A single optical chip can measure a source of photons, opening up new quantum developments

Quantum computing: A multidisciplinary team has created an efficient method to measure qudits, a kind of photon source, on a single optical chip, using already available experimental and computational resources. Although the word ‘qudit’ may seem like a typo, this lesser-known relative of the qubit, or quantum bit, has the ability to carry more data and is more resistant to noise, two fundamental characteristics needed to improve the performance of quantum networks, quantum key distribution systems and, eventually, the quantum internet. Unlike traditional computer bits, which classify data as one or zero, qubits can contain one, zero or both values. This is due to superposition, a phenomenon that allows several quantum states to exist simultaneously.

The ‘d’ in qudit refers to the variety of levels or values that can be encoded on a photon. Conventional qubits only have two levels, but by adding more, they become qudits. Researchers at the Swiss Federal Institute of Technology in Lausanne, or EPFL, Purdue University and the US Department of Energy’s Oak Ridge National Laboratory completed the characterization of an entangled pair of eight-level qubits that formed a 64-dimensional quantum space, quadrupling the previous record for discrete-frequency modes. Their findings were published in the journal Nature Communications.

New computing era

“We have always known that it is possible to encode qudits of level 10 or 20 or even higher using photon colors or optical frequencies, but the problem is that measuring these particles is very difficult,” said Hsuan-Hao Lu, a research associate at ORNL. “This is the value of the research: we have found an efficient and innovative technique that is relatively easy to do experimentally.” The qudits are even more difficult to measure.

Despite these challenges, pairs of two qudits in the form of photons trapped in their frequencies are suitable for carrying quantum information because they can follow a predetermined path through the optical fibre without being significantly modified by their environment. “We combined state-of-the-art frequency container fabrication with light sources, then used our technique to characterize quantum entanglement of high-dimensional qudits with a level of precision never shown before,” said Joseph Lukens, Wigner Fellow and researcher at ORNL.

The researchers began their experiments by shining a laser into a micro-ring resonator, a circular device on a chip manufactured by EPFL designed to generate non-classical light. This powerful photon source occupies 1 square millimetre of space – comparable in size to the tip of a sharp pencil – and allowed the team to generate quantum frequency bin pairs. The researchers are now fine-tuning their measurement method in preparation for a series of experiments. By sending signals through optical fibre, they aim to test quantum communication protocols such as teleportation, a method for transporting quantum information, and entanglement exchange, which is the process of entangling two previously unrelated particles.