Julien Basset

Microwave Emission from Hybridized States in a Semiconductor Charge Qubit

We explore the microwave radiation emitted from a biased double quantum dot due to the inelastic tunneling of single charges. Radiation is detected over a broad range of detuning configurations between the dot energy levels, with pronounced maxima occurring in resonance with a capacitively coupled transmission line resonator. The power emitted for forward and reverse resonant detuning is found to be in good agreement with a rate equation model, which considers the hybridization of the individual dot charge states.

Measurement of Quantum Noise in a Carbon Nanotube Quantum Dot in the Kondo Regime

The current emission noise of a carbon nanotube quantum dot in the Kondo regime is measured at frequencies ν of the order or higher than the frequency associated with the Kondo effect k(B)T (K)/h, with TK the Kondo temperature. The carbon nanotube is coupled via an on-chip resonant circuit to a quantum noise detector, a superconductor-insulator-superconductor junction. We find for hν ≈ k(B)T(K) a Kondo effect related singularity at a voltage bias eV ≈ hν, and a strong reduction of this singularity for hν ≈ 3k(B)T(K), in good agreement with theory. Our experiment constitutes a new original tool for the investigation of the nonequilibrium dynamics of many-body phenomena in nanoscale devices.

Single-electron double quantum dot dipole-coupled to a single photonic mode

We have realized a hybrid solid-state quantum device in which a single-electron semiconductor double quantum dot is dipole coupled to a superconducting microwave frequency transmission line resonator. The dipolar interaction between the two entities manifests itself via dispersive and dissipative effects observed as frequency shifts and linewidth broadenings of the photonic mode respectively. A Jaynes-Cummings Hamiltonian master equation calculation is used to model the combined system response and allows for determining both the coherence properties of the double quantum dot and its interdot tunnel coupling with high accuracy. The value and uncertainty of the tunnel coupling extracted from the microwave read-out technique are compared to a standard quantum point contact charge detection analysis. The two techniques are found to be consistent with a superior precision for the microwave experiment when tunneling rates approach the resonator eigenfrequency. Decoherence properties of the double dot are further investigated as a function of the number of electrons inside the dots. They are found to be similar in the single-electron and many-electron regimes suggesting that the density of the confinement energy spectrum plays a minor role in the decoherence rate of the system under investigation.

Emission and absorption quantum noise measurement with an on-chip resonant circuit.

Using a quantum detector, a superconductor-insulator-superconductor junction, we probe separately the emission and absorption noise in the quantum regime of a superconducting resonant circuit at equilibrium. At low temperature the resonant circuit exhibits only absorption noise related to zero point fluctuations, whereas at higher temperature emission noise is also present. By coupling a Josephson junction, biased above the superconducting gap, to the same resonant circuit, we directly measure the noise power of quasiparticles tunneling through the junction at two resonance frequencies. It exhibits a strong frequency dependence, consistent with theoretical predictions.

Evaluating charge noise acting on semiconductor quantum dots in the circuit quantum electrodynamics architecture

We evaluate the charge noise acting on a GaAs/GaAlAs based semiconductor double quantum dot dipole-coupled to the voltage oscillations of a superconducting transmission line resonator. The in-phase (I) and the quadrature (Q) components of the microwave tone transmitted through the resonator are sensitive to charging events in the surrounding environment of the double dot with an optimum sensitivity of 8.5×10−5 e/Hz. A low frequency 1/f type noise spectrum combined with a white noise level of 6.6×10−6 e2/Hz above 1 Hz is extracted, consistent with previous results obtained with quantum point contact charge detectors on similar heterostructures. The slope of the 1/f noise allows to extract a lower bound for the double-dot charge qubit dephasing rate which we compare to the one extracted from a Jaynes-Cummings Hamiltonian approach. The two rates are found to be similar emphasizing that charge noise is the main source of dephasing in our system.

Current-Induced Reorientation of Exchange Bias on a Nanoscale

We demonstrate how local heating by an electric current can induce and reorient the exchange bias on a nanoscale. In our experiments we use point contacts ~10 nm in size to inject current densities as high as 10 into F/N/F/AFM exchange-biased spin valves (EBSV) where two ferromagnetic (F) layers are separated by a nonmagnetic (N) metal spacer and one of the Fs is biased by an adjacent antiferromagnetic (AFM) layer. At low currents the spin valves exhibit the usual giant magnetoresistance (GMR) when two F layers switch from parallel to antiparallel orientation. At highest applied currents the Joule heating in the contact becomes significant and in combination with static magnetic field can induce and repeatedly reorient the exchange bias in a small contact volume . The strength of exchange bias induced in the point contact was found to depend on the polarity of the applied current. We tentatively attribute this polarity dependence to spin-transfer torques arising near F/AFM interface at high currents.

Measuring the Degeneracy of Discrete Energy Levels Using a GaAs/AlGaAs Quantum Dot

We demonstrate an experimental method for measuring quantum state degeneracies in bound state energy spectra. The technique is based on the general principle of detailed balance and the ability to perform precise and efficient measurements of energy-dependent tunneling-in and -out rates from a reservoir. The method is realized using a GaAs/AlGaAs quantum dot allowing for the detection of time-resolved single-electron tunneling with a precision enhanced by a feedback control. It is thoroughly tested by tuning orbital and spin degeneracies with electric and magnetic fields. The technique also lends itself to studying the connection between the ground-state degeneracy and the lifetime of the excited states.

Equilibrium free energy measurement of a confined electron driven out of equilibrium

We study out-of equilibrium properties of a quantum dot in a GaAs/AlGaAs two-dimensional electron gas. By means of single electron counting experiments, we measure the distribution of work and dissipated heat of the driven quantum dot and relate these quantities to the equilibrium free energy change, as it has been proposed by C. Jarzynski [Phys. Rev. Lett. {\bf78}, 2690 (1997)]. We discuss the influence of the degeneracy of the quantized energy state on the free energy change as well as its relation to the tunnel rates between the dot and the reservoir.

High-frequency quantum admittance and noise measurement with an on-chip resonant circuit

By coupling a quantum detector, a superconductor-insulator-superconductor junction, to a Josephson junction \textit{via} a resonant circuit we probe the high frequency properties, namely the ac complex admittance and the current fluctuations of the Josephson junction at the resonant frequencies. The admittance components show frequency dependent singularities related to the superconducting density of state while the noise exhibits a strong frequency dependence, consistent with theoretical predictions. The circuit also allows to probe separately the emission and absorption noise in the quantum regime of the superconducting resonant circuit at equilibrium. At low temperature the resonant circuit exhibits only absorption noise related to zero point fluctuations, whereas at higher temperature emission noise is also present.

Spin-transfer interactions in exchange-biased spin valves

We extend to a new antiferromagnetic alloy, IrMn, point-contact based studies of the effect of a large current density on the exchange bias at antiferromagnet/ferromagnet (AFM/F) interfaces. Similarly to the case of AFM=CoFe, a negative current density ∼1012A∕m2 injected through the F=CoFe into an IrMn∕CoFe interface was found to increase the exchange bias, while a positive current decreased it. The model used to describe the data for FeMn∕CoFe, based on predicted current-induced torques on AFM, can explain the new data.