A protocol for storing and releasing a single photon in an embedded eigenstate — a quantum state that is virtually unaffected by loss and decoherence — could advance the development of quantum computers. Created by researchers at City University of New York (CUNY), the protocol could facilitate a new approach to capturing and preserving photons. The team’s calculations suggest that it is possible for a pair of photons to impinge on a cavity-atom coupled system, and that atomic nonlinearity would allow one photon to be perfectly trapped and preserved in the system while the other is reemitted. The researchers theoretically explored the excitation and release of single-photon embedded eigenstates based on atomic nonlinearities by investigating a system composed of an atom and a partially open cavity. The open cavity would normally allow light trapped in the system to leak out; but the researchers showed that under certain conditions, destructive interference phenomena could prevent leakage and allow a single photon to be hosted in the system indefinitely. This embedded eigenstate could be helpful for storing information without degradation. The closed nature of this protected state also created a barrier to exterior stimuli, so that single photons could not be injected into the system. The team overcame this limitation by exciting the system at the same time with two or more photons. In realistic systems, additional imperfections would prevent perfect confinement of photons. But the researchers’ calculations showed that, even considering realistic losses, their protocol could allow single photons to be stored for much longer than the time needed to excite the embedded eigenstate, in contrast with single-cavity or single-atom configurations. The researchers believe that the principle could be extended to experimentally feasible systems composed of two cavities with one cavity containing the atom. “We proposed a system that acts as a closed box when excited by a single photon, but it opens up very efficiently when we hit it with two or more photons,” researcher Michele Cotrufo said. “Our theory shows that two photons can be efficiently injected into the closed system. After that, one photon will be lost and the other will be trapped when the system closes. The stored photon has the potential to be preserved in the system indefinitely.” The researchers also showed that the stored excited photon could later be released on demand by sending a second pulse of photons. “Our work demonstrates that [it] is possible to confine and preserve a single photon in an open cavity and have it remain there until it’s prompted by another photon to continue propagating,” professor Andrea Alù said. The group is now exploring avenues to experimentally verify its theoretical work. The team’s findings have the potential to enable the on-demand generation of entangled photonic states and quantum memories. The research was published in Optica, a publication of OSA, The Optical Society (https://doi.org/10.1364/OPTICA.6.000799).