Researchers are leveraging photonics to develop and scale the hardware necessary to tackle the stringent requirements of quantum information technologies. By exploiting the properties of photonics, researchers point to the benefits of scaling quantum hardware. If or when successful, researchers say quantum hardware at scale will enable long-range networks, interconnections between multiple quantum devices, and large-scale photonic circuits for quantum computing and simulation.

An interdisciplinary team of researchers from Denmark, Germany, and the UK is focusing on the best ways to use photonics and exploit its properties to develop a platform that can scale quantum hardware, Phys.Org reported. To this end, the team developed an integrated photonic platform based on thin-film lithium niobate, whose single crystals are important materials for optical waves and are an ideal modulator for low-loss mode.

Then, researchers interfaced the integrated photonic platform with deterministic solid-state single-photon sources based on quantum dots (semiconductor crystals) in nanophotonic waveguides. The resulting photons produced are processed with low-loss circuits, which according to the researchers are programmable at speeds of several gigahertz. Researchers state that fast reprogrammable low-loss optical circuits are key for performing tasks in photonic quantum information processing.

The high-speed platform paved the way for researchers to achieve several key photonic information processing functionalities. The first processing functionality researchers observed during experiments was on-chip quantum interference. Researchers used the Hong-OuMandel (HOM) effect, which is characterized as when two-photon interference is observed.  Figure 1 displays the on-chip HOM experiments performed that tested the performance of the platform for photonic quantum information processing.

Another processing functionality the team demonstrated that is key to photonic information processing is an integrated single-photon router. Researchers demonstrated a fully on-chip photon router for the quantum dot-emitted photons. To accomplish this, they leveraged the platform’s capability to integrate fast phase shifters with quantum emitter wavelengths to showcase the integrated single-photon router.

The team also implemented a universal four-mode interferometer, made up of a network of 6 Mach-Zehnder interferometers and 10 phase modulators, as shown in Figure 2.  Programmable multimode quantum photonic interferometers are paramount for the implementation of essential functionalities of photonic quantum technologies. And, the researchers said they interferometers are able to realize circuits for quantum computational advantage experiments or analog quantum simulation.

In a research paper published by Science Advances, researchers detailed their development of the high-speed, integrated photonic platform based on thin-film lithium niobate. The paper is entitled “High-speed thin-film lithium niobate quantum processor driven by a solid-state quantum emitter.”

The authors argue the results showed that integrated photonics with solid-state deterministic photon sources is a promising option to scale quantum technologies in multiple phases. Going forward, the platform can be further optimized to reduce coupling and propagation loss. In particular, fault-tolerant quantum computing architectures (with loss levels of ≲10% per photon) are a step closer to reality.

The interdisciplinary team of researchers all come from international institutions including the Center for Hybrid Quantum Networks (Hy-Q), Niels Bohr Institute, University of Copenhagen (Denmark); Institute of Physics, University of Muenster (Germany); CeNTech—Center for Nanotechnology (Germany); SoN—Center for Soft Nanoscience (Germany);  Wolfson Institute for Biomedical Research, University College London (UK); Ruhr University Bochum (Germany); and Heidelberg University (Germany).