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A Potential Way to Build Next-Generation Quantum Simulators

Researchers from the University of Bristol and the Technical University of Denmark have discovered a potential new method to develop next-generation quantum simulators that integrate light and silicon micro-chips.

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

In the plan to build quantum machines that can compete and overcome traditional supercomputers in solving particular issues, the scientific community is confronting two major technological challenges.

The first is the potential to develop large quantum circuits that can process the information on a large scale, and the second is the potential to develop a huge number of single quantum particles that can both encode and propagate the quantum data through such circuits.

In order to develop an advanced quantum technology that can overcome traditional machines, both these demands must be met.

Silicon quantum photonics is a highly potential platform to deal with such issues. This technology involves generating and processing the information carried by photons—single particle of lights—in silicon micro-chips.

These devices direct and control light at the nanoscale with the help of integrated waveguides—the analog of optical fibers at the nanometer-scale.

Importantly, the production of photonic chips involves the same methods employed for developing electronic micro-chips in the semiconductor industry, facilitating the fabrication of quantum circuits at a large scale.

The research group, in the University of Bristol’s Quantum Engineering Technology (QET) Labs, has recently presented silicon photonic chips embedding quantum interferometers made up of nearly a thousand optical components, orders of magnitude higher than what was possible before a few years.

However, the main question that remained unsolved was whether these devices have the potential to produce an adequate number of photons to carry out valuable quantum computational operations. The Bristol-led study, reported in the journal Nature Physics on July 2nd, 2019, shows that this question has a positive solution.

By investigating recent technological advancements in silicon quantum photonics, the research group has shown that even small-scale silicon photonic circuits can produce and process several photons unparalleled in integrated photonics.

Actually, because of defects in the circuit like photon losses, earlier illustrations in integrated photonics have been typically restricted to experiments with just two photons generated and processed on-chip, and it was only in 2018 that four-photon experiments were described using complex circuitry.

In the research, the group demonstrated that even simple circuits can create experiments with up to eight photons—which is two times greater than the prior record in integrated photonics—by enhancing the design of each integrated component.

Furthermore, their analysis demonstrated that by increasing the circuit complexity, which is a strong potential of the silicon platform, experiments with over 20 photons can be done, a system where photonic quantum machines are anticipated to exceed the best traditional supercomputers.

In addition, the research explores possible applications for such near-term photonics quantum processors entering a system of quantum advantage.

Particularly, by reconfiguring the kind of optical non-linearity in the chip, the researchers showed that silicon chips can be employed to carry out a range of quantum simulation operations, called boson sampling issues.

For certain protocols such as the Gaussian Boson Sampling, this new demonstration is first in the world.

Moreover, the group showed that silicon quantum devices will be able to overcome industrially relevant issues, using such protocols. Above all, they demonstrate how the chemical challenge of determining the vibrational transitions in molecules experiencing an electronic transformation can be simulated on the type of devices using Gaussian Boson Sampling.

Our findings show that photonic quantum simulators surpassing classical supercomputers are a realistic near-term prospect for the silicon quantum photonics platform.

Dr Stefano Paesani, Study Lead Author, Centre for Nanoscience and Quantum Information, University of Bristol

He continued, “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 pharmaceutics and the engineering of molecular states able to generate energy more efficiently.”

The results obtained make us confident that the milestone of quantum machines faster than any current classical computers is within reach of the integrated quantum photonics platform.

Dr Raffaele Santagati, Study Co-Author, University of Bristol

While it is true that also other technologies have the capability to reach such regime, for example, trapped ions or superconducting systems, the photonics approach has the unique advantage of having the near-term applications we investigated. The photonic path, although perilous, is set, and is very much worth pursuing,” he further stated.

The project was guided by Professor Anthony Laing, Associate Professor of Physics at Bristol.

In quadrupling the number of photons both generated and processed in the same chip, the team has set the scene for scaling up quantum simulators to tens of photons where performance comparisons with today’s standard computing hardware become meaningful.

Professor Anthony Laing, Associate Professor of Physics, University of Bristol

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