Solving the quantum observation problem

Dr Hird’s research sits at the intersection of quantum communication, computing and sensing, all built around photons, “tiny packets of light”. His core focus is a class of devices known as quantum memories, which can store a photon and insulate it from the constant interference of the environment. Most people know the thought experiment used to explain quantum physics — in which Schrödinger's theoretical cat is both alive and dead at the same time until someone opens the box to check. In the same way, in quantum systems, the surroundings are constantly “trying to peek”, says Dr Hird, collapsing delicate superpositions in the way that opening Schrödinger’s cat box destroys the paradox.

A recent twist in Dr Hird’s work is to embed these quantum memories inside an interferometer – a well‑established tool in physics used to detect minuscule changes. “A famous example of an interferometer is LIGO,” he notes, referencing the large-scale observatory built to detect cosmic gravitational waves. In Dr Hird’s case, the interferometer is repurposed to do something conceptually simple but technically formidable: confirm that a single photon has passed a given point without destroying it in the process.

Normally, detecting a photon is an act of destruction. “We’re very good at detecting photons,” Dr Hird says, “but the normal way we detect photons is we absorb them and we can’t use them again.” His device, by contrast, “enables you to confirm that a successful outcome has happened (the photon exists and has passed the point), and allows you to keep that state safe without opening the box.”

This so‑called quantum non‑demolition measurement has two immediate applications. One is in secure quantum communications, especially channels such as satellite‑to‑ground links where a large percentage of data is lost or distorted en route. Current UK satellites sending single photons to ground stations see “less than 1%” of photons successfully received. A non‑destructive checker in the channel could help synchronise and stabilise such systems without sacrificing the very photons that carry the secure information.

The second lies inside quantum computers themselves, where many operations succeed only probabilistically. Dr Hird likens it to rolling dice and trying to get all sixes at once. His device lets you recognise and “hold on” to each successful roll while you wait for the others to fall into place, offering “verification of success of subsystems” without collapsing the underlying quantum state.

On paper, the work goes further still. In the course of writing up the device, Dr Hird realised that by feeding in a particular kind of quantum state, the output would be a novel type that, to his knowledge,  “has never been realised before”. Dr Hird and his team would be the first to describe the mechanism for producing this state, and exploring applications of it opens up a new line of research.

All of this is anchored in the Birmingham Quantum Hub, one of the UK’s five national quantum hubs, which Dr Hird describes as “incredibly strong, particularly in sensing, timing, and imaging”. The programme has been running for more than a decade, with partners ranging from defence contractors and national infrastructure players to telecoms operators.

If the Quantum Technology group at the University of Birmingham provides the scientific ecosystem, the University’s Enterprise function helps make the work investable. Dr Hird has filed an initial patent application and is now updating it after discovering “about three other applications” for the device while preparing his paper. He is frank that this work on securing Intellectual Property rights would have been “not quite completely impossible, but incredibly hard work” without the help of the business development team at Enterprise.

The team have also assisted in setting up protected discussions with organisations such as the National Physical Laboratory and prospective commercial partners on how Dr Hird’s device might fit into their roadmaps. Dr Hird says that support has meant he has been able to “advance the fundamental science and deepen our understanding, while simultaneously exploring new avenues around industry applications.” His motivation to commercialise is straightforward: “Ultimately, as a scientist, I want people to use my research.”

Quantum physics may have a long theoretical history but today’s new discoveries in the field, like Dr Hird's, look set to rapidly find real-world applications.