First to quantize the indistinguishability of multiple photons, laying the foundation for large-scale optical quantum computers

The indistinguishability of photons is an important property that will be needed for future optical quantum computers. Previous methods could only quantize the indistinguishability of two photons at a time. Now, a team of researchers from France and Italy has developed a new optical device that can quantize the indistinguishability of multiple photons for the first time.The related results were published in Physical Review X on September 2 under the title "Quantizing the indistinguishability of n photons with a circularly integrated interferometer" [1].

01New device: Precise quantization of photon indistinguishability

Photons can be used to perform complex calculations, but they must be identical, or nearly so. A new device, made by a team from Politecnico di Milano, can determine the extent to which several photons emitted by a "source" are indistinguishable: while previous methods could only roughly estimate the indistinguishability of photons, the new method provides a precise measurement. The device is essentially an arrangement of interconnected waveguides that can be used as a diagnostic tool in a quantum optics laboratory.

In optical quantum computing, sequences of photons interact with each other in complex optical circuits. Back in 2019, a team at CSU demonstrated the "quantum superiority" of an optical quantum computer in solving the "bosonic sampling problem" [2]: this means that predicting a set of input bosons (normally photons) after a certain The most effective way to sample the range of possible distributions is to demonstrate the physicality of the calculations by experiment.

In order for these calculations to work, the photons must have the same frequency, the same polarization, and the same time of arrival at the device. Researchers can easily check whether two photons are indistinguishable with an interferometer: in this interferometer, two waveguides (one for each photon) are close enough together; however, one photon can jump into the adjacent waveguide. Thus, if the two photons are completely indistinguishable, they will always end up together in the same waveguide.

02Four-photon systems: verifying the indistinguishability of photons

For larger sets of photons, this pairwise test becomes impractical because it must be repeated for all possible two-photon combinations. The researchers have devised approximate methods, but they only give upper and lower bounds on indistinguishability. Andrea Crespi of Politecnico di Milano, the first author of this paper, said, "When you have more than two photons, it is not so easy to assess whether they are identical."

Figure 1 This diagram shows a conceptual device for measuring indistinguishability. Photons enter from the left and are guided by the waveguide to the output on the right. At specific points (black bars), photons can jump to neighboring waveguides. The probability of such jumps depends on the indistinguishability of the photon.

Crespi and his colleagues came up with a simple method: determine the indistinguishability of multiple photons by having them interact in a highly coordinated array of waveguides. For their first demonstration, the team constructed a four-photon system. They started with a glass plate and used laser writing to "imprint" eight high-density tubes to guide the photons through the plate. These waveguides act as an eight-lane highway for photon "drivers," who can change lanes at specific points where adjacent lanes touch. For example, lane 2 is in contact with lanes 1 and 3 at a specific location. A similar "bridge" also connects lanes 1 and 8, so each lane touches both neighbors.

Using a semiconductor source called a quantum dot, the team repeatedly fed four photons into the odd numbered lanes (1, 3, 5, 7) and at the end of the highway, recorded which lanes were occupied by photons.

Eventually, they observed many lane arrangements, such as (1, 3, 5, 6) and (2, 4, 6, 8). Next, the researchers heated one of the lanes with a laser, gradually changing its refractive index, which caused oscillations in the probability of some of the final lane arrangements: these oscillations imply that interference effects are affecting the lane changes.

Figure 2 Schematic diagram of the experimental setup. A quantum dot-based single photon source (QDSPS) is periodically excited and emits a string of single photons. The photons are separated into four spatial modes (spatial modes) using a demultiplexer based on a periodically driven acousto-optic modulator (AOM). The polarization of the photons is controlled with a wave plate and a delayed fiber is used to ensure the simultaneous arrival of the four photons. The photons are detected at the output of the interferometer using a superconducting nanowire single photon detector (SNSPD) and the coincidence of the four photons is recorded using a correlator.

Figure 3 Output states detected when four identical photon states (1, 3, 5, 7) are input.

The team showed theoretically that the amplitude of the oscillations gives a true indistinguishability, which is a number from 0 to 1, where 1 corresponds to the exact same photon. They found an indistinguishability of 0.8, implying 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 other photons.

 Figure 4 By rotating the polarization of the half-wave plate (relative to the experimental reference position θ = 0° in Figure 3), the indistinguishability of photon A is reduced with respect to the other photons. By rotating the half-wave plate by θ(a) = 11°, θ(b) = 19°, θ(c) = 31°, and θ(d) = 45°, the visibility of the "oscillation" is reduced because the measured minimum two-photon Hong-Ou-Mandel interference visibility (extracted single-photon indistinguishability) is reduced. When photon A is almost completely distinguishable, the amplitude contrast of the measured four-photon oscillations decreases to almost zero.

03Significant experimental results

It is conceivable that the technique could work with more photons, but the number of measurements needed to see the lane alignment change grows exponentially with the number of photons. So Crespi acknowledges that this will be unrealistic for future optical computers that handle 100 or more photons. Still, he foresees their device as a breakthrough approach to quantum optics experiments. Our experiment adds a tool to the quantum optics experimenter's toolbox," he said. "

Lu Chaoyang, a quantum information expert at the University of Science and Technology of China, said [3], "This paper reports a useful method to diagnose photonic quantum circuits by measuring multiphoton indistinguishability. This is an important metric that is very sensitive to experimental imperfections." Wolfgang Löffler, a quantum optics expert at Leiden University in the Netherlands, said, "This is a very clever interferometer design." He is also impressed by the optical system that generates, and separates, the photon sequences: getting all the devices to work together is a major effort.

Reference links：

[1]https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.031033

[2]https://physics.aps.org/articles/v12/s146

[3]https://physics.aps.org/articles/v15/135