The next generation quantum sensor

Monday, March 4, 2019

Researchers at the Institute for Quantum Computing (IQC), led by faculty member Michael Reimer, have developed a new quantum sensor based on semiconductor nanowires that can detect single particles of light with high speed, timing resolution and efficiency over an unparalleled wavelength range, from ultraviolet to near-infrared.

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The world-first device outperforms existing commercial technologies and promises enormous possibilities for sensing applications including dose monitoring for cancer treatment, 3D imaging, quantum communication and remote sensing.

“A sensor needs to be very efficient at detecting light. In applications like quantum radar, surveillance, and nighttime operation, very few particles of light—called photons—return to the device,” says Reimer, also an assistant professor in the electrical and computer engineering department at the University of Waterloo. “In these cases, you want to be able to detect every single photon coming in.”

Artist's representation of the interaction of incident single photon pulses and a tapered semiconductor nanowire array photodetector. Each nanowire is shaped in an optimal way to strongly absorb the incoming light. A blue glow shows an artistic impression of the absorption process and avalanche effect in the nanowire p-n junction.

Artist's representation of the interaction of incident single photon pulses and a tapered semiconductor nanowire array photodetector. Each nanowire is shaped in an optimal way to strongly absorb the incoming light. A blue glow shows an artistic impression of the absorption process and avalanche effect in the nanowire p-n junction.

The next generation quantum sensor designed in Reimer’s Quantum Photonics Devices Lab is so fast and efficient that it can absorb and detect a single photon and refresh for the next one within nanoseconds.

The researchers created an array of tapered nanowires that turn incoming photons into electric current that can be amplified and detected. The semiconducting nanowire array achieves its high speed, timing resolution and efficiency thanks to the quality of its materials, the number of nanowires, doping profile and the optimization of the nanowire shape and arrangement.

The researchers used advanced modelling software to optimize the shape of the nanowires. The tapered design allows for high bandwidth detection because shorter wavelengths of light are absorbed at the top of the nanowire and longer wavelengths are absorbed near the bottom. Their design also allows photons to be absorbed right at the positive-negative (p-n) junction of the nanowire that generates the electric field necessary to amplify the absorbed photon. Absorption at the p-n junction means less time travelling along the nanowire and higher timing resolution, which is necessary for detecting each individual photon with high timing accuracy.

Animation representing photon absorption in a nanowire. Animation created by Brad van Kasteren.

The team’s collaborators at the Eindhoven University of Technology in the Netherlands found that etching the nanowires resulted in higher optical quality than growing them from the bottom-up. These nanowires allow for a very low dark current—the default noise before photon absorption—meaning that the researchers achieve an excellent signal-to-noise ratio.

Reimer and his team optimized the arrangement of the nanowire array to achieve a fast response time.  “We found that if the nanowires were too far apart, light would just pass through undetected,” explains Reimer. “If they were too close together, then they acted as a bulk material and reflected the light. With the right spacing and the shape, the array doesn’t reflect and instead absorbs all the light.”

Remote sensing, high-speed imaging from space, acquiring long-range high-resolution 3D images, quantum communication, and singlet oxygen detection for dose monitoring in cancer treatment are all applications that could benefit from the kind of robust single photon detection that this new quantum sensor provides.

Single photon detectors using superconducting nanowires have been developed to meet many of the demands of these applications, but their portability is limited due to their cryogenic operating temperatures. This constraint relegates these devices to the lab.

Single photon avalanche photodiodes currently on the market overcome the problem of temperature, but require trade-offs between timing resolution and efficiency. The new array of nanowires developed by Reimer and his team require no such compromises. They detect light with high efficiency and high timing resolution, and achieve these results at room temperature. Reimer notes that operation beyond the lab will likely benefit from a simple and portable Peltier cooler lowering the temperature of the device somewhere below zero, a far cry from the roughly three Kelvin required by available superconducting devices.

Unlike commercially available single photon detectors that only absorb a narrow bandwidth of light, this new sensor can detect a broad spectrum from ultraviolet (UV) to near infrared (IR).  Reimer predicts this range can be broadened even further towards telecommunication wavelengths.

“This device uses Indium Phosphide (InP) nanowires. Changing the material to Indium Gallium Arsenide (InGaAs), for example, can extend the bandwidth even further while maintaining performance,” says Reimer. “It’s state of the art now, with the potential for further enhancements.”

Absorption spectrum of InP nanowire array and InGaAs nanowire array showing near-unity absorption over an unprecedented wavelength range.

Absorption spectrum of InP nanowire array and InGaAs nanowire array showing near-unity absorption over an unprecedented wavelength range.

This extended range will allow detection of telecommunication and IR wavelengths, expanding the range of feasible commercial applications even further.

With an efficient, high speed, broadband, room temperature quantum sensor at their fingertips, what could be next for these researchers?

Once the prototype is packaged with the right electronics and portable cooling, the sensor is ready for testing beyond the lab.  “A broad range of industries and research fields will benefit from a quantum sensor with these capabilities,” says Reimer. “This efficient, high speed, broadband, room temperature quantum sensor opens the door to exciting new possibilities for photon detection.” 

The nanowire array was designed and fabricated in the Netherlands by collaborators at the Eindhoven University of Technology, and tested in the Quantum Photonic Devices lab at IQC. Tapered InP nanowire arrays for efficient broadband high-speed single photon detection was published in Nature Nanotechnology March 4. This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund (CFREF).

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