Ultra-thin devices light the way to miniaturised entangled photon sources

Stacked layers of very thin materials offer a novel way of creating entangled photon pairs that could be integrated into future PICs, paving the way for quantum computing at a much more compact scale.

By Leevi Kallioniemi, PhD student, and Xiaodan Lyu, research fellow, Nanyang Technological University, Singapore

Quantum entanglement has astounded and divided physicists since it was first conceived last century.

One of many counterintuitive effects predicted by quantum mechanics, the phenomenon involves two particles becoming linked, or entangled, meaning that a change in one particle’s state will inevitably and immediately change the state of the other, even if they are infinitely far apart. Many pioneers of quantum theory initially rejected this idea.

Most famously, Albert Einstein described it as “spooky action at a distance,” highlighting how nonsensical it seemed.

However, quantum theory has held up to scientific scrutiny; experiments conducted in the past century have repeatedly confirmed its predictions, and quantum entanglement has been demonstrated with many particles, including pairs of electrons and pairs of photons.

(From left) Gao Weibo, Lyu Xiaodan, Liu Zheng and Leevi Kallioniemi are part of the NTU Singapore team that found a new way to produce entangled pairs of photons with very thin materials. (Credit: NTU Singapore)

Not only has the phenomenon been verified, but it is also now vital to quantum computing. This field of research aims to create powerful computers that could supercharge efforts to tackle all kinds of complex problems, from drug discovery to climate change. Yet the technology is still developing, and researchers face many hurdles in realising its potential benefits.

Now, our team at Nanyang Technological University, Singapore (NTU Singapore) has made an advancement that could pave the way for significant progress in quantum computing, by offering an innovative method of creating entangled photons.

While many kinds of particles can be entangled, photons have the advantages that they are stable and naturally tend to travel across distances, lending themselves to signal transmission. For a pair of photons to be entangled, both must vibrate in sync when produced. Entangling two particles of light can be a relatively simple process in a modern laboratory.

Two thin flakes of niobium oxide dichloride stacked on each other and photographed under a light microscope. One flake’s crystalline grain (grey flake) is positioned perpendicularly to the grain of the other flake (green flake). (Credit: NTU Singapore)

Conventionally, researchers create entangled photons at room temperature by shining a laser on crystals that are a few millimetres thick. The crystals interact with photons from the laser and then produce pairs of lower-energy photons, a process called spontaneous parametric down conversion. Scientists then use optical gear to ensure the generated photons in a pair remain linked. But for quantum applications that may need to manipulate these particles in small devices, such a set-up is too large. Optical equipment can also be cumbersome to align and adds further bulk and complexity to the set-up.

However, our research team, led by Gao Weibo, a professor in NTU’s School of Electrical & Electronic Engineering and School of Physical & Mathematical Sciences, has made a discovery that addresses this problem. We have produced entangled photons from crystalline materials that are 1000 times thinner than the crystals used in current state-of-the art devices. This means that devices for quantum applications, such as those for quantum information and photonic quantum computing, could get simpler and more compact in the future.

A blue laser set-up for generating entangled pairs of photons in NTU Singapore’s experiments. (Credit: NTU Singapore)

The best of both worlds
This is not the first time that scientists have sought to replace bulky crystal sources of entangled photons with thinner materials. But making sources of linked photons smaller comes up against a major issue: as the materials get thinner, they produce photons at a lower rate. In fact, the rate falls so much that these thin materials become far less useful for computing applications.

A new crystalline material called niobium oxide dichloride (NbOCl₂) has the potential to resolve this. Exhibiting unique optical and electrical properties, recent research suggests that even when this material is very thin, it can produce photon pairs efficiently. But there is still another problem: the pairs of photons generated by a single flake of NbOCl₂ are not entangled by default, so they cannot be used for quantum applications.

Gao’s idea for a solution to this was inspired by a longstanding method published in 1999 for producing linked photons using thick and bulky crystalline materials. The technique involves stacking two flakes of thick crystals, aligned so that their crystalline grains are perpendicular to each other.

Shining a laser at the crystals stimulates the production of photons in each flake, but in different polarisations, due to the different alignments of the flakes. This results in the photon pairs being entangled in terms of their polarisation; determining the polarisation of one particle will immediately determine that of the other.

Gao hypothesised that the same process would produce entangled photon pairs from NbOCl₂ flakes. If it worked, this would dramatically scale down the core components of the set-up; the NbOCl₂ flakes have a combined thickness of just 1.2 micrometres – 80 times thinner than a strand of hair, and around 1000 times thinner than the bulky, millimetres-thick crystals traditionally used.

