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Nature Communications

Photon trapping in a time cavity flying at the speed of light

Ihar Babushkin, Oliver Melchert, Uwe Morgner, Ayhan Demircan

Published Aug 29, 2026• 6 citations

Abstract

Abstract A refractive index step, moving at the speed of light, can serve as a trap for weak light waves. Here we shed light on highly unusual features of such trapping, making it fundamentally different from conventional “resting” cavities. First, such trapping can be described as time-tapping rather than space-trapping. Trapping in time provides superior quality Q, but this quality does not anymore impose very narrow spectral lines, breaking thereby the fundamental time-bandwidth limit inherent to most traps of any physical nature. Although the trapped states are almost entirely disconnected from the outer world, a nonreciprocal connection channel involving resonant radiation can be opened and tunably manipulated. Nonreciprocity breaks the balance between the energy currents directed into the trap and back, allowing to load the trap faster than it is possible for reciprocal systems. Exotic, up to now unobserved in optics states such as halo states, and phenomena as anomalously huge delays in transmission occur naturally in a time trap. Beatings between the trapped-in-time modes provide a novel tool for ultrafast shaping of both spectral and temporal waveforms for arbitrary weak wavepackets, even for single photons. Altogether, this promises a highly eﬀicient platform for energy storage, pulse compression, waveform shaping and frequency conversion for ultrafast classical and quantum optics.

DOI: 10.1038/s41467-026-77059-1

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Abstract A refractive index step, moving at the speed of light, can serve as a trap for weak light waves. Here we shed light on highly unusual features of such trapping, making it fundamentally different from conventional “resting” cavities. First, such trapping can be described as time-tapping rather than space-trapping. Trapping in time provides superior quality Q, but this quality does not anymore impose very narrow spectral lines, breaking thereby the fundamental time-bandwidth limit inherent to most traps of any physical nature. Although the trapped states are almost entirely disconnected from the outer world, a nonreciprocal connection channel involving resonant radiation can be opened and tunably manipulated. Nonreciprocity breaks the balance between the energy currents directed into the trap and back, allowing to load the trap faster than it is possible for reciprocal systems. Exotic, up to now unobserved in optics states such as halo states, and phenomena as anomalously huge delays in transmission occur naturally in a time trap. Beatings between the trapped-in-time modes provide a novel tool for ultrafast shaping of both spectral and temporal waveforms for arbitrary weak wavepackets, even for single photons. Altogether, this promises a highly eﬀicient platform for energy storage, pulse compression, waveform shaping and frequency conversion for ultrafast classical and quantum optics.

Most present-day resonant systems, throughout physics and engineering, are characterized by a strict time-reversal symmetry between the rates of energy coupled in and out of the system, which leads to a trade-off between how long a wave can be stored in the system and the system bandwidth. Any attempt to reduce the losses of the resonant system, and hence store a (mechanical, acoustic, electronic, optical, atomic, or of any other nature) wave for more time, will inevitably also reduce the bandwidth of the system. Until recently, this time-bandwidth limit has been considered fundamental, arising from basic Fourier reciprocity. A recent theory suggested that it might in fact be overcome by breaking Lorentz reciprocity in the resonant system, reinvigorated a debate about whether (or not) this was indeed the case. Here, we report an experimental realization of a cavity where, inducing nonreciprocity by breaking the time invariance, we do overcome the fundamental time-bandwidth limit of ordinary resonant systems by a factor of 30, in full agreement with accompanying numerical simulations. We show that, although in practice experimental constraints limit our scheme, the time bandwidth product can be arbitrarily large, simply dictated by the finesse of the cavity. Our experimental realization uses a simple macroscopic, time-variant, fiber-optic cavity, where we break Lorentz reciprocity by non-adiabatically opening the cavity, injecting a pulse of large bandwidth, and then closing the cavity, storing the pulse which can be released on-demand at a later time. Our results open the path for designing resonant systems, ubiquitous in physics and engineering, that can simultaneously be broadband (i.e., ultrafast) and possessing long storage times, thereby unleashing fundamentally new functionalities in wave physics and wave-matter interactions.

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We show that light pulses can be stopped and stored all-optically, with a process that involves an adiabatic and reversible pulse bandwidth compression occurring entirely in the optical domain. Such a process overcomes the fundamental bandwidth-delay constraint in optics, and can generate arbitrarily small group velocities for any light pulse with a given bandwidth, without the use of any coherent or resonant light-matter interactions. We exhibit this process in optical resonator systems, where the pulse bandwidth compression is accomplished only by small refractive index modulations performed at moderate speeds. The optically achievable ultra low speeds can also generate extremely large non-linearities using non-resonant interactions, and thus enable decoherence-free single photon quantum gates.

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