Single-photon detection advance to speed up quantum communications

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Scientists at the National Institute of Standards and Technology (NIST) in the US have devised a method they say can detect individual photons at a rate 10 times faster than existing technology – with lower error rates, higher detection efficiency and less noise.

It is hoped that this advance could improve internet data speeds, because the speed at which communications systems can operate depends on how fast – and how accurately – detectors on the receiving end can discriminate and process photons travelling through fibre optic cables.

In fibre optic communications, information travels in the form of a stream of light pulses typically travelling through fibre optic cables.

Each pulse can be as faint as a single photon, the smallest possible unit (quantum) of light.

limited to much lower speeds,’ said the NIST group leader Alan Migdall. ‘Combining that ultimate sensitivity with the ability to achieve the counting of photons at high rates has been a long-standing challenge. We are pushing both performance limits all in the same device.’

The NIST innovation entails a redesign of the control electronics system surrounding a workhorse detector called a single photon avalanche diode (Spad), in which an incoming photon triggers a tiny but measurable burst of current across a semiconductor.

A voltage is applied across the semiconductor. When a photon hits the detector, its absorbed energy kicks an electron off an atom in the semiconductor – the same photoelectric effect that generates electricity in solar panels.

That loose electron is accelerated by the applied voltage, and causes a sort of chain reaction in which large numbers of adjacent atoms release an ‘avalanche’ of electrons just as a small added stress can prompt an entire mountainside of snow to  collapse. That avalanche current is the output signal. Finally, the device is reset by quenching the current with a counter-voltage and restoring the initial applied voltage. Because the avalanche involves such a large number of electrons, getting the entire system back to a quiet state where it is ready to detect another photon is challenging. 

A conventional Spad can detect from 1 million to 10 million photons per second, which is not enough to meet the increasing needs of modern communications. Raising the rate, however, has been problematic because of the many trade-offs involved.

For example, the thickness of the absorption layer that the incoming photon hits, determines how likely the device is to catch that incoming photon: thick absorbers (around 0.1mm) have a higher probability of photon capture because of their greater depth; thinner layers have a greater chance that the photon will pass through undetected.

But the thicker the absorber, the higher the applied voltage needs to be. And higher voltages can produce larger avalanches – large enough to overheat the device, reducing detection efficiency as well as increasing the risk of spurious ‘afterpulses’, in which leftover electrons trapped in the semiconductor set off a secondary avalanche after the Spad is reset.

Image: Sean Kelley/NIST

To reduce afterpulses, it is necessary to reset the system in two nanoseconds or less. Conventional modules that detect the current and then apply the quench cannot operate that fast, historically limiting the performance of thick-absorber Spads to about 10 million counts per second or fewer. It has generally been assumed that thick-absorber Spads are unsuited to higher rate counts.

To overcome those problems in a thick-absorber device, the NIST team – which reported its results in March in Applied Physics Letters – began experimenting with an advanced electronics system for a commercially available thick-absorber Spad.

Like many such systems, the Spad is ‘gated’ off and on repeatedly – that is, it is reset continuously by an applied alternating voltage at some frequency. As a result, the longest time period during which the Spad can produce an avalanche is the gate interval. ‘Typical gating frequencies for these types of Spads has been limited to no higher than 150 megahertz,’ said NIST associate Michael Wayne, first author on the journal article.

‘That means that the Spad is able to avalanche for six or seven nanoseconds,’ Wayne said. ‘While this may not seem like a long time, it is long enough for the device to both become fully saturated with charge – which increases unwanted afterpulsing – and to become hot enough at high count rates to lower its detection efficiency. Gating at a higher frequency – thus shortening the maximum duration of an avalanche – would lessen both of these effects. But because the avalanche is not allowed to grow for as long, it can become too small to detect over the “noise” caused by opening and closing the gate.’

To overcome that problem, the team developed a method similar to noise-cancelling headphones: applying a radio frequency signal that exactly offsets the noise. That allowed them to operate the Spad at one billion cycles per second (1GHz).

Subtracting out the noise, said project leader Joshua Bienfang, ‘we are able to reveal extremely small avalanches. Additionally, the high frequency means that the gate is open for only 500 picoseconds. This results in a reduction in average avalanche current by about a factor of 500, lowering both the afterpulsing and self-heating effects, and allowing us to count at rates up to 100 million per second.’

The new Spad design could find practical uses in the applications of quantum communication and quantum computation. ‘Both offer capabilities not possible with conventional communication and computation. And both applications would benefit from faster, lower-noise single photon detectors,’ Migdall said.

This novel design is likely to impact a number of quantum applications,' he continued. 'They range from single-photon sensing, where faster count rates and lower noise reduce the time for existing measurements, to the emerging quantum internet, which relies critically on single-photon detection for quantum communication and quantum computation. Both of those can be expected to have a very substantial impact on our society and economy.’

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