World’s smallest quantum light sensor on a silicon chip

World’s smallest quantum light sensor on a silicon chip

Technology News |
By Nick Flaherty

Researchers at the University of Bristol have developed the world’s smallest room temperature quantum light sensor onto a silicon chip.

The quantum photonic sensor circuit occupies 80 by 220 micrometres on a commercially available 250nm BiCMOS process.

“These homodyne detectors pop up everywhere in applications across quantum optics,” said Professor Jonathan Matthews, who led the research and is Director of the Quantum Engineering Technology Labs at Bristol. “They operate at room temperature, and you can use them for quantum communications, in incredibly sensitive sensors — like state-of-the-art gravitational wave detectors — and there are designs of quantum computers that would use these detectors.”

The chip was designed and characterized in-house with fabrication outsourced to the Leibniz Institute for High Performance Microelectronics (IHP) using the SG25H5_EPIC process with  germanium-based photodiodes with f3dB > 60 GHz and vertically integrated heterojunction bipolar transistors (HBTs) with a specified transition frequency fT = 220 GHz and a breakdown voltage of 1.7 V. 

In 2021 the Bristol team showed how linking a photonics chip with a separate electronics chip can increase speed of quantum light detectors — now with a single electronic-photonic integrated chip, the team have further increased speed by a factor of 10 whilst reducing footprint by a factor of 50.

The performance is enabled by monolithic electronic-photonic integration, which goes below the capacitance limits of devices made up of separate integrated chips or discrete components. It exceeds the bandwidth of quantum detectors with macroscopic electronic interconnects, including wire and flip chip bonding.

“The key to measuring quantum light is sensitivity to quantum noise,” said researcher Dr Giacomo Ferranti. “Quantum mechanics is responsible for a minute, fundamental level of noise in all optical systems. The behaviour of this noise reveals information about what kind of quantum light is travelling in the system, it can determine how sensitive an optical sensor can be, and it can be used to mathematically reconstruct quantum states. In our study it was important to show that making the detector smaller and faster did not block its sensitivity for measuring quantum states.”

The efficiency of the detector needs to improve, and there is work to be done to trial the detector in lots of different applications. We measure a 15.3-gigahertz 3-decibel bandwidth with a maximum shot noise clearance of 12 decibels and shot noise clearance out to 26.5 gigahertz, when measured with a 9–decibel-milliwatt power local oscillator. 

A 1550nm continuous-wave tuneable laser is amplified with an erbium-doped fibre amplifier is used as a local oscillator (LO). A variable optical attenuator adjusts the LO power and noise measurements are recorded using a Keysight N9020B MXA electronic spectrum analyzer (ESA) with a 26.5-GHz bandwidth.

“We built the detector with a commercially accessible foundry in order to make its applications more accessible,” said Matthews. “It is critical that we as a community continue to tackle the challenge of scalable fabrication of quantum technology. Without demonstrating truly scalable fabrication of quantum hardware, the impact and benefits of quantum technology will be delayed and limited.“

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