We demonstrate experimentally the self-healing of spatiotemporally structured single-photon wave packets. After traversing an opaque obstruction, the single-photon spatial profile (from an entangled-photon pair) is rapidly reconstituted, which may be useful for resilient quantum communication.
Space-time wave packets (STWPs) are a new class of spatiotemporally structured light fields that are endowed with a host of unique properties, including diffraction-free propagation at controllable group velocity, self-healing, and dispersion-free propagation in dispersive media. These characteristics make STWPs useful candidates for free-space optical communications, particularly for quantum communications. We demonstrate here the first single-photon STWP produced in two different configurations from entangled-photon pairs produced through spontaneous parametric downconversion. In the first configuration, a spatiotemporal spectral modulator is placed in the path of the signal photon, whose arrival is heralded by detection of the idler photon, which we refer to as local STWP synthesis. In the second configuration, the signal photon after the modulator is detected to herald the arrival of the idler photon, which we refer to as remote STWP preparation. In both configurations, we observe the predicted diffraction-free propagation of single-photon STWPs. These results pave the way for quantum communications using spatiotemporally structured quantum states of light.
We realize an experimental implementation of one step of a quantum random walk at the single-photon level. After a single step, it is already possible to observe the difference between the quantum and classical random walk. The single photon is obtained using twin photons from parametric down-conversion, in which the detection of one photon of the idler prepares a single-photon state of the signal. We used two different experimental setups, one based on an interferometer and a second using a birefringent crystal. The physical process behind these effects is the spatial reshaping of the single-photon wave packet, which is the spatial analog of the time reshaping observed in tunneling experiments.
The possibility to manipulate the wave packets of single photons or biphotons has enriched quantum optics and quantum information science, with examples ranging from faithful quantum-state mapping and high-efficiency quantum memory to the purification of single photons. Here we demonstrate another fascinating use of wave packet manipulation on restoring quantum interference. By modulating the photons' temporal wave packet, we observe the revival of postselected entanglement that would otherwise be degraded or lost due to poor quantum interference. Our study shows that the amount of the restored entanglement is only limited by the forms of modulation and can achieve full recovery if the modulation function is properly designed. Our work has potential applications in long-distance quantum communication and linear optical quantum computation, particularly for quantum repeaters and large cluster states.