(by LION/EA)
Leiden physicists have developed a way to address how accurately a superconducting single photon detector (SSPD) can be characterized by detector tomography. SSPDs are not fully understood, and tomography is a key element to determine how these devices detect light. A better understanding of these detectors promises better detector design with improvements in optical fiber communication, night vision goggles, telescopes and cancer treatment. The research team of Dr. Michiel de Dood published their latest results on tomography in the Journal of Applied Physics.

A regular CCD camera captures billions of light particles—called photons—each second. The plentitude of photons creates a macroscopic view of the everyday world we live in and allows us to take pictures with an ordinary camera. But in some cases, you might want to capture just a few photons. For example in quantum communication, where you exchange photons one at a time to share a secret key. If an outsider has been eavesdropping, the intended recipient will know because quantum mechanics dictates that a measurement changes the information stored in the photon. In the act of eavesdropping, the outsider leaves a clear trace. And just image how much night vision goggles would benefit from being able to detect single infrared photons. Astronomers would be delighted to spot extremely dim galaxies, and doctors could trace singlet oxygen particles that break down cancer cells.

For all these applications we need a device that is designed to capture single photons at non-conventional wavelengths. An example of such a detector is an SSPD, similar to the one that first author Qiang Wang used in his research; a series of tiny niobium nitride wires—just a hundred nanometers (nm) wide and 4 nm thick—cooled down to a temperature of 3.2 Kelvin to ensure that the wire is superconducting. In this state, electric currents flow without electrical resistance. Absorption of a photon weakens the superconductivity of the wire and may induce a transition where the wire becomes resistive. The current through the wire leads to a measurable voltage that comprises the detection of single photons. Surprisingly however, not every absorbed photon triggers the detector. So in addition to not all photons being absorbed, there is also an inefficiency in the electronic detection process of photons that were already absorbed.

Given their huge potential, scientists are eager to improve and use SSPDs. ‘Before these detectors can reach their full potential, we need to understand better how they work,’ says De Dood. ‘For that, we need new methods to characterize them, and I think detector tomography is the best way. It enables us to distinguish absorption inefficiency from electronic detection inefficiency. Before drawing firm conclusions, we need to determine the accuracy and practical limitations of the new method.’ And now De Dood’s research team has found a way to do precisely that. It is one step ahead in the process of understanding the mechanisms behind SSPDs.