Insider Brief
- Sparrow Quantum and Ruhr University Bochum developed a deterministic source that delivers more than 500 million usable photons per second into optical fiber.
- The source operates at 1 gigahertz with more than 50% fiber efficiency while maintaining high photon purity and indistinguishability without spectral filtering.
- The increased photon supply could enable experiments involving 10 to 20 photons and support applications in quantum computing, networking, communications and metrology.
PRESS RELEASE — Quantum computers are being built in several ways: superconducting circuits, trapped ions, neutral atoms, semiconductor spins and photonics. Photonic quantum systems have one fundamental advantage: their information carriers can travel through optical fibre, making the same technology relevant to both quantum processors and quantum networks.
What photonic quantum systems need in exchange is a supply of photons that behaves: each one identical to the last, each arriving when the machine asks rather than when physics happens to oblige. For much of the field’s history, that has meant probabilistic sources: excite a material, wait for the photon-generation event to occur, and discard the occasions when it does not.
A lottery is survivable when you need one photon and punishing when you need many. Every photon has to arrive together with all the others, so losses multiply rather than add: a source that works well enough for one photon can be hopeless for ten. That is why the source sets the ceiling for everything built on top of it. Improve it once, at the point of generation, and the improvement carries through the whole system.
In work carried out with Ruhr-Universität Bochum, Sparrow Quantum‘s deterministic source now operates at 1 GHz, delivering more than 500 million usable photons per second into a single-mode fibre — the highest single-photon flux reported to date. The significance is not speed alone: the source combines gigahertz repetition rate with more than 50% fibre efficiency and high single-photon purity and two-photon indistinguishability.
Bright enough for a power meter
Reported source figures are often taken after spectral filtering, which removes the light that would otherwise drag the numbers down. These were not. Sparrow Quantum measured everything the source emits and reports the lower limit rather than the best case.
That decision is also what makes the next result possible. Quantum light at these levels is normally far too faint to register on a conventional optical power meter, so single photons are typically counted one at a time with highly sensitive detectors. This stream carries more than 100 picowatts of optical power — enough to read on an ordinary power meter. The source may also find near-term use in metrology, as a photon-flux standard and for calibrating single-photon detectors. Since the full emission is being collected, the power at the fibre is a direct measure of the source’s fibre efficiency, without the detector calibrations and correction factors that make published efficiency figures so hard to compare.
The source is driven as hard as the emitter physically allows: at one gigahertz there is no room left to fit another pulse between the existing ones. Speed of this kind can come at the expense of photon quality. Here it did not, again with the full unfiltered emission collected. Conditions and values are given in the preprint [link].
The demonstration that has been waiting
The demonstrated source can also be harvested in multi-photon experiments. Multi-photon protocols for linear optical quantum computing, quantum-enhanced machine learning, quantum key distribution and quantum networking exist in the literature in numbers, designed and analysed but seldom executed, because assembling many indistinguishable photons at once has been impractical outside a handful of laboratories. Supply is what changes that. Split across ten channels by time-space demultiplexing, the source still delivers tens of millions of photons per second to each. That is enough coincidence rate to make ten-photon experiments practical rather than aspirational, and enough for interference between roughly 10 and 20 photons, beyond the scale practical with conventional commercial sources.
“As an end-user, I always knew what the source was stopping me from doing,” said Juan C. Loredo, VP of Innovation at Sparrow Quantum and a co-author, who used earlier generations of the company’s sources as a senior researcher at the University of Vienna. “You design the protocol you want, then you cut it down to what the photon rate will support, and then you wait days for enough data. This is the first time I have looked at a photon flux and thought the limit is somewhere else now.”
“Every gain here came from serious engineering rather than from redefining the technology,” said Peter Lodahl, founder and Chief Quantum Officer of Sparrow Quantum. “We drove the source as hard as the emitter physically allows, collected every photon it produced, and the quality held.”
More photons is just the beginning. Larger photonic quantum systems need those photons to be entangled — linked so that they form a shared quantum state — and generating and controlling entanglement at scale is the next major challenge.
“Making individual photons behave has taken years of engineering,” said Lodahl. “The next challenge is making them work together through entanglement, with the same level of control and reliability.”
