All-fiber Control of Entanglement:
Recovering Correlations via Mechanical Perturbations in a Multimode Fiber

In quantum optical communications, single photons can be used as a unit of quantum information. However, one can supercharge their capacity to carry information by encoding high-dimensional quantum dits, or qdits, into their transverse shape. They allow having more than two levels per unit of information as it is the case for bits. In fiber optical communications, it requires using multimode fibers to harness the spatial degrees of freedom to encode the qduts. However, when propagating through a real-life multimode fiber, the transverse shape of the photons gets scrambled because of mode mixing and modal interference. This scrambling of transverse shape is typically rectified using free-space spatial light modulators. But, this remedy prevents us from achieving a truly resilient all-fiber operation and requires a careful alignment and lab-graded stability hindering real-life implementation. In [R. Shekel et al., Arxiv 2306.02288 (2023)], the authors introduce an all-fiber method for controlling the shape of single photons and spatial correlations between entangled photon pairs. They do so by implementing carefully controlled mechanical perturbations to the fiber.

Real-time shaping of entangled photons by classical control and feedback

[O. Lib, G. Hasson and Y. Bromberg, Sci. Adv. 6 (2020)]

Wavefront shaping offers the possibility to compensate for the effect of propagation through heterogeneous media. However, when using a single or a few photons, the feedback signal is typically too weak to allow real-time wavefront shaping applications, which limits applications for quantum communications using entangled photons. In this paper from the team of Yaron Bromberg at the Hebrew University of Jerusalem, the authors overcome this challenge by using as feedback the classical signal of the pump that follows the same path as the entangled photon. It allows adapting in real-time the pump wavefront to compensate for the aberrations/scattering introduced by a heterogeneous dynamic sample.

Unscrambling entanglement through a complex medium

[N. H. Valencia, S. Goel, W. McCutcheon, H. Defienne and M. Malik, Nature Physics (2020)]

Quantum properties of light may enable unconditionally secure optical communications. In this respect, high-dimensional entangled states offer a way of exceeding the limitations of current approaches to quantum communication (e.g. larger information capacity and increased noise resilience). For example, the orbital angular momentum of photons was first used to establish high-dimensional quantum key distribution (HD-QKD) protocols in free-space, but with a limited range due to diffraction and the presence of atmospheric turbulence. Alternatively, multimode optical fibers (MMF) can be used to transport information encoded in parallel across many modes over large distances, and with limited losses. However, the complex mode mixing process occurring during light propagation through the fiber scrambles the encoded information, making it unusable by the receiver. In their work, N. H. Valencia and co-workers demonstrate the transport of six-dimensional spatial-mode entanglement through a 2m-long commercial MMF, by compensating the random mode mixing effect using a transmission matrix-based wavefront-shaping technique. Such an ability to certify the presence of high-dimensional entanglement between two parties (Alice and Bob) is an essential step towards the implementation of practical HD-QKD protocols in optical fibers.