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Right-click to download interview with Faraz Najafi and Dirk Englund (14.6 MB MP3)

“We would like to one day build a photonic quantum processor on a chip, and single-photon sources and detectors are crucial components for such a chip,” explains team member Faraz Najafi at MIT.

While classical computers store and process information as "bits" that can have one of two states ("0" or "1"), a quantum computer exploits the ability of quantum particles to be in "superposition" of two or more states at the same time. “While a single quantum bit (qubit) can be in two states simultaneously, two qubits can be in four states simultaneously, and so forth,” explains co-team leader Dirk Englund of MIT. “What is more, the number of states that the 'quantum registers' occupy simultaneously grows exponentially with the number of qubits in it.”

Entangled photons and hundreds of qubits

Information processing based on such quantum devices could, in principle, outperform classical computers at certain tasks, such as simulating inherently quantum mechanical processes in nature, breaking cryptographic codes or implementing highly parallel machine learning, he adds. Another important aspect of such quantum systems is that the quantum particles can also become “entangled”. Entanglement allows particles to share a much closer relationship than classical mechanics allows, so data is transferred instantaneously between entangled particles – regardless of how far apart they are.

Photons could be ideal for information processing because they can easily be entangled (compared with other physical particles) and because they can be moved around easily. Photons also travel great distances through optical fibres or even air without losing their quantum nature.

Real-world quantum computers will require up to hundreds of qubits to work because they need to go through numerous controlled quantum operations. To scale up such systems, the single photons would ideally need to be supplied deterministically – that is, one by one – and detected individually too. These photons also need to be detected efficiently.

Enter SNSPDs

Superconducting nanowire single-photon detectors (SNSPDs) are one of the most promising single-photon detectors available today. However, they are very sensitive to nanoscale defects, and only a few out of every 100 deposited on a chip using standard manufacturing techniques function properly.

Now, researchers led by Englund and Karl Berggren, also of MIT, have developed a technique in which they can build these detectors separately and then integrate functioning detectors into an optical chip. The optical chips can be fabricated separately using standard chip manufacturing techniques. Englund and Berggren teamed up with Solomon Assefa of IBM’s TJ Watson Research Center in New York for this part of the work. Their technique can be used to not only build denser and larger detector arrays, but the finished devices are also more sensitive to incoming photons. Indeed, the team succeeded in building detectors that could register 20% of incoming photons – this was an improvement of about 10 times compared with previous approaches.

Making a photonic quantum processor from the “ground up”

“Our process is about bringing two components together: the high-speed SNPSD and a photonic waveguide that channels the light onto our photonic chip,” Najafi tells nanotechweb.org. “We fabricate the SNSPD and the waveguide separately – that way, we are able to use processes routinely employed in the semiconductor industry to obtain a good waveguide.”

The researchers made hundreds of SNPSDs on thin micron-sized membranes and tested every detector individually to find out which worked the best. They then picked up these good devices and transferred them onto a waveguide under an optical microscope.

The technique allows the team to integrate large SNSPD arrays onto photonic chips made from different materials, and could help make a photonic quantum processor from the “ground up”.

Encouraged by its preliminary results, Englund and colleagues say that they are now working on larger on-chip systems that integrate single-photon sources, programmable couplings between them, and the same types of detectors for readout.

The work is detailed in Nature Communications doi:10.1038/ncomms6873.