Audrey Cottet

Manipulating the Quantum State of an Electrical Circuit

We have designed and operated a superconducting tunnel junction circuit that behaves as a two-level atom: the “quantronium.” An arbitrary evolution of its quantum state can be programmed with a series of microwave pulses, and a projective measurement of the state can be performed by a pulsed readout subcircuit. The measured quality factor of quantum coherenceQ ϕ ≅ 25,000 is sufficiently high that a solid-state quantum processor based on this type of circuit can be envisioned.

Positive Cross Correlations in a Three-Terminal Quantum Dot with Ferromagnetic Contacts

We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads, for instance, to positive zero-frequency cross correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect with respect to polarization of the contacts and tunneling rates.

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.

Nanospintronics with carbon nanotubes

One of the actual challenges of spintronics is the realization of a spin transistor allowing control of spin transport through an electrostatic gate. In this paper, we report on different experiments which demonstrate gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyse spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts.

Implementation of a combined charge-phase quantum bit in a superconducting circuit

We discuss a qubit circuit based on the single Cooper-pair transistor (which consists of two ultrasmall Josephson junctions in series) connected in parallel with a large Josephson junction. The switching of this junction out of its zero-voltage state is used to readout the qubit. We report measurements of the discriminating power of the readout process, and we discuss its back-action on the qubit.

Positive cross-correlations due to dynamical channel blockade in a three-terminal quantum dot

We investigate current fluctuations in a three-terminal quantum dot in the sequential tunneling regime. In the voltage-bias configuration chosen here, the circuit is operated as a beam splitter, i.e., one lead is used as an input and the other two as outputs. In the limit where a double occupancy of the dot is not possible, a super-Poissonian Fano factor of the current in the input lead and positive cross correlations between the current fluctuations in the two output leads can be obtained, due to dynamical channel blockade. When a single orbital of the dot transports current, these effects can be obtained by lifting the spin degeneracy of the circuit with ferromagnetic leads or with a magnetic field. When several orbitals participate in the electronic conduction, lifting spin degeneracy is not necessary. In all cases, we show that a super-Poissonian Fano factor for the input current is not equivalent to positive cross correlations between the outputs. We identify the conditions for obtaining these two effects and discuss possible experimental realizations.

Out-of-equilibrium charge dynamics in a hybrid circuit quantum electrodynamics architecture

(Dated: December 23, 2013)The recent development of hybrid cQED allows one to study how cavity photons interact witha system driven out of equilibrium by fermionic reservoirs. We study here one of the simplestcombination : a double quantum dot coupled to a single mode of the electromagnetic eld. Weare able to couple resonantly the charge levels of a carbon nanotube based double dot to cavityphotons. We perform a microwave read out of the charge states of this system which allows us tounveil features of the out of equilibrium charge dynamics, otherwise invisible in the DC current.We extract relaxation rate, dephasing rate and photon number of the hybrid system using a theorybased on a master equation technique. These ndings open the path for manipulating other degreesof freedom e.g. the spin and/or the valley in nanotube based double dots using microwave light.

Photon-mediated interaction between distant quantum dot circuits.

Engineering the interaction between light and matter is an important goal in the emerging field of quantum opto-electronics. Thanks to the use of cavity quantum electrodynamics architectures, one can envision a fully hybrid multiplexing of quantum conductors. Here we use such an architecture to couple two quantum dot circuits. Our quantum dots are separated by 200 times their own size, with no direct tunnel and electrostatic couplings between them. We demonstrate their interaction, mediated by the cavity photons. This could be used to scale up quantum bit architectures based on quantum dot circuits or simulate on-chip phonon-mediated interactions between strongly correlated electrons.

Spin quantum bit with ferromagnetic contacts for circuit QED.

We theoretically propose a scheme for a spin quantum bit based on a double quantum dot contacted to ferromagnetic elements. Interface exchange effects enable an all electric manipulation of the spin and a switchable strong coupling to a superconducting coplanar waveguide cavity. Our setup does not rely on any specific band structure and can in principle be realized with many different types of nanoconductors. This allows us to envision on-chip single spin manipulation and readout using cavity QED techniques.

General boundary conditions for quasiclassical theory of superconductivity in the diffusive limit: Application to strongly spin-polarized systems

Boundary conditions in quasiclassical theory of superconductivity are of crucial importance for describing proximity effects in heterostructures between different materials. Although they have been derived for the ballistic case in full generality, corresponding boundary conditions for the diffusive limit, described by Usadel theory, have been lacking for interfaces involving strongly spin-polarized materials, e.g. half-metallic ferromagnets. Given the current intense research in the emerging field of superconducting spintronics, the formulation of appropriate boundary conditions for the Usadel theory of diffusive superconductors in contact with strongly spin-polarized ferromagnets for arbitrary transmission probability and arbitrary spin-dependent interface scattering phases has been a burning open question. Here we close this gap and derive the full boundary conditions for quasiclassical Green functions in the diffusive limit, valid for any value of spin polarization, transmission probability, and spin-mixing angles (spin-dependent scattering phase shifts). Our formulation allows also for complex spin textures across the interface and for channel off-diagonal scattering (a necessary ingredient when the numbers of channels on the two sides of the interface differ). As an example we derive expressions for the proximity effect in diffusive systems involving half-metallic ferromagnets. In a superconductor/ half-metal/superconductor Josephson junctionwe find ϕ0-junction behavior under certain interface conditions.