Editorial Feature

What to Know About Photonic Quantum Computers

Quantum computers are the next-generation computing devices that leverage quantum effects to perform calculations that are either not feasible or extremely time-consuming on current digital supercomputers.

What to Know About Photonic Quantum Computers

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Photonic quantum computers are a subset of quantum computing methods that use photons as the fundamental data units.

Quantum Computing Landscape

A lot of businesses, such as Google, IBM, and several startups, rely on superconducting qubits. The primary problem with these devices is their integration because they can be quite huge and cumbersome. A big disadvantage of this method is that it must be cooled close to zero Kelvin in order to function, which makes scaling the system up to a higher number of qubits extremely difficult.

Other qubit platforms, such as cold neutral atoms and trapped ions, also face challenges in complex laser cooling architectures. On the other hand, there are photonic quantum computers, which use even smaller particles of light to store and manipulate the information.

Quantum Processing With Photons

A photon can also exist in a superposition of two states, that is both zero and one at the same time, necessary for qubit generation. Typically, qubits are initiated by a bulky external laser. To perform operations, mirrors and phase shifters are used to create entanglement between photons. This entanglement is the source of difficulty in developing smaller and more portable quantum computers.

Photon-based computers are much more adaptable to their surroundings and do not interact with interference from environmental fluctuations as much as those based on electrons. This allows them to operate at room temperature without the need for sophisticated cooling methods. This is also a major plus in terms of scaling, as it makes it much easier to scale factorial quantum computers to larger numbers of qubits.

Traditionally, the equipment used for photonic manipulation relied on huge lasers that were bulky and numerous large beam splitters to generate quantum states in order to develop photonic quantum computers. Bulky devices make it difficult to scale, and eventually, the objective is to develop a device that fits in our pocket or on the table, not a computer that requires the entire lab.

Photonic Quantum Computing on a Chip

Recent research advancements have revealed ground-breaking photonic quantum computers. A new photonic chip that runs at ambient temperature has been introduced, which is a significant step towards creating a portable and scalable quantum computer.

The photonic quantum computer chip combines all necessary features into a tiny, one-centimeter-square chip.

With this photonic chip, researchers were able to integrate all the complex optics on the chip. This includes a laser for generating pairs of photons for entanglement, micro-rings for control and manipulation and waveguides for transport of photons. For example, the size of the laser was reduced to 1000 times that of an external bulky device. The laser in the chip is made of indium phosphide, while the filters are made of silicon nitride.

Once the light is generated and quantum states are established, a series of components within the micro-ring structure is used to filter noise from the system. The chip generates around 3 W of power and is capable of entangling up to 800 pairs of photons per second. This new technology is promising in terms of negating the challenges of integration.

As an example, a fully functioning quantum computer based on such a chip can complete a calculation task in just 36 microseconds—a minuscule fraction of a second—whereas the fastest supercomputers available today would require 9,000 years to complete the same operation. This staggering difference is a window into the potential power technology based on quantum mechanics.

Other Photonic Quantum Computing Developments

A Dutch firm associated with the inception of QuiX Quantum recently showed a 20-qubit photonic chip, and in 2022, the Canadian company Xanadu proposed a 216-qubit photonic quantum computer chip. These are examples of basic photonic quantum computing chips currently available. They emphasize that their qubits are squeezed states of photons that are essentially optimized phases, and they employ a four-layer technology to incorporate the quantum seed and then act on it.

The Canadian government invested 40 million Canadian dollars in the company earlier this year for the sampling method. Although the technology can be scaled up to a million cubits, it has at least made the photonic quantum computer available for use in the cloud for everyone. Although this is far behind IBM's 400 or perhaps soon-to-be thousands of chips, the delay may not matter too much as further developments enter the fray.

This is also what the company Psi-Quantum is pursuing by partnering with GlobalFoundries, a leading semiconductor producer, to manufacture chips for photonic quantum computing. Psi-Quantum wants to have the facility for chip fabrication in place by the middle of the decade and then have the million-qubit quantum processor shortly after that.

German research efforts in photonic quantum computing are also growing. A public-private collaboration of 14 organizations is working on a project titled PhoQuant to construct a photonic quantum computer with up to 100 qubits by 2026.

Challenges and Advantages

The primary problems of quantum computing include compact, scalable, resistant to noise, and many other advantages over other forms of computing. These are the things that make technology so alluring. Current fiber networks, based on the communication band photons, can be integrated with quantum technology, allowing the construction of computers and potentially creating a quantum internet that will be far more powerful and safe than any of the many other established current uses.

Photons can be shifted in frequency using non-linear optical techniques that suit the platform to be transformed into the already established communications infrastructure.

Scalability is made possible by developed infrastructure for manufacturing devices. Quantum computing, especially scalable, portable, and functional devices like those made from photonic chips, can eventually create a better world by simulating the behavior of complex biological systems and cell organs with great accuracy. The pharmaceutical industry and the financial industry are already employing quantum computing routines to speed up better identification and projections. The next decade of quantum computing developments stands to deliver transformational capabilities in many sectors.

What to Know About Silicon Nitride in Quantum Computers

References and Further Reading

Mahmudlu, H., Johanning, R., van Rees, A. et al. Fully on-chip photonic turnkey quantum source for entangled qubit/qudit state generation. Nat. Photon. 17, 518–524 (2023). https://doi.org/10.1038/s41566-023-01193-1

D. Smith, C. Taballione, M. C. Anguita, M. De Goede, P. Venderbosch, B. Kassenberg, H. Snijders, J. P. Epping, R. van der Meer, P. W. H. Pinkse, H. van den Vlekkert, and J. J. Renema, "A Universal 20-mode Quantum Photonic Processor in Silicon Nitride," in Quantum 2.0 Conference and Exhibition, Technical Digest Series (Optica Publishing Group, 2022), paper QW4B.2.

Madsen, L.S., Laudenbach, F., Askarani, M.F. et al. Quantum computational advantage with a programmable photonic processor. Nature 606, 75–81 (2022). https://doi.org/10.1038/s41586-022-04725-x

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Written by

Ilamaran Sivarajah

Ilamaran Sivarajah is an experimental atomic/molecular/optical physicist by training who works at the interface of quantum technology and business development.

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