By exploring complex integrated circuits, photonic states can be generated and processed at larger scales. Credit: Dr Stefano Paesani, University of Bristol

Scientists find a new way to build on-chip quantum simulators

torsdag 04 jul 19


Leif Katsuo Oxenløwe
Professor, Gruppeleder
DTU Fotonik
45 25 37 84


Yunhong Ding
DTU Fotonik
45 25 65 87

Nature Physics

Generation and sampling of quantum states of light in a silicon chip’ by S. Paesani, Y. Ding, R. Santagati, L. Chakhmakhchyan, C. Vigliar, K. Rottwitt, L. Oxenløwe, J. Wang, M. Thompson and A. Laing in Nature Physics.

Scientists from DTU and the University of Bristol have found a promising new way to build the next generation of quantum simulators combining light and silicon micro-chips.

On the path to develop quantum machines able to surpass the problem solving capabilities of  classical supercomputers, the scientific community is facing two main technological challenges.

The first is the ability to process information on a massive scale, requiring large quantum circuits, and the second is the ability to create a large number of single quantum particles that can be individually encoded and carry the quantum information through such circuits.

Both these requirements need to be satisfied in order to  enter into the regime often referred to as quantum supremacy, where quantum technology can solve problems classical machines can not.

A very promising platform for tackling such challenges is silicon quantum photonics. In this platform, the information is carried by photons, single particle of lights, which are generated and processed in silicon micro-chips. Fabrication of photonic chips relies on the same techniques used for fabricating electronic micro-chips in the semiconductor industry, making the fabrication of quantum circuits on a massive scale realistic.

A collaboration between the DNRF Centre for Silicon Photonics for Optical Communications (SPOC), and University of Bristol’s Centre for Nanoscience and Quantum Information / Quantum Engineering Technology (QET) Labs, recently arrived at a demonstration of silicon photonic chips embedding quantum interferometers composed of almost a thousand optical components, an order of magnitude higher that what was possible just a few years ago.

However, the big question that remained unanswered was if these devices were also able to produce the number of individual photons large enough to perform really useful quantum computational tasks. The research by the SPOC centre and the  QET Labs, published in the journal Nature Physics, demonstrates that this question has a positive answer. 

Performing useful quantum computational tasks
By exploring recent technological developments in silicon quantum photonics, the team has demonstrated that even small-scale silicon photonic circuits can generate and process a large number of individual photons, unprecedented in integrated photonics.

In fact, due to imperfections in the circuit such as photon losses, previous demonstrations in integrated photonics have been mostly limited to experiments with only two photons generated and processed on-chip, and only last year, four-photon experiments were reported using complex circuitry.

“In this work, by improving the design of each integrated component, we have shown that even simple circuits can produce experiments with up to eight photons, doubling the previous record in integrated photonics. Moreover, our analysis shows that by scaling up the circuit complexity, which is an essential capability of the silicon platform, experiments with more than 20 photons are possible. With more than 20 quantum particles, in this case photons, we’re entering a regime where photonic quantum machines are expected to surpass the best classical supercomputers”, explains Leif K. Oxenløwe, Professor at DTU Fotonik and leader of the SPOC centre.

The study also investigates possible applications for such near-term photonic quantum processors with a clear quantum advantage.

In particular, by reconfiguring the type of optical nonlinearity in the chip, they demonstrated that silicon chips can be used to perform a variety of quantum simulation tasks, known as boson sampling problems.

For some of these protocols, for example the Gaussian Boson Sampling, this new demonstration is a world-first. 

The team also demonstrated that, using such protocols, silicon quantum devices will be able to solve industrially relevant problems. In particular, they show how the chemical problem of finding the vibrational transitions in molecules undergoing an electronic transformation can be simulated on their type of devices using Gaussian Boson Sampling. 

Lead author Dr. Stefano Paesani from the University of Bristol’s QET Labs, says:

“Our findings show that photonic quantum simulators surpassing classical supercomputers are a realistic near-term prospect for the silicon quantum photonics platform.The development of such quantum machines can have potentially ground-breaking impacts on industrially relevant fields such as chemistry, molecular designing, artificial intelligence, and big-data analysis. Applications include the design of better pharmaceuticals and the engineering of molecular states able to generate energy more efficiently.”

Senior Researcher Dr. Yunhong Ding from DTU’s SPOC centre supervised the project. He elaborates:

“It is amazing that silicon photonic integrated chips are so powerful for quantum applications. Our silicon quantum photonic chip makes us confident that quantum supremacy is within reach in the near future based on our unique silicon photonic integrated platform.
There are competitive technologies, such as superconducting systems. So far there is no winner yet. The photonic approach has some unique advantages, and superconducting systems others, which make it all the more promising and exciting. For instance, being associated with the widespread silicon integration technology, photonic chips are very promising for a lot of quantum applications.”