Researchers at TMOS have designed and demonstrated a new device that uses an ultra-thin metasurface to measure photon pairs. The work was reported today in Optica.

An increasing number of emerging quantum applications operate using optical technologies. Essentially, photons carry information at the speed of light and over long distances, making them good candidates for fast and secure communications and quantum computing. Many of these applications require photons that are identical (indistinguishable). When the photons are not identical, it can lead to errors in the data and quantum technologies become less reliable.

Currently, quantum photon sources are regularly taken offline to be tested and adjusted using an interferometer. This requires comparing photons multiple times using different configurations, a process that is time consuming and requires relatively bulky equipment that can accommodate the various physical arrangements.

Real-time analysis of photon indistinguishability that can be conducted within a device while it operates could improve the precision of quantum technologies.

The new device, designed by TMOS researchers at the Australian National University, does all the necessary measurements in a single pass.

Co-lead author Jihua Zhang says, “This metasurface-enabled multiport interferometer can determine if a photon pair’s properties are identical in a single shot. It doesn’t need multiple measurements using phase or time delays because the multiport structure allows the device to run measurements concurrently. This enables realtime and accurate characterization.”

One essential advantage is that this multiport interfrerometer is single-element, which not only reduces the size but also make it ultrastable when compared with previous multiport interferometers in the free-space optical setup. The use of meta-optics further decreases the size, weight and power of the device, as well as the cost of production. Flat optics, as meta-optics has become known, is key to miniaturising optical systems, which will in turn lead to the miniaturisation of devices we use day to day.

Co-lead author Jinyong Ma says, “We created a static, dielectric metasurface grating without any reconfigurable elements. The grating was designed using multi-factor topology optimization, which is essentially adjusting the surface pattern so that it interacts with light in a specific way. After successful simulations, fabrication and a one-off calibration, we were able to successfully characterise the similarity of the photons’ spatial mode, polarization and spectra.”

Chief Investigator Andrey Sukhorukov, who leads the research from the Australian National University, says, “The success of our experimental trials suggests that the work could be further developed to also measure the indistinguishability of other photon properties, such as orbital angular momentum. It could underpin ultracompact and power-efficient optical elements that would be especially suited for portable and satellite-based free space quantum photonic technologies.”

For more information about this research, contact connect@tmos.org.au

Single-shot characterization of photon indistinguishability with dielectric metasurfaces

Jihua Zhang, Jinyong Ma, Neuton Li, Shaun Lung, and Andrey A. Sukhorukov

Optica 23rd June 2024

Characterizing the indistinguishability of photons is a key task in quantum photonics, underpinning the tuning and stabilization of the photon sources and thereby increasing the accuracy of quantum operations. The protocols for measuring the degree of indistinguishability conventionally require photon-coincidence measurements at several different time or phase delays, which is a fundamental bottleneck towards fast measurements and real-time monitoring of indistinguishability. Here, we develop a static dielectric metasurface grating without any reconfigurable elements that realizes a tailored multiport transformation in the free-space configuration without the need for phase locking and enables single-shot characterization of the indistinguishability between two photons in multiple degrees of freedom including time, spectrum, spatial modes, and polarization. Topology optimization is employed to design a silicon metasurface with polarization independence, high transmission, and high tolerance to measurement noise. We fabricate the metasurface and experimentally quantify the indistinguishability of photons in the time domain with fidelity over 98.4%. We anticipate that the developed framework based on ultrathin metasurfaces can be further extended for multi-photon states and additional degrees of freedom associated with spatial modalities.