To achieve this set-up with NbOCl₂ flakes, Gao drew on a technique called van der Waals engineering, which involves stacking thin sheets of materials. These sheets are held together by weak natural interactions called van der Waals forces, which act between the thin layers. Researchers can harness these weak forces to stack multiple thin layers of the materials at different angles relative to each other to alter their optical properties and create new ones.

Besides shrinking the size of the crystals themselves, this substitution also eliminates the need for external optical equipment. After photons are created in a pair in the traditional set-up, the way they travel inside the thick crystal flakes can cause them to vibrate out of sync. Extra optical instruments are therefore needed to keep the particles synchronised and ensure the pair remain linked.

But Gao realised that, since the NbOCl₂ crystal flakes used would be much thinner than the bulkier crystals from previous studies, the photon pairs generated would travel a smaller distance inside the NbOCl₂ flakes.
As a result, the photons would be more likely to remain in sync with each other, and additional optical equipment would no longer be needed, helping to simplify and reduce the bulk of the set-up.

To test his hypothesis, Gao collaborated with Liu Zheng, a professor from NTU’s School of Materials Science & Engineering, and led a research team to conduct a series of experiments.

The results proved Gao’s theory, finding that the stacked NbOCl₂ flakes produced photon pairs that behaved very similarly to perfectly entangled photons; observations showed that they had a fidelity – a measurement of how closely they resemble an ideal entangled state – of 86 percent. In future, these entangled photons could be used as photonic quantum bits, or qubits, for quantum computing.

Potential for quantum computing
Quantum computers promise to make complex calculations and discern patterns in large sets of data much more quickly than conventional computers. They could take just minutes to complete computations that would take today’s supercomputers millions of years.

Such processing capabilities have the potential to transform how we tackle many pressing challenges facing humanity and accelerate the discovery of solutions. For example, we could harness this computing power to make better sense of complex weather phenomena and find new antibiotics much faster than before.

According to theory, quantum computers should be able to do this because they can perform multiple calculations at once, instead of working through them one at a time like standard computers. This ability is thanks to tiny switches called qubits, which are quantum entangled particles that can be in both the on and off positions simultaneously. Standard computers, by contrast, have switches that can only be in either the on or off state at any time, but not both.

Currently, many approaches to building quantum computers use electrons as qubits. But for this to work, the electrons need ultra-low temperatures approaching the coldness of outer space. Maintaining such conditions requires a lot of energy and resources, making these efforts costly, more complex, and less accessible.

Photons are promising alternative options for qubits. Since they can be produced in entangled pairs at room temperature instead of needing extreme conditions, photonic qubits could make quantum computing cheaper, more energy efficient, and more practical.

Photonics specialist Sun Zhipei, a professor at Finland’s Aalto University and co-principal investigator at the Research Council of Finland’s Center of Excellence in Quantum Technology, said the new technique for producing entangled photons “is a major advancement, potentially enabling the miniaturisation and integration of quantum technologies.”

Sun, who was not involved with the research, added: “This development has potential in advancing quantum computing and secure communication, as it allows for more compact, scalable and efficient quantum systems.”

Furthermore, entangled photons could enable instant communication, since these particles of light are like synchronised clocks that show the same time regardless of the distance between them.

Future directions
Although our advancement is a significant step towards using photons as qubits, more breakthroughs are needed before we can realise its potential in photonic quantum computing devices. For one thing, the technologies to integrate very thin crystalline materials into photonic quantum computer chips need to be developed. Our current method of producing linked photons must also be improved to a level that is suitable for practical applications.

Going forward, our team at NTU is planning to optimise the set-up so that it can create entangled photons at a faster rate than we have achieved so far, in order to make it useful for quantum computing. Boosting the rate of production will likely improve the fidelity of the light particles too, so that they behave even more closely to ideally entangled photons.

We are exploring whether we can increase the number of entangled photons generated by etching small patterns and grooves on the surface of the NbOCl₂ crystal flakes. The team is also considering stacking other materials with the flakes, such as 3R-phase transition metal dichalcogenides, which are emerging materials reported to be capable of generating photon pairs efficiently.

We could also improve the fidelity of the entangled photons by further adjusting our set-up, for example by changing the thickness of the flakes. Another factor we are planning to investigate is how much the flakes’ crystalline grains line up with the direction of the laser light’s polarisation.

While there is more work to be done, this discovery sets out an exciting vision for miniaturised, room-temperature sources of high-quality, entangled photon pairs that could underpin the quantum computers of the future.