The regime of ultra-strong light-matter interaction (USC) has attracted considerable interest recently, as it predicts fascinating quantum phenomena such as the presence of virtual photons in the ground state. We explore a new approach to probe the quantum correlations in USC systems based on an electrical transport experiment. Namely, we consider the case where the light-matter coupled system constitutes the electromagnetic environment of an ultra-small tunnel junction operating in the regime of Dynamical Coulomb Blockade (DCB). Unlike optical spectroscopy, such probe can be seen as a non-demolition measurement of the USC system ground state.

It is well known since the seminal work by Purcell that light-matter interaction can be substantially modified by coupling the quantum transition with a single mode electromagnetic resonator. When the light-matter coupling energy, ħΩ exceeds the dissipation rates of the system, energy is periodically exchanged with the resonator at a frequency Ω also known as the vacuum Rabi frequency. The system enters the strong coupling regime where the cavity mode is split into two light-matter coupled (polariton) states separated by an energy 2ħΩ. The last decade has seen the emergence of a yet stronger interaction regime, where the coupling constant Ω becomes comparable to the frequency of the matter excitation ωm. This regime with Ω/ωm ≃ 1 is known as ultra-strong light-matter coupling (USC)[1] and sets new frontiers for cavity quantum electrodynamics Indeed, coupling with light can be so strong that it theoretically leads to important changes of the material properties such as the emergence of new phases and the possibility to trigger superconductivity. The USC regime also brings in fascinating quantum phenomena. As the coupling constant Ω grows, the polariton states become squeezed radiation states [1]. The ground state of the system is also completely changed, and acquires a population of virtual photons [1]. Under certain conditions, these can be extracted from the system. The effect of converting such virtual into real photons is known as the dynamical Casimir effect, which is analogous to the Hawking radiation from black holes.

Up to date, USC has been observed in many spectroscopic experiments, however such experiments do not provide direct access to the quantum optical properties of the light-matter coupled states. A basic system that allows probing the quantum fluctuations of an electromagnetic resonator is an ultra-small tunnel junction with a capacitance CT, such as the charging energy of a single electron e²/2CT is comparable to the oscillation quantum ħωLC [2]. When the ratio e²/CTħωLC is sufficiently large, the tunneling of single electron is conditioned by the quantum fluctuations of the electromagnetic environment, leading to the suppression of low voltage conductance. This effect is known as Dynamical Coulomb Blockade (DCB) [2].

We investigate a THz quantum-meta-material architecture which takes advantage of the DCB in order to probe the quantum-optical properties of the polariton quasi-particles. This can be achieved by using a THz meta-material resonator with highly sub-wavelength capacitive parts that interacts with highly-doped quantum wells in the USC regime. The lumped element nature of the THz meta-atom allows implementing a tunnel junction in one of the capacitors, which experiences the polaritonic environment in the DCB regime. The ongoing experimental effort is supported by a theoretical investigation [3].

[1] C. Ciuti, G. Bastard, and I. Carusotto, "Quantum vacuum properties of the intersubband cavity polariton field", Phys. Rev. B 72, 115303 (2005).
[2] G.-L. Ingold and Y. V. Nazarov, in Single Charge Tunneling, edited by H. Grabert and M. H. Devoret, of NATO ASI Series B: Physics Vol. 294 (Plenum, New York, 1992), pp. 21–107
[3] U. Iqbal, C. Mora and Y.Todorov, “Dynamical Coulomb blockade: An all electrical probe of the ultrastrong light-matter coupling regime”, Phys. Rev. Research 6, 033097 (2024).

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