Current research projects
We study electronic transport in quantum matter systems and especially the dynamics of such systems. Below you can find the projects we are currently working on. If you are interested in joining any of these projects as an intern, PhD or postdoc, please don't hesitate to contact the indicated contact person(s).

Quantum electrodynamics of conductors
At very low temperatures, we have shown that fluctuations in the electrical current across a tunnel junction result in the emission of quantum-correlated photons in the GHz range. We are now studying emission at higher frequencies, in the optical domain, which is interesting for two main reasons: First, it enables the generation of quantum states of photons with easily controllable coherence condutors. Second, single photon detection will allow a detailed study of the differences between the optical and electric detection of quantum phenomena.

References: 2012, 2009, 2008, Kirtley et al.
Contacts: Julien Gabelli

The life and death of spins and charges in superconductors
We begin by asking a very simple question: what happens to spin-polarised electrons when they enter a superconductor? Unlike in 'normal' metals, both spin and charge imbalances (non-equilibrium accumulation) can appear in superconductors. In a recent experiment, we showed that charge relaxes much faster than spin. To do this, we used hybrid (superconductor-ferromagnet) nanostructures to make devices reminiscent of polariser/analyser setups in optics. As the spin relaxation time we measured was quite long (>10ns), we can take advantage of the separation between the spin and charge degrees of freedom to perform spin-dependent quantum transport experiments.

Reference: Quay et al.
Contacts: Charis Quay, Marco Aprili

Topological states of microwave light
The aim of this project is the realization of a quantum simulator to study topological effects in quantum fluids. To do so we will use the circuit-QED tools to generate an interacting photonic gas submitted to an artificial gauge field. The system will consist in a collection of coupled microwave cavities incorporating superconducting qubits. The photons trapped in this network should behave as electrons in a bi-dimensional gas submitted to a high magnetic field. Our approach will be to revisit the rich physics of the quantum Hall effect, first in the integer regime, finally in the fractional one where open questions still remain.

Contacts: Julien Gabelli, J├ęrome Estève

Shot-noise scanning tunneling microscopy
In a tunnel junction, the power of the current fluctuations, Si, is linear with the tunneling current, where the current noise is called shot-noise. In a non-interacting system, the shot-noise is Poissonian as each electron crossing the tunnel barrier is independent from the others. As soon as the electrons in one of the terminals (i.e. the tip or the sample in the case of STM) are interacting with one another, the shot-noise will be directly affected. In addition to addressing dynamics, shot-noise also allows for directly measuring the charge locally as the shot-noise power spectrum is proportional to the charge transferred. We have developed cryogenic circuitry operating in the MHz regime that enables our home-built scanning tunneling microscope to study dynamics at the atomic scale of correlated electron systems through shot-noise measurements.

Contacts: Freek Massee, Marco Aprili

High frequency dynamics of SNS junctions
We probe the dynamics of superconductor - normal metal superconductor junctions by combining dc and microwave measurements. Our results on the effect of a microwave excitation on the critical and retrapping currents measured by dc I(V) characteristics reveal how they are respectively affected by elastic and inelastic relaxations.

Contacts: Julien Basset, Marco Aprili

The effect of a fractal environment on a quantum conductor
In a waveguide with a fractal structure, the Fourier mode decomposition is impossible because of self-similarity. Thus, the concept of photon (as a mode of the electromagnetic field) must be redefined and the existence of fractal dimensions has many implications for quantum electrodynamics in such waveguides. In atomic physics, for example, this means a change in the spontaneous emission rate (Purcell effect). In the context of quantum electron transport, a similar phenomenon exists for tunnel junctions: the dynamic Coulomb blockade resulting in the non-linearity of the current-voltage characteristic of the junction. Experimentally, it is obviously not possible to realize a waveguide to all interative fractal orders. However, the wavelengths of the investigated 'modes' introduce a characteristic cutoff and numerical simulations show that a limited number of iterations should be sufficient to achieve the desired regime, where the expected spectrum is the triadic Cantor set.

Contact: Julien Gabelli

Quantum statistics of surface plasmons excited by a tunnel junction
When a voltage of a few volts is applied between two metals separated by a thin insulating barrier, a tunnel current flows. This well-known electrical phenomenon is accompanied by another optical one: the generation of light. The emitted light is broadband and can be seen as the radiation due to the high frequency component of current fluctuations. In a tunnel junction, these fluctuations are linked to the discrete nature of the electron, the shot noise.
Up-to-date it is known that photon emission in tunnel junctions is related to the excitation, by electrons, of non-radiative modes called surface plasmon-polaritons. The coupling between the plasmons located at the metal/air interface and the far field photons is subsequently realized by scattering on surface roughness. This second step is however quite ineffective such that the number of emitted photons by unit of electron varies from 10-4 to 10-6 in the optical range (l = 0.5-1.5┬Ám). This makes the optical measurements difficult to realize though doable with state-of-the-art photon detectors.
In this project we will overcome the problem related to plasmon/photon conversion by realizing a direct near-field measurement of plasmons. To this end we will use nanoscale superconducting detectors placed on-chip. By directly studying the relationship between the quantum statistics of current fluctuations in a tunnel junction and the resulting plasmon emission, one expects to prove that the electron statistics can be imprinted on plasmons.

References: Kirtley et al.
Contacts: Julien Gabelli, Julien Basset

Past projects

Microwave cooling of the Josephson Phase
As is well-known for atoms and ions and recently applied to opto-mechanical oscillators, radiation pressure can be used to reduce Brownian motion, resulting in cooling mechanisms and lower effective temperatures. There is a straightforward analogy with a Josephson junction irradiated with microwave photons: the coupling of the Josephson junction to a high finesse microwave cavity produces sidebands in the cavity resonances and microwave radiation tuned on these sidebands can cool the phase through anti-Stokes photon scattering. We have demonstrated this in an experiment where the effective temperature was obtained by measuring the width of the junction critical current distribution (see Figure). Our experiment addresses the out-of-equilibrium manipulation of the Josephson phase by microwave photons.
It is instructive to point out that this same experiment simulates some experimental aspects of a large-scale apparatus as the VIRGO interferometric gravitational wave detector. Furthermore, cooling a macroscopic quantum observable by using microwaves may represent an appealing alternative to standard cryogenic refrigeration.

Spin and phase dynamics
Via a direct coupling between the magnetic order parameter and the singlet Josephson supercurrent, we detect spin-wave resonances, and their dispersion, in ferromagnetic Josephson junctions in which the usual insulating or metallic barrier is replaced with a weak ferromagnet. Quantum transport may provide an alternative for high sensitive spin dynamics detection.
Measuring the full counting statistics of avalanche transport
We have measured the noise of an avalanche diode. We show how from the statistics of the current fluctuations we can deduce the statistics of the mechanism that governs the avalanche. See our paper for more details.

Bifurcation and phase relaxation in a Josephson junction
We have demonstrated a novel way to attain a regime characteristic of nonlinear oscillators: bifurcation. Specifically, we show that in a Josephson junction, if we excite the system with a fast pulse of current, it enables with a finite probability a jump to a previously unattainable state. By varying the excitation frequency, and measuring the jump probability, we determine directly the phase relaxation time as in a pump-probe experiment.
Third cumulant of quantum noise
We measure the third cumulant of current fluctuations at high frequency (6 GHz) to be deep into the quantum regime (frequency much greater than voltage and temperature). This quantity is critical in the understanding of how quantum noise can be measured by classical aparatus. First results can be found here.

The noise susceptibility of a quantum conductor
We have answered the following question: how fast can one modulate quantum noise? See our measurement and theory.