Simplifying high-dimensional quantum information processing using photons

A team of researchers has developed a technique that makes high-dimensional quantum information encoded in light more practical and reliable.

This advancement, published in Physical Review Letters, could pave the way for more secure data transmission and next-generation quantum technologies.

Quantum information can be stored in the precise timing of single photons, which are tiny particles of light.

However, traditional methods can require extremely complex and unstable measurement techniques to resolve these arrival times, making practical applications unwieldy and tedious.

The new study, jointly led by Griffith University researchers Dr. Simon White and Dr. Emanuele Polino from the Quantum Optics and Information Laboratory (QOIL), within Griffith’s Queensland Quantum and Advanced Technologies Research Institute (QUATRI), introduces a simpler, more stable approach using a quantum effect called Hong-Ou-Mandel (HOM) interference, which precisely measures the timing of photons without the usual technical headaches.

HOM interference occurs when two identical photons meet at a beam splitter and quantum effects cause them to behave in a special way.

“Think of it as the universe’s version of an awkward handshake that actually achieves something useful,” Dr. White said.

This effect has been used in many quantum applications, but now the researchers successfully applied it to time-bin quantum encoding, which is a method of storing information in the time at which photons arrive.

“Photons are ideal carriers of quantum information, and encoding information in a photon’s arrival time is a great way to send a quantum message,” Dr. White said.

“We show how to simplify the measurement of these messages so detectors don’t need to resolve the individual time of arrival; instead we only need to observe the interference.”

To further enhance this method, the team combined HOM interference with a technique known as a quantum walk, which describes the movement of single photons over different paths in time.

This combination allows for the generation and measurement of high-dimensional quantum signals called qudits.

Unlike classical bits, which can only be 0 or 1, or regular qubits, which can be in combinations of 0 and 1 simultaneously, qudits are units of quantum information taking more than two possible values.

This feature can significantly increase the amount of information that can be processed and transmitted, and can help ensure secure communications can be trusted.

“Through optical experiments, our team demonstrated the reliability of both the state generation and measurement techniques, which are scalable beyond two dimensions, and we achieved an impressive fidelity of over 99%,” Dr. Polino said.

Additionally, the Griffith research team successfully highlighted how their protocol generated quantum entanglement—a key quantum phenomenon where properties are strongly correlated—between different properties of single photons.

“Entanglement is a key property of quantum mechanics,” Dr. Polino said.

“Demonstrating the presence of entanglement is important as it gives insight into how these quantum properties can be used in the future.”

“Sending secure quantum signals is a difficult task, but encoding using time-based qudits makes that task easier and more robust,” Dr. White concludes.

“By improving the stability, versatility, and simplicity of time-bin quantum encoding, this breakthrough brings us closer to scalable quantum technologies.

“This work helps us better see the foundational properties of quantum particles and opens new possibilities for secure communication, advanced quantum simulation, and real-world quantum applications. And honestly, we think that is pretty important.”