Much like energy, mass, and other physical quantities that obey the laws of conservation, quantum information can neither be created nor destroyed. But when a quantum system interacts with the environment, its state information can be lost and difficult to recover. Systems such as high-dimensional entangled states of light are more resistant to decoherence than ones based on light polarization, for example. But the correlations between two photons tend to break down when one or both are transmitted through a noisy channel, such as a multimode fiber-optic cable. Natalia Herrera Valencia and Mehul Malik, both of Heriot-Watt University in Edinburgh, and their colleagues have now demonstrated that they can preserve the quantum coherence of two photons by determining how the transmitted photon was scattered from the entangled quantum state itself.

The team’s experimental setup is depicted in the figure below. Two photons entangled in their transverse position–momentum are produced from a nonlinear crystal (NLC) illuminated by a UV laser. Then a polarizing beamsplitter (PBS) separates the two photons, with one measured locally and the other sent through a 2-meter-long multimode fiber (MMF). The position–momentum state of the two photons was then displayed by two spatial light modulators (SLMs) and measured with some photodiodes to determine how the fiber affected the entangled quantum state. Those data are contained in the fiber transmission matrix—an array of complex numbers that connect the photon’s input modes to the set of observed output modes.

To remove the scattering effects of the noisy environment, Valencia and her colleagues used the transmission matrix to scramble the untransmitted photon. They demonstrated that a six-dimensional entangled state of light could be maintained with an accuracy of 84%. Theoretical calculations suggest that it’s possible for each photon to travel through its own independent scattering channel and for the consequent scattering effects to be removed. That possibility and the observed results would help make possible secure quantum cryptographic communications. (N. H. Valencia et al., *Nat. Phys.*, 2020, doi:10.1038/s41567-020-0970-1.)