Nature Photonics review makes the case for ultrathin crystals in reprogrammable quantum light chips

A review surveys other groups' experiments. It has no control group, so the useful test is narrower: do the studies the authors cite show the bottleneck is where they say it is? They name the gap themselves. "Nonetheless, a missing link for the goal of programmable quantum photonics remains: multifunctionality," the authors said ^[4]. "For many applications, one needs a system that can perform multiple functions, which requires multiple components to be reconfigured simultaneously." ^[4] Their example asks whether a chip could "alter the source wavelength as well as its polarization and route the emitted photons to different detectors" ^[5].

In the authors' account, light is worth the effort because photons have low decoherence and can carry information encoded in multidimensional quantum states ^[15]. Getting the full benefit takes sources that control frequency, amplitude, phase and polarization. Those controls are what allow states such as quantum vortices and vector beams ^[6].

The materials argument holds up best for switching. The review calls dynamic modulation a "critical block" for programmable quantum photonics ^[18]. It describes the traditional ways of manipulating phase and frequency as bulky, lossy and slow to switch ^[11]. The authors believe 2D materials may eventually be versatile enough to meet every criterion ^[13]. "Essentially every component of a future quantum circuit can be engineered using 2D material components," Aharonovich said ^[14].

The case for sources is weaker. The most widely used sources today rely on nonlinear processes such as spontaneous parametric down-conversion, and the authors see considerable potential in single-photon sources instead ^[7]. Crozier is clearly taken with what stacking can do. "It's remarkable how much control is possible simply by changing how these ultrathin materials are stacked on top of one another," he said ^[9]. He also said: "However, van der Waals crystals do not yet compete with current leading sources, for example, quantum dots and color centers, in terms of brightness, photon purity and coherence: more work is needed." ^[10]

The phys.org account does not include brightness, purity or coherence figures, so a reader cannot tell how big that gap is or how much work "more work" means. I think the review makes a stronger case for 2D materials as switches than as emitters. That view holds only if a 2D modulator on a chip is shown to meet the coupling, switching and scalability criteria together, which the review says no platform does yet ^[12].

The authors are also setting out their own centre's direction. "Programmable quantum photonic systems are an important future development for the center," Neshev said ^[17].