While generating a crowd of photons is as easy as flipping a light switch, it’s trickier to make a pair of indistin­guishable photons. And it takes yet more work to endow that pair with a quantum mechanical link, the entanglement. Now, researchers at the Joint Quantum Institute and their colleagues describe a new way to make entangled twin particles of light and to tune their properties using a method con­veniently housed on a chip, a potential boon for quantum techno­logies that require a reliable source of well-tailored photon pairs.

The researchers, led by Mohammad Hafezi, designed the method to harness the advantages of topological physics. Topo­logical physics explores previously untapped physical phenomena using the mathematical field of topology, which describes common traits shared by different shapes. Researchers have made several major disco­veries by applying this approach, which describes how quantum particles can move in a parti­cular material or device by analyzing its broad charac­teristics through the lens of topological features that correspond to abstract shapes. The topo­logical phenomena that have been revealed are directly tied to the general nature of the material; they must exist even in the presence of material impurities that would upset the smooth movement of photons or electrons in most other circum­stances.

Their new method builds on previous work, including the genera­tion of a series of dis­tinguishable photon pairs. In both the new and old experiments, the team created a checker­board of rings on a silicon chip. Each ring is a resonator that acts like a tiny race track designed to keep certain photons traveling round and round for a long time. But since individual photons in a resonator live by quantum rules, the photons can sometimes just pass unchanged through an inter­vening wall and proceed to speed along a neighboring track. The repeating grid of rings mimics the repeating grid of atoms that electrons travel through in a solid, allowing the researchers to design situa­tions for light that mirror well known topo­logical effects in electronics. By creating and exploring different topological environments, the team has developed new ways to mani­pulate photons. “It's exactly the same mathematics that applies to electrons and photons,” says Sunil Mittal, a JQI post­doctoral researcher. “So you get more or less all the same topo­logical features. All the mathematics that you do with electrons, you can simply carry to photonic systems.”

In the current work, they recreated the anomalous quantum Hall effect that opens up paths for electrons on the edge of a material. These topo­logical edge states can reliably transport electrons while leaving routes through the interior easily disrupted and im­passable. Achieving this particular topological effect requires that localized magnetic fields push on electrons and that the total magnetic field when averaged over larger sections of the material cancels out to zero. But photons lack the electrical charge that makes electrons susceptible to magnetic shoves, so the team had to recreate the magnetic push in some other way. To achieve this, they laid out the tracks so that it is easier for the photons to quantum mechanically jump between rings in certain directions. This simulates the missing magnetic influence and creates an environ­ment with a photonic version of the anomalous quantum Hall effect and its stable edge paths.

For this experiment, the team sent two laser beams of two different colors into this carefully designed environ­ment. Inside a resonator, a photon from each of the beams spon­taneously combine. The researchers then observed how the photons reformed into two indis­tinguishable photons, travelled through the topo­logical edge paths and were eventually ejected from the chip.

Since the new photons formed inside a resonator ring, they took on the traits – polari­zation and spatial mode – of the photons that the resonators are designed to hold. The only trait left that the team needed to worry about was their frequencies. The researchers matched the frequencies of the photons by selecting the appro­priate input frequencies for the two lasers based on how they would combine inside the silicon resonators. With the appro­priate theoretical under­standing of the experiment, they can produce photons that are quantum mechani­cally indistin­guishable.

The nature of the formation of the new photons provides the desired quantum characteristics. The photons are quantum mechani­cally entangled due to the way they were generated as pairs; their combined proper­ties—like the total energy of the pair—are constrained by what the original photons brought into the mix, so observing the property of one instantly reveals the equivalent fact about the other. Until they are observed they don’t exist as two individual particles with distinct quantum states for their frequencies. So, they are in a super­position state. The resulting combi­nation of being indistin­guishable and entangled is essential for many potential uses of photons in quantum technologies. An additional lucky side effect of the researcher’s topo­logical approach is that it gives them a greater ability to adjust the frequencies of the twin photons based on the frequencies they pump into the chip and how well the frequencies match with the topo­logical states on the edge of the device.

“This is not the only way to generate entangled photon pairs but they are not tunable,” Mittal says. “So once you fabri­cate your device, it is what it is. If you want to change the bandwidth of the photons or do something else, it's not possible. But in our case, we don't have to design a new device. We showed that, just by tuning the pump frequencies, we could tune the inter­ference properties. So, that was very exciting.” The combi­nation of the devices being tunable and robust against manu­facturing imper­fections make them an appealing option for practical appli­cations, he say. The team plans to continue exploring the potential of this technique and related topo­logical devices and possible ways to further improve the devices such as using other materials to make them. (Source: JQI)

Reference: S. Mittal et al.: Tunable quantum interference using a topological source of indistinguishable photon pairs, Nat. Phot., online 10 May 2021; DOI: 10.1038/s41566-021-00810-1

Link: Joint Quantum Institute, NIST & University of Maryland, College Park, USA