Measure photon similarity
&ball; Physics 15, 135
A new optical device measures photon indistinguishability, an important property for future light-based quantum computers.
Photons can be used to perform complex calculations, but they must be identical or nearly identical. New device can determine how indistinguishable multiple photons emitted from a source are . Previous methods only gave a rough estimate of indistinguishability, but the new method offers an accurate measure. The device, which is essentially an arrangement of interconnected waveguides, could work as a diagnostic tool in a quantum optics lab.
In optical quantum computing, sequences of photons are made to interact with each other in complex optical circuits (see Synopsis: Quantum Computers Approach Milestone for Boson Sampling). For these calculations to work, the photons must have the same frequency, the same polarization and the same time of arrival in the device. Researchers can easily check if two photons are indistinguishable by sending them through a type of interferometer in which two waveguides – one for each photon – come close enough that a photon can jump into the neighboring waveguide. . If the two photons are perfectly indistinguishable, then they always end up together in the same waveguide.
For larger sets of photons, this type of pairwise testing becomes impractical, since it must be repeated for all possible combinations of two photons. Researchers have developed approximate methods, but they only give upper and lower bounds on indistinguishability. “When you have more than two photons, it’s not so easy to assess whether they’re the same,” says Andrea Crespi from the Polytechnic University of Milan.
Crespi and his colleagues have developed a simple method to determine the indistinguishability of multiple photons by letting them interact in a highly coordinated array of waveguides. As a first demonstration, the team built a four-photon system. They started with a glass plate and used a laser-writing technique to “imprint” eight high-density tubes to guide photons through the plate. These waveguides are like an eight-lane highway for photon “conductors” that can change lanes at specific points where neighboring lanes touch. For example, channel 2 touches channels 1 and 3 at specific locations. A similar “bridge” also connects lanes 1 and 8, so each lane touches two neighbors.
Using a semiconductor source called a quantum dot, the team repeatedly fed four photons into the odd lanes (1, 3, 5, 7) and recorded the lanes occupied by a photon after Highway. Many final lane arrangements have been observed, such as (1, 3, 5, 6) and (2, 4, 6, 8). Next, the researchers heated one of the lanes with a laser to gradually change its refractive index, which induced an oscillation in the probabilities for some of the final lane arrangements. These oscillations implied that interference effects influenced the lane changes.
The team theoretically showed that the amplitude of the oscillations gives the so-called true indistinguishability, which is a number from 0 to 1, where 1 corresponds to perfectly identical photons. They found an indistinguishability of 0.8, which means that their system had some imperfections. The researchers also showed that they could make the oscillations disappear by rotating the polarization of an input photon, thus distinguishing it from others.
The technique may possibly work with more photons, but the number of measurements needed to see the variation in lane layout increases exponentially with the number of photons. Crespi therefore admits that this would not be practical for future optical computers dealing with 100 photons or more. Still, he envisions their device as a way to troubleshoot a quantum optics experiment when there is doubt about the indistinguishability of input photons. “Our experiment adds a tool to the quantum optics experimenter’s toolbox,” he says.
“This paper presents a useful method for diagnosing photonic quantum circuits by measuring multiphoton indistinguishability, an important metric that is highly sensitive to experimental imperfections,” says quantum information specialist Chao-Yang Lu of the University of Science and technologies from China. “It’s a very clever interferometer design,” says quantum optics expert Wolfgang Löffler from Leiden University in the Netherlands. He is also impressed by the optical system that generates and separates the sequence of photons. “Making it all work together is a major effort,” says Löffler.
Michael Schirber is corresponding editor for physics journal based in Lyon, France.
- Mr. Bridge et al.“Quantify not-the indistinguishability of photons with an integrated cyclic interferometer”, Phys. Rev. X 12031033 (2022).