Jérôme Faist is a Swiss Physicist and professor at the Institute of Quantum Electronics at ETH Zürich, where he also heads the FIRST Center for Micro- and Nanoscience. He studied physics and optoelectronics at the Swiss Federal Institute of Technology Lausanne, with early research focused on vertical-cavity surface-emitting lasers and optical modulators.
Following his doctoral studies, Faist conducted postdoctoral research at IBM Rüschlikon and later joined Bell Laboratories, Lucent Technologies, where he worked in Federico Capasso’s group. During this period, he made seminal contributions to intersubband physics and played a central role in the development of the quantum cascade laser (QCL). Together with collaborators, he successfully demonstrated the first experimental QCL using molecular-beam epitaxy, establishing a fundamentally new class of semiconductor lasers whose emission wavelength can be engineered from the mid-infrared to the terahertz range.
Faist later became Full Professor at the University of Neuchâtel, where his research concentrated on mid- and far-infrared intersubband lasers and where he founded the spin-off company Alpes Lasers to commercialize QCL technology for scientific, industrial, and medical applications. At ETH Zürich, his research continues to push the performance of quantum cascade lasers, including frequency-comb operation and room-temperature devices for advanced spectroscopic applications.
Beyond laser technology, Faist is internationally recognized for his pioneering work on light-matter interaction in solid-state systems. His research addresses coherent intersubband transitions in strong magnetic fields, vacuum fluctuations in terahertz metamaterials, and ultra-strong coupling phenomena, providing widely adopted experimental platforms and conceptual frameworks in condensed matter physics and material science.
His contributions have been recognized by numerous awards, including the National Swiss Latsis Prize. Jérôme Faist is an IEEE Member and an Optical Society of America Fellow and is the author of the monograph Quantum Cascade Lasers (Oxford University Press). His work has had a lasting impact on infrared photonics, spectroscopy, and the control of quantum states in solid-state systems.
Semiconductor quantum walk combs from the near-infrared to the terahertz
The random walk is a fundamental concept in mathematics and physics that describes a walker taking random steps in a discretized space. While the spread of a classical random walker is characterized by a square root standard deviation after N steps it is surpassed by its quantum counterpart, in which the standard deviation of the position scales linearly with the number of steps. We recently showed that the concept of quantum walk[1] can be applied to the problem of generating an optical frequency comb, an array of single frequencies locked into equidistance by a non-linear optical process. These optical combs have proven to be of great use for a number of applications such as optical synthesis, metrology and sensing.
In our case, the quantum walk occurs in the synthetic dimension formed by the modes of a resonator where a phase modulation induced, for instance, through the electro-optical effect or the laser’s pump can introduce coupling in the chain, giving rise to continuous-time quantum walk dynamics. This system can be analogously mapped to a quantum harmonic oscillator [2]. We demonstrate both theoretically and experimentally that the full potential of the synthetic frequency lattice can be unlocked by employing ultrafast saturable gain. The giant nonlinearity of such gain locks the resonator modes and effectively counteracts the dispersion. This is in great contrast with the situation of slow gain where, as was shown many years ago, only the fundamental mode is the Gaussian-shaped fundamental mode of the parabolic potential [2].
Laser based on semiconductors active regions are, in general, excellent candidates to realize quantum walk combs, as they exhibit very naturally both the phase modulation as well as the fast gain saturation which are both key for the operation of the device. It is also true in interband devices despite the relatively long spontaneous emission time[3]. The coupling between the real and imaginary part of the gain, responsible for the linewidth enhancement factor, will ensure that a modulation of the gain will also generate the phase modulation necessary for the proliferation of the modes.
We report the operation of such combs across the electromagnetic spectrum from the Terahertz to the near-infrared, demonstrating the universal character of the quantum walk comb in semiconductor lasers.
Quantum walk combs are equally relevant for applications in sensing or telecommunication as well as a platform for emulation of quench dynamics, taking advantage of the liquid-like properties of light in a fast gain medium [4]. We show also that, because their spectral envelop can be controlled electrically, quantum walk combs enable new avenues into broadband spectroscopy.
Bibliography
[1] I. Heckelmann, M. Bertrand, A. Dikopoltsev, M. Beck, G. Scalari, and J. Faist, “Quantum walk comb in a fast gain laser,” Science, vol. 382, no. 6669, pp. 434 438, Oct. 2023, doi: 10.1126/science.adj3858.
[2] H. Haus, “A theory of forced mode locking,” IEEE Journal of Quantum Electronics, vol. 11, no. 7, pp. 323–330, Jul. 1975, doi: 10.1109/JQE.1975.1068636.
[3] B. Marzban, L. Miller, A. Dikopoltsev, M. Bertrand, G. Scalari, and J. Faist, “A Quantum Walk Comb Source at Telecommunication Wavelengths,” Nov. 13, 2024, arXiv: arXiv:2411.08280. doi: 10.48550/arXiv.2411.08280.
[4] A. Dikopoltsev et al., “Collective quench dynamics of active photonic lattices in synthetic dimensions,” Nat. Phys., pp. 1–7, May 2025, doi: 10.1038/s41567 025-02880-2.
