Nicolas Roch

Coupling a quantum dot, fermionic leads, and a microwave cavity on a chip.

We demonstrate a hybrid architecture consisting of a quantum dot circuit coupled to a single mode of the electromagnetic field. We use single wall carbon nanotube based circuits inserted in superconducting microwave cavities. By probing the nanotube dot using a dispersive readout in the Coulomb blockade and the Kondo regime, we determine an electron-photon coupling strength which should enable circuit QED experiments with more complex quantum dot circuits.

Generating Entangled Microwave Radiation Over Two Transmission Lines

Using a superconducting circuit, the Josephson mixer, we demonstrate the first experimental realization of spatially separated two-mode squeezed states of microwave light. Driven by a pump tone, a first Josephson mixer generates, out of quantum vacuum, a pair of entangled fields at different frequencies on separate transmission lines. A second mixer, driven by a π-phase shifted copy of the first pump tone, recombines and disentangles the two fields. The resulting output noise level is measured to be lower than for the vacuum state at the input of the second mixer, an unambiguous proof of entanglement. Moreover, the output noise level provides a direct, quantitative measure of entanglement, leading here to the demonstration of 6 Mebit · s(-1) (mega entangled bits per second) generated by the first mixer.

Superconductivity in a single-C60 transistor

Single-molecule transistors have enabled studies of magnetism and other correlated nanoscale behaviour, but superconductivity has not been observed with this approach. It is now shown that superconducting junctions on both sides of a C60 molecule induce superconductivity across the whole device.

Widely Tunable, Nondegenerate Three-Wave Mixing Microwave Device Operating near the Quantum Limit

We present the first experimental realization of a widely frequency tunable, nondegenerate three-wave mixing device for quantum signals at gigahertz frequency. It is based on a new superconducting building block consisting of a ring of four Josephson junctions shunted by a cross of four linear inductances. The phase configuration of the ring remains unique over a wide range of magnetic fluxes threading the loop. It is thus possible to vary the inductance of the ring with flux while retaining a strong, dissipation-free, and noiseless nonlinearity. The device has been operated in amplifier mode, and its noise performance has been evaluated by using the noise spectrum emitted by a voltage-biased tunnel junction at finite frequency as a test signal. The unprecedented accuracy with which the crossover between zero-point fluctuations and shot noise has been measured provides an upper bound for the noise and dissipation intrinsic to the device.

Persistent control of a superconducting qubit by stroboscopic measurement feedback

Making a system state follow a prescribed trajectory despite fluctuations and errors commonly consists of monitoring an observable (temperature, blood-glucose level, etc.) and reacting on its controllers (heater power, insulin amount, etc.). In the quantum domain, there is a change of paradigm in feedback, since measurements modify the state of the system, most dramatically when the trajectory goes through superpositions of measurement eigenstates. Here, we demonstrate the stabilization of an arbitrary trajectory of a superconducting qubit by measurement-based feedback. The protocol benefits from the long coherence time (T2 > 10 � s) of the 3D transmon qubit, the high efficiency (82%) of the phasepreserving Josephson amplifier, and fast electronics that ensure less than 500 ns total delay. At discrete time intervals, the state of the qubit is measured and corrected in case an error is detected. For Rabi oscillations, where the discrete measurements occur when the qubit is supposed to be in the measurement pointer states, we demonstrate an average fidelity of 85% to the targeted trajectory. For Ramsey oscillations, which do not go through pointer states, the average fidelity reaches 76%. Incidentally, we demonstrate a fast reset protocol that allows us to cool a 3D transmon qubit down to 0:6% in the excited state.

Observation of the Underscreened Kondo Effect in a Molecular Transistor

We present the first quantitative experimental evidence for the underscreened Kondo effect, an incomplete compensation of a quantized magnetic moment by conduction electrons, as originally proposed by Nozières and Blandin. The device consists of an even charge spin S=1 molecular quantum dot, obtained by electromigration of C60 molecules into gold nanogaps and operated in a dilution fridge. The persistence of logarithmic singularities in the low temperature conductance is demonstrated by a comparison to the fully screened configuration obtained in odd charge spin S=1/2 Coulomb diamonds. We also discover an extreme sensitivity of the underscreened Kondo resonance to the magnetic field that we confirm on the basis of numerical renormalization group calculations.

Cotunneling through a magnetic single-molecule transistor based on N@C-60

We present an experimental and theoretical study of a magnetic single-molecule transistor based on

Stabilizing Spin Coherence Through Environmental Entanglement in Strongly Dissipative Quantum Systems

\mathrm{N}@{\mathrm{C}}_{60}

V-shaped superconducting artificial atom based on two inductively coupled transmons

connected to gold electrodes. Particular attention is paid to the regime of intermediate molecule-lead coupling, where cotunneling effects manifest themselves in the Coulomb-blockade regime. The experimental results for the differential conductance as a function of bias, gate voltage, and external magnetic field are in agreement with our analysis of the tunneling rates and provide evidence of magnetic signatures in single-

Kondo effects in a C60 single-molecule transistor

\mathrm{N}@{\mathrm{C}}_{60}