyes may potentially be good photosensitizers in DSSC. To justify the simulation basis, N3 dye was also simulated under the similar conditions. Simulated absorption spectrum, HOMO, LUMO, and band gap values of N3 were compared with the experimental values. We also computed the electronic structure properties and absorption spectra of dye/(TiO2)8 systems to elucidate the electron injection efficiency at the interface. This work is expected to give proper orientation for experimental synthesis.

Driven Dynamics and Rotary Echo of a Qubit Tunably Coupled to a Harmonic Oscillator

We have investigated the driven dynamics of a superconducting flux qubit that is tunably coupled to a microwave resonator. We find that the qubit experiences an oscillating field mediated by off-resonant driving of the resonator, leading to strong modifications of the qubit Rabi frequency. This opens an additional noise channel, and we find that low-frequency noise in the coupling parameter causes a reduction of the coherence time during driven evolution. The noise can be mitigated with the rotary-echo pulse sequence, which, for driven systems, is analogous to the Hahn-echo sequence.

Effect of pinning on the response of superconducting strips to an external pulsed current

Using the anisotropic time-dependent Ginzburg–Landau theory we study the effect of ordered and disordered pinning on the time response of superconducting strips to an external current that switched on abruptly. The pinning centers result in a considerable delay of the response time of the system to such abrupt switching on of the current, whereas the output voltage is always larger when pinning is present. The resistive state in both cases are characterized either by dynamically stable phase-slip centers/lines or expanding in-time hot-spots, which are the main mechanisms for dissipation in current-carrying superconductors. We find that hot-spots are always initiated by the phase-slip state. However, the range of the applied current for the phase-slip state increases significantly when pinning is introduced. Qualitative changes are observed in the dynamics of the superconducting condensate in the presence of pinning.