Emerging Quasi-Particle Unites Microwave and Optical Realms

In a recent article published in Nature Communications on September 18, researchers from the Paul-Drude-Institut in Berlin, Germany, and the Instituto Balseiro in Bariloche, Argentina, showcased a groundbreaking discovery. They revealed that the intermingling of confined quantum fluids of light and GHz sound results in the emergence of a previously elusive quasi-particle called the phonoriton. This unique entity combines aspects of a quantum of light (photon), a quantum of sound (phonon), and a semiconductor exciton. This newfound revelation paves a path for effectively converting information between optical and microwave domains, promising significant advancements in photonics, optomechanics, and optical communication technologies.

The study drew inspiration from a common phenomenon—the transfer of energy between interconnected oscillators, akin to two pendulums linked by a spring. When specific coupling conditions, known as the strong-coupling (SC) regime, are met, energy continually oscillates between these interconnected entities, altering their frequencies and decay rates. This scenario is analogous to when dealing with photonic or electronic quantum states, where the SC regime is crucial for controlling and exchanging quantum states.

In real-world applications, hybrid quantum systems necessitate coherent information transfer between oscillators with vastly different frequencies. A notable example is within networks of quantum computers. While the most promising quantum computers operate using microwave qubits (operating at a few GHz), quantum information is optimally transferred using near-infrared photons (100s of THz). Consequently, achieving a bidirectional and coherent transfer of quantum information between these domains is imperative. Directly converting microwave qubits to photons often proves highly inefficient. Therefore, an alternative approach is to mediate this conversion through a third particle, efficiently coupling to both the microwave qubits and photons—enter the GHz lattice vibrations (phonon), a suitable candidate for this role.

The theoretical basis for strong coupling between light and phonons was laid in 1982 by Keldysh and Ivanov, predicting that semiconductor crystals could blend photons and phonons through another quasi-particle: the exciton-polariton. Polaritons arise from the robust coupling of photons and excitons, and when a phonon is introduced, it can couple two polariton oscillators with frequencies differing by precisely the frequency of the phonon. In the SC regime, this coupling leads to the creation of a novel quasi-particle—the phonoriton—a blend of an exciton, a photon, and a phonon.

However, the emergence of the phonoriton necessitates stringent experimental conditions, resulting in limited reports on its formation. Apart from the scientific significance of uncovering this novel semiconductor excitation, the phonoriton holds promise as a new intermediary for coherent microwave-to-optical frequency conversion.

In their research, Alexander Kuznetsov and team generated polaritons within a patterned microcavity resonator, as depicted in Figure 2(a). The microcavity’s thicker regions served as effective traps for both 370 THz polaritons and 5 to 20 GHz phonons. This trapping significantly amplified the interaction between the two particles, a critical requirement for phonoriton formation.

By injecting additional polaritons optically into the trap, the researchers created two polariton condensates distinguished by a remarkably bright and narrowly spectral emission line (sub-GHz). Unlike conventional lasers, polaritons exhibit robust interparticle interactions, hence earning the moniker “quantum fluids” of light. These interactions allowed precise tuning of the energy split between the two light fluids by controlling their densities with an external laser.

When the energy split aligned with the phonon energy, the two polariton fluids synchronized, as illustrated in Figure 2(b). This synchronization stemmed from a blend of non-linear polariton-polariton interactions and the efficient transfer of polaritons between the light fluids mediated by phonon absorption and emission. The phonon-induced coupling between polariton states surpassed their decay rate, marking the emergence of the phonoriton.

To further manipulate the system, the authors employed a piezoelectric transducer atop the microcavity and surrounding the trap to administer microwaves and introduce 7 GHz phonons into the trap. In the presence of these injected phonons, the phonoriton spectrum transformed into a series of narrow resonances (phonon sidebands), illustrated in Figure 2(c). The sidebands on either side of the central peak corresponded to coherent emission (left) and absorption (right) of phonons, effectively showcasing bidirectional microwave-to-optical conversion. Intriguingly, unlike conventional optomechanical systems where phonons directly interact with photons, here, the interaction strength scales with both the polariton and phonon populations.

In summary, the work by Alexander Kuznetsov and his colleagues harnessed the tailored resonances of photonic, electronic, and phononic states within patterned semiconductor microcavities. Through this, they demonstrated the existence of phonoritons and showcased coherent bidirectional microwave-to-optical conversion within a semiconductor system.

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