The advent of quantum technologies is now considered to be an essential part of the Fourth industrial revolution. In the very recent years we have witnessed an impressive progress on quantum computing platforms developed by industrials as Google or IBM. On the other hand, quantum cryptography is considered as a strategic technology for optical communications in the future. The quantum nature of light plays paramount role in these developments, as optical links could also be part of future quantum computation networks.
My research area, the Mid-Infrared (MIR, lambda=3-30µm) and the TeraHertz (THz, lambda=30-300µm) spectral ranges have important applications for thermal imaging, both for civil and military use, molecule spectroscopy and environmental monitoring of CO2 or pollutants. The THz radiation holds promises for applications in numerous areas, such as medicine, security, astronomy, imaging and even arts and has been recently envisioned for future 6G wireless communications by Samsung. Such applications are supported by semiconductor optoelectronic devices such as quantum cascade lasers and detectors, which are based on tunnel-coupled quantum wells. The laws of quantum mechanics are thus fully exploited in order to design and control the electronic transport across the device. However, up to date, there has not been any applications that rely on the quantum nature of the infrared photons. The current state of the art of infrared detectors is mostly responsible for this, as the detector technology developed in this part of the spectrum is far less performant as compared to visible or microwave technology.
In a nutshell the object of the ERC project is to bring the MIR and THz detector technology to a point where the detectors become sensitive to the quantum state of the infrared light. For that, I am specifically exploring optoelectronic devices, where a detector quantum region is coupled with an electromagnetic resonator which can be metamaterial or a microcavity. In such devices, we can reach a highly non-perturbative regime of interaction, the ultra-strong light-matter coupling regime (USC). Such devices have the potential to allow the first time experimental observation of intrinsically quantum features of the ultra-strong coupling regime, such as quantum vacuum radiation (dynamical Casimir effect) and squeezing of polaritons states. In these devices, generically acting as detectors, the light-matter coupled states (polaritons) will be efficiently converted into electrical signals.
There are three approaches which correspond to three different type of devices that are actively explored. For the first approach, we study quantum detectors of radiation where the polariton-assisted fermionic transport are expected to occur. The impact of the USC is studied on directly measurable detector characteristics, such as the photocurrent spectra and the photoconductive gain. Within the ERC project, we recently conducted experimental and theoretical research which allows elucidating the operation of such detectors. These result will appear in Nature Communications.
In a second approach, we study metamaterial-coupled detectors supplied with a single electron transistor read-out, which will increase their sensitivity to the level of a single photo-generated electron. The read-out mechanism is based on the Charge Sensitive Phototransistor (CSIP) which is analogue to the single electron transistors from mesoscopic physics, but uses of the same quantum-well based technology as unipolar devices. In these devices, the photon absorption in a quantum well inserted in the gate-source region triggers an electron transfer into an almost depleted source-drain channel, which changes dramatically its conductance. The extreme sensitivity of such detectors, down to the single photon level, will be exploited to reveal the quantum statistics of infrared light.
In a third approach, we exploit there dimensional infrared metamaterials which provide extreme electromagnetic confinement. In our metamaterial architectures the volume of the capacitance C can be reduced down to nanometric sizes, while keeping the circuit resonance in the desired frequency range. In this limit, the single electron charging energy e²/2C becomes an important fraction of the circuit photon energy ћLC. This is precisely the condition to observe the Dynamical Coulomb blockade (DCB) which occurs when a tunnel junction is inserted in the capacitor, as described in seminal work by Devoret et al. In DCB, the fundamental quantum fluctuations of the electromagnetic field lead to a partial suppression of the junction conductance. As pointed out recently by Souquet and co-authors the shape of the current-voltage characteristics of the junction becomes strongly correlated to the quantum state of the circuit . DCB is feasible in our metamaterial architectures, with the possibility to distinguish different states of the resonators, such as, for example, a thermal ground state or an externally driven coherent state. Squeezed radiation states also bring specific signatures in the current-voltage of the junction.
