Real-time gigahertz free-space quantum key distribution within an emulated satellite overpass.

Satellite quantum key distribution (SatQKD) intermediated by a trusted satellite in a low-Earth orbit to ground stations along the satellite's path allows remote users to connect securely. To establish a secure connection, a SatQKD session must be conducted to each user over a dynamically changing free-space link, all within just a few hundred seconds. Because of the short time and large losses under which the QKD protocol will be implemented, it has not yet been possible to form a complete key by transmitting all the relevant information required within a single overpass of the satellite. Here, we demonstrate a real-time QKD system that is capable of forming a 4.58-megabit secure key between two nodes within an emulated satellite overpass. We anticipate that our system will set the stage for practical implementations of intercontinental quantum secure communications that can operate over large networks of nodes and enable the secure transmission of data globally.

Entangled photon-pair sources based on three-wave mixing in bulk crystals.

Entangled photon pairs are a critical resource in quantum communication protocols ranging from quantum key distribution to teleportation. The current workhorse technique for producing photon pairs is via spontaneous parametric down conversion (SPDC) in bulk nonlinear crystals. The increased prominence of quantum networks has led to a growing interest in deployable high performance entangled photon-pair sources. This manuscript provides a review of the state-of-the-art bulk-optics-based SPDC sources with continuous wave pump and discusses some of the main considerations when building for deployment.

Observing sub-Poissonian statistics of twisted single photons using oscilloscope.

Heralded single photon sources (HSPSs) from spontaneous parametric down-conversion are widely used as single photon sources. We study the photon number statistics of an HSPS carrying orbital angular momentum in our laboratory and observe the sub-Poissonian statistics using only photodetectors and an oscilloscope.

Direct transfer of pump amplitude to parametric down-converted photons.

In general, the spatial distribution of individual photons (signal or idler) generated by spontaneous parametric down-conversion (SPDC) does not evidently show any particular spatial mode structure because of their randomness in generation and the incoherent nature. Here, we numerically showed that all individual photons generated by the SPDC process carry the transverse amplitude as that of the pump and then confirmed it experimentally. The pump amplitude is revealed in SPDC when individual photons are spatially filtered from the total SPDC distribution. This is observed simply by imaging the photons that are filtered using a minimum-sized aperture. The phase measurements showed that the observed mode distribution does not possess the transverse phase distribution as that of the pump.