Quantum computers promise breakthroughs in chemistry, communication and materials. In principle, such a computer could solve in a few hours what today’s fastest supercomputers would need millions of years for.
The crucial ingredient is entanglement – a quantum connection that makes particles behave as a single whole, no matter how far apart they are. Quantum computers work only because they exploit this entanglement and make their particles cooperate as one system.
“There are many ways to build such a machine. Most approaches use trapped atoms or ions, which can entangle particles more directly – but they are slow and difficult to scale,” says Peter Lodahl, Professor and Group Leader of the Quantum Photonics Group at the Niels Bohr Institute, University of Copenhagen, and founder and Chief Quantum Officer at Sparrow Quantum ApS.
A new study in Nature Communications shows how a fusion method for photonic quantum computers can overcome this barrier.
Photons are fast, easy to guide in optical circuits and ideal for sending information over long distances – but they do not interact in a stable or predictable way, which makes it difficult to entangle them. At the University of Copenhagen, it took a cross-disciplinary team – from nanofabrication to quantum optics – to make it work in practice.
“We use a single electron spin in the quantum dot to control the emitted photons. The first photon is sent through a fibre loop so that it can meet the next one when both reach a beamsplitter,” Lodahl explains. “In this way, we fuse the entanglement between the electron’s quantum states – its magnetic spin – across time.”
Even more striking, the team showed that this process keeps information alive longer than the spin’s natural memory. As Lodahl puts it: “You can think of it as a kind of quantum memory – information is stored in the light for a moment and then transferred back into the spin. The information is essentially teleported into the new spin state, so it continues beyond the original particle.”
Large quantum networks can be built using just one light source. In today’s all-optical approaches, thousands of separate photon sources may be needed to scale up. “Here,” Lodahl emphasises, “we can achieve the same with a single quantum dot that we simply reuse again and again.”
Quantum computers only work if their particles act as a team. But in photonics, where the players are photons, that is a nightmare. Photons are hard to control: when they meet, they sometimes become entangled and sometimes not – a probabilistic, random process governed by the laws of quantum mechanics.
“That makes it extremely difficult to get two of them to do anything together reliably – and these two-particle gates are the basic building blocks of quantum computers,” says Peter Lodahl.
Because of this, the researchers had to rethink the entire architecture of photonic quantum computing. A promising alternative is fusion-based photonic quantum computing. Instead of trying to realise deterministic two-qubit photonic gates, small entangled clusters of photons are created and then fused into larger networks.
The problem? In most laboratories, creating these initial entangled states is still probabilistic – like rolling dice each time. Sometimes it works, often it does not. “It is a bit like building a house of cards where each new card only sticks one time out of ten,” Lodahl explains.
That makes scaling painfully slow. As the Copenhagen team points out, “the real challenge is to generate these first resource states reliably. Relying entirely on probabilistic sources greatly increases the amount of hardware required, and it quickly becomes unmanageable.” Researchers around the world have spent years trying to overcome this bottleneck – and this breakthrough shows a new way forward.
The breakthrough comes from using quantum light sources – emitters – tiny devices such as semiconductor quantum dots that can emit entangled photons on demand. No dice rolls, no maybes: every time you press the button, you get the pair you need.
Until now, researchers have only been able to entangle photons created at the same time. But that would require juggling many light sources at once – a task that quickly turns into a technical circus.
This study shows a new path: temporal fusion. Instead of using many emitters, the same one is reused again and again, fusing photons created at different moments. The result is an unusual form of time-separated entanglement – a way of stitching light together across moments – that makes scaling far simpler.
As Peter Lodahl puts it: “This brings theory into practice. Reliable light sources and temporal fusion together may be the crucial ingredients for building truly large-scale photonic quantum technologies.”
He also notes that the result builds on many years of teamwork – from constructing the experimental set-up to refining the protocols.
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