A paper co-authored by seven Physics & Informatics (PHI) Lab-affiliated scientists and four colleagues at Stanford and CalTech, described as a “mini review,” is targeting the big goal of rethinking quantum engineering as a whole. (Other affiliations of the co-authors include Yale, Cornell, and the University of Massachusetts.) Titled “Mesoscopic ultrafast nonlinear optics–The emergence of multimode quantum non-Gaussian physics” and published in Optica, it argues that instead of relying on the interactions between light and non-optical systems as a means to engineer quantum states of light beyond the conventional framework, it’s time for “quantum optics, in optics.” They recommend “looking at features and dynamics rather than states and gates, or phenomena and functions rather than protocols and algorithms.”
This opportunity to disrupt the status quo arrives thanks to the dramatic progression of nonlinear optics into the mesoscopic region, with an energy scale as low as dozens to hundreds of photons, where systems can be solely described by neither classical physics nor quantum mechanics. Since the first demo sixty years ago of second-harmonic generation, a process where two photons with the same frequency combine to form a new photon with twice the frequency (half the wavelength) of the original photons, the co-authors note that “nonlinear optics has become significantly more nonlinear, traversing nearly a millionfold improvement in energy efficiency.” Put another way, they say that four successive waves of technological revolutions – phase-matching techniques, waveguides, nonlinear nanophotonics, and now dispersion engineering – have led to a “reduction of nearly eight orders of magnitude in the energy scale needed to access optical nonlinearities.”
The upshot? We are now approaching the limit prescribed by quantum mechanics, i.e., the energy scale of a single photon. That opens up the “tantalizing possibility” of unlocking the physics of “strong coupling,” a milestone widely regarded as essential for accessing the full potential of quantum phenomena and for developing new technologies in quantum information processing, sensing, and beyond. The question is how to get there. Experimental platforms for quantum electrodynamics (QED) are one conventional option, but with the drawbacks of cryogenic cooling and complex setups. Nonlinear nanophotonics seem like a better approach, with their “unique portfolio of room-temperature operability, lithographic scalability, and compatibility with long-distance telecommunications.” But not so fast, this paper warns.
To fully appreciate the potential of “mesoscopic nonlinear optics,” aside from the experimental challenges, we also need to tackle theoretical challenges. That is, we do not fully understand yet, even theoretically, how light behaves in this regime. The mesoscopic regime is at the cross-over point between classical and quantum physics. Thus, to understand the physics in this intermediate regime, we need to establish new theoretical frameworks that seamlessly bridge the gap between the classical and quantum worlds. A second challenge is computational. That is, the physics in this regime is so complicated that even numerical simulation looks naively impossible. Consider the multimode nature of ultrafast quantum nonlinear optics. (The multiplicity of quantum pulse modes relates to the complexity and flexibility of light in temporal and spectral domains.) A generic quantum pulse with a moderate number of 1000 modes, for instance, each populated with at most one photon, would require a 21000 dimensional state space. To fully leverage the potential of this mesoscopic regime, we thus need a novel theoretical framework to understand such complicated dynamics.
The rest of this paper considers conceptual and experimental challenges of developing a deeper understanding of the physics that characterize nonlinear optics in the mesoscopic regime and speculates on new opportunities involving quantum measurements, quantum computing, wave phenomena, and non-Gaussian quantum light-matter interactions, among others. (Non-Gaussian, in that the interactions involve states and operations that do not adhere to normal Gaussian probability distributions and, as a result, can lead to richer, genuine quantum phenomena.) Their vision of how this transition from classical to quantum ultrafast nonlinear optics will play out in the near future amounts to a pro-optics (“quantum optics, in optics”) research manifesto.
“The confluence of classical, quantum, and multi-mode physics suggests a unique opportunity for engineering methodologies from classical optics, quantum optics, and ultrafast optics to overlap and synergize,” these 11 physicists state. “And we argue that a new program of research should be dedicated to exploring these opportunities.”