Generation of high quality single photons for quantum computing

MIT researchers have developed a way to generate more single photons at room temperature to carry quantum information. The design, they say, is promising for the development of practical quantum computers.

Quantum emitters generate photons that can be detected one by one. Computers and consumer quantum devices could potentially exploit certain properties of these photons as quantum bits ("qubits") to perform calculations. While conventional computers process and store information in bits of 0 or 1, qubits can be 0 and 1 simultaneously. This means that quantum computers could potentially solve insoluble problems for conventional computers.

A major challenge, however, is to produce unique photons with identical quantum properties, called "indiscernible" photons. To improve indistinguishability, emitters carry light through an optical cavity where photons bounce, a process that allows them to match their properties to the cavity. In general, the longer the photons remain in the cavity, the more they correspond.

But there is also a compromise. In large cavities, quantum emitters generate photons spontaneously, leaving only a small fraction of photons remaining in the cavity, rendering the process ineffective. Smaller cavities extract higher percentages of photons, but photons are of lower quality or "distinguishable".

In an article published today in Letters of physical examination, the researchers divided a cavity into two, each with a specific task. A smaller cavity manages the efficient extraction of photons, while a large, attached cavity stores them a little longer to improve their indistinctness.

Compared to a single cavity, the researchers' coupled cavity generated photons with an indistability of nearly 95%, compared with an indistability of 80%, with an efficiency about three times greater.

"In short, two are better than one," says the first author, Hyeongrak "Chuck" Choi, a graduate student of the Electronics Research Laboratory at MIT (RLE). "What we found is that in this architecture, we can separate the roles of the two cavities: the first cavity simply focuses on the collection of photons for high efficiency, while the second focuses on the Indistinguishability in a single channel A cavity playing both roles can not meet both metrics, but two cavities perform both simultaneously. "

Dirk Englund, Associate Professor of Electrical and Computer Engineering, Researcher at RLE and Head of the Quantum Photonics Laboratory; Di Zhu, a graduate student of RLE; and Yoseob Yoon, a graduate student in the Department of Chemistry.

Relatively new quantum emitters, called "single-photon emitters," are created by defects in otherwise pure materials, such as diamonds, doped carbon nanotubes, or quantum dots. The light produced from these "artificial atoms" is captured by a tiny photonic crystal optical cavity, a nanostructure acting as a mirror. Some photons escape, but others bounce around the cavity, forcing photons to have the same quantum properties, mainly various frequency properties. When they match, they come out of the cavity through a waveguide.

But single-photon emitters also experience tons of environmental noise, such as grating vibrations or electrical charge fluctuations, that produce different wavelengths or phases. Photons with different properties can not be "interfered", so their waves overlap, resulting in interference patterns. This pattern of interference is basically what a quantum computer observes and measures to perform computational tasks.

The indistinguishability of photons is a measure of the potential for photon interference. In this way, it is useful to simulate their use for quantum computing. "Even before photon interference, with impossible indistinguishability, we can specify the photon interference capability," says Choi. "If we know this capability, we can calculate what will happen if they use it for quantum technologies, such as quantum computers, communications or repeaters."

In the researchers' system, a small cavity is attached to a transmitter, which, in their studies, was an optical defect in a diamond, called "Silicon Inactivity Center", a silicon atom replacing two atoms of carbon in a diamond network. The light produced by the defect is collected in the first cavity. Because of its light-focusing structure, photons are extracted at very high rates. Then the nanocavity channels the photons into a second, larger cavity. There, the photons bounce for a while. When they reach great indistinctness, the photons come out through a partial mirror formed of holes connecting the cavity to a waveguide.

It's important to note, says Choi, that no cavities should meet rigorous design requirements in terms of efficiency or indistinguishability of traditional cavities, called "quality factor (factor Q ). " The higher the Q factor, the lower the loss of energy in the optical cavities. But cavities with high Q factors are technologically difficult to manufacture.

In the study, the coupled cavity of researchers produced photons of superior quality to any possible single-cavity system. Even when its Q-factor was about one-hundredth of the quality of a cavity system, they could achieve the same indistinguishability with three times the efficiency.

The cavities can be adjusted to optimize efficiency with respect to indistinguishability and take into account any constraints on the Q factor, depending on the application. This is important, adds Choi, because current emitters operating at room temperature can vary greatly in quality and properties.

Then the researchers test the ultimate theoretical limit of multiple cavities. One more cavity would still handle the initial extraction efficiently, but would then be bound to multiple photonic cavities of different sizes to achieve optimal indistelability. But there will probably be a limit, says Choi: "With two cavities, there is only one connection, so it can be effective, but if there are multiple cavities, multiple connections can make it inefficient. we are currently studying the fundamental limit for cavities for use in quantum computing ".

This story is republished with the permission of MIT News (web.mit.edu/newsoffice/), a popular site that covers the news of MIT's research, innovation, and teaching. .