The researchers created a hybrid chip using two different types of semiconductor technology
Light-based quantum technology could enable nearly impenetrable communication networks and computers to solve problems that are currently unsolvable. Today, generating exotic optical quantum states requires bulky equipment, which limits the scalability of any potential applications. Researchers say a new device that squeezes all the necessary components onto a chip smaller than a coin could be the solution.
Most of the eye-catching results in quantum computing have come from processors that use superconducting qubits, but companies like Xanadu and PsiQuantum are betting that encoding quantum data in photons could be a more powerful approach. Photons are also an obvious choice for quantum communication networks because they are compatible with existing fiber optic technology and their quantum states are relatively stable.
A key capability for both applications is the ability to produce entangled photon pairs, in which the quantum states of two particles are essentially linked, no matter how far apart they are. This makes it possible to link together many qubits in a quantum processor, resulting in exponential acceleration on certain problems. It is also key to connecting nodes in quantum communication networks in a way that is immune to eavesdropping attempts.
Advances in photonics have made it possible to entangle photon pairs on chips, rather than using bulky optical devices, which could greatly extend the range of the technology. But Michael Kues, a professor at Leibniz University in Hanover, Germany, said the photons themselves must still be produced by conventional lasers. "You have everything on this little chip, but you still have this big laser," he said. "That makes it a little clunky and not really mass-produced, which limits scalability."
In a new paper published April 17 in Nature Photonics, Kues and his colleagues unveiled the first photon chip that integrates all the key components needed to generate entangled photon pairs. Their approach relies on combining two different types of semiconductor technology to create a hybrid chip that can both generate laser light and convert that light into high-quality entangled photons. "Now it's all in a tiny device smaller than a euro coin," Kues said.
The first part of the chip has an optical amplifier made of indium phosphide, a material already widely used to make semiconductor lasers. The material isn't good for generating entangled photons, though, so the researchers connected that part of the chip through waveguides to a second part made of silicon nitride. It has a series of "microring resonators" - circular waveguides through which the laser is forced to travel - that gradually get smaller.
The first two rings are designed to filter out noise in the laser signal that would interfere with the quantum states the researchers are trying to create. As the light circulates around the final ring, two laser photons of different frequencies are annihilated, creating a pair of entangled photons. The chip consumes about 3 watts of power and is capable of producing 8,200 pairs of entangled photons per second at optical communication wavelengths (1,550 nanometers is the most commonly used wavelength for optical fibers).
"All the pieces of this work have been demonstrated before, but, integrating all of them together is an achievement," said Amr Helmy, a professor of photonics at the University of Toronto. In particular, he points to the integration of innovative filtering mechanisms on the same chip as other components as a major achievement. However, Helmy says this appears to have come at the expense of performance, with the brightness of the output and the quality of the quantum states falling short of previous work.
Still, researchers say the ability to shrink the size of a quantum light source 1,000 times could help take the technology out of the lab and make it easier to deploy in the real world. In addition to its more obvious uses in quantum computing and quantum networking, Kues says the compact shape may also be useful for certain quantum sensing applications.
However, although both semiconductor technologies the team is relying on are already in use in industry, the process of integrating the two can be a potential barrier to mass production. Currently, two parts of the chip have to be manually aligned to ensure waveguides match, which would be a difficult process to scale up, Kues said. "We've now shown that the feature works, but there's still a lot of research to be done to actually build it," he added.
Source: Laser Net