Inside Psiquantum's Light-Based Utility-Scale Quantum Computer

Inside Psiquantum's Light-Based Quantum Computer

Dr Ben Miles delivers an in-depth tour of Psiquantum, a secretive quantum company aiming to deploy the world’s first utility-scale quantum computer built on photonics. The video explains how qubits are encoded in photon paths on a silicon chip, how single photons are generated via nonlinear optics in ring resonators, how heralding photons signal photon creation, and how dynamic switching and time encoding enable scalable computation. It also covers the cryogenic infrastructure, detectors, and the distributed modular architecture that could connect many chips across a facility.

Key insights

• Qubits are encoded in the path of photons on silicon photonic chips, offering long coherence and natural interconnectivity.
• Single photons are produced by spontaneous four wave mixing in ring resonators, with heralding photons signaling successful generation.
• Time encoding and a space-to-time conversion enable scalable qubits across networks of chips while mitigating phase disturbances.
• The system operates at 2 Kelvin in modular cryostats, aiming for millions of qubits through a distributed, factory-scale approach.

Introduction

This article summarises a deep dive into Psiquantum's approach to building a practical quantum computer using photonics. The core idea is to move away from traditional matter-based qubits and instead encode quantum information in light that travels through silicon waveguides. This design aims to scale by connecting many photonic chips in a modular, factory-style architecture, all while maintaining the delicate quantum states necessary for computation.

Photonic Qubits on Silicon

Psiquantum builds its qubits on silicon chips using lithography to create waveguides. Photons are guided through these on-chip structures, with information encoded in their spatial paths. A single photon can be placed into a superposition of two paths by a 50-50 beamsplitter, and quantum interference allows precise control of the photon's final state as it traverses the circuit. The massless, fast nature of photons makes them excellent for moving quantum information across chips and even between different modules in a large facility.

Photon Generation and Heralding

A critical challenge is generating exactly one photon per clock cycle. Psiquantum solves this by powering a pumped laser at 1550 nanometers into a series of ring resonators. The circulating light interacts with silicon electrons nonlinearly, enabling spontaneous four wave mixing to create photon pairs: a signal photon and a herald photon. The herald photon indicates an auxiliary source produced the intended signal photon, which will enter the main quantum circuit. Filtering removes the pump and unwanted photons, leaving a single photon to seed computation. The generation events are rare, so many resonators operate in parallel to boost the chance that a photon enters the world line within a cycle.

Heralding and Clocking Their Clock

Crucially, the herald photon does not need to be captured to clock the system. Instead, the two photons produced per event allow the system to infer which source line produced the signal photon without destroying it. The arrival of the herald photon signals the switch to connect the corresponding source line to the world line, enabling a reliable, clocked flow of photons through the quantum computer. This approach helps maintain a steady rhythm of photon-based operations across thousands of potential sources.

Switching and BTO Materials

To route photons with ultra-fast timing, they employ a fast electro-optic material, barium titanate oxide (BTO). BTO supports the Pockels effect, enabling rapid changes in refractive index and thus fast, local control of photons. Growing and integrating BTO with silicon over a decade has been a substantial manufacturing challenge because of lattice mismatches that can introduce defects. Psiquantum has developed layering techniques to gradually transition from silicon to oxide to BTO, a meticulous, large-scale fabrication process performed layer by layer with extreme control over temperature and environment.

The Maxander Interferometer and Thermo-Optic Control

Qubits are manipulated in a programmable optical circuit built from a mesh of Maxander interferometers, two 50-50 beam splitters arranged to form programmable gate-like operations. A heating element on one path introduces a phase delay via the thermo-optic effect, allowing dynamic control over the interference pattern and thus the qubit’s evolution. However, heating silicon is slow, and crosstalk between adjacent paths is a major issue. Psiquantum integrates the BTO-based control into the interferometer architecture to deliver rapid, localized phase shifts with reduced thermal crosstalk, enabling a scalable photonic quantum processor with many qubits and gates.

Detectors and Cryogenics

Measurement of photons relies on superconducting nanowire detectors made from niobium nitride. When cooled below their superconducting transition, a single photon can trigger a detectable resistive hotspot, yielding a voltage pulse that marks a quantum event. To achieve high detection efficiency, the detectors must be integrated with silicon waveguides and aligned with nanometer precision. Psiquantum operates its chips inside Mark 2 cryostats, four-meter-tall, vacuum-packed, radiation-shielded modules cooled to approximately 2 Kelvin. This temperature enables robust performance and is warmer than other quantum computing platforms, which often require millikelvin temperatures, while still maintaining extremely low phonon activity that could decohere delicate quantum states.

Time Encoding and Quantum State Transport

One of the most interesting concepts in the video is the time encoding of qubits. The space of a qubit is encoded not just in which path a photon takes but in the relative timing of two paths. By deliberately delaying one path by about 1 nanosecond, they create a time-bin encoded qubit. A key advantage is that time differences accumulate slowly compared to the speed of light, making the qubit robust against environmental fluctuations such as temperature drift and fiber-length changes. When the photon arrives at the next chip, the same interferometric circuit decodes the time qubit back into a spatial qubit for continued processing. This time-based encoding is proposed as a major scaling strategy for distributed photonic quantum computing across many modules and even across facilities.

Quantum Tunneling Demonstration and Scaling Up

The video showcases a bold demonstration dubbed quantum tunneling. Two chips housed in separate PSI cubes are connected by 250 meters of optical fiber running through a tunnel. In the experiment, a photon pair is generated on the first chip; the herald photon is detected to trigger a switch that connects the source line to the world line. The time-encoded qubit is then decoded on the second chip and detected by a superconducting nanowire detector. Across thousands of runs, the recovered quantum state matched the original with 99.7 percent fidelity. This result indicates that photonic quantum computing can preserve quantum information across multiple chips and long transmission distances, supporting a distributed, modular, utility-scale architecture rather than a single-chip device.

Scalability and Factory-Scale Vision

Psiquantum envisions a scalable, distributed system made of modular Mk2 cryostats, each hosting multiple photonic chips. Some chips focus on photon generation, others perform long computations, and others handle result detection. The architecture would interconnect modules through optical fibers, forming one large, facility-wide quantum computer. With deployments announced in Australia and Chicago, each site could house hundreds of Mk2 cryostats and multiple photonic chips, gradually advancing toward millions of qubits. The ultimate aim is to move beyond a single-chip laboratory setup to a practical, high-volume quantum computing platform capable of addressing real-world problems.

Conclusion

The video presents a cohesive narrative of engineering ingenuity, showing how quantum computation can be built out of light, silicon, and cutting-edge materials science. By combining a robust approach to qubit encoding with scalable generation, switching, and detection, the Psiquantum pathway offers a compelling route toward utility-scale quantum computing that leverages distributed photonic architectures rather than forcing every qubit into a single chip.