May 14 2019
A technique for producing more single photons at room temperature for transferring quantum information has been developed by researchers at MIT. According to the researchers, the design has the potential to enable the development of practical quantum computers.
MIT researchers have designed a new single-photon emitter that generates, at room temperature, more of the high-quality photons that could be useful for practical quantum computers, quantum communications, and other quantum devices. (Image credit: MIT)
Quantum emitters produce photons that can be detected one at a time. Commercial quantum computers and devices could possibly leverage specific properties of those photons as quantum bits (“qubits”) to perform computations. In contrast to classical computers that store and process information in bits of either 0s or 1s, qubits can be 0 and 1 at the same time. This suggests that quantum computers could possibly overcome challenges that are unmanageable for classical computers.
However, an important challenge is to produce single photons with identical quantum characteristics—called “indistinguishable” photons. In order to enhance the identicalness between the photons, light is funneled by emitters through an optical cavity in which the photons bounce back and forth—a process that assists in matching their properties with that of the cavity. In general, if the photons stay longer in the cavity, they match more.
However, there is also a trade-off. In huge cavities, photons are spontaneously produced by quantum emitters, leading to only a small fraction of photons staying in the cavity, thereby rendering the process less efficient. Although higher percentages of photons are extracted by smaller cavities, the photons are of lower quality, or “distinguishable.”
In a study reported in Physical Review Letters on May 14th, 2019, the scientists split a cavity into two, each with an assigned task. A smaller cavity takes care of the effective extraction of photons, whereas an attached large cavity stores them for a longer time to improve the indistinguishability.
When compared to a single cavity, the coupled cavity designed by the researchers produced photons with nearly 95% indistinguishability, than the usual 80% indistinguishability, with nearly three times higher efficiency.
In short, two is better than one. What we found is that in this architecture, we can separate the roles of the two cavities: The first cavity merely focuses on collecting photons for high efficiency, while the second focuses on indistinguishability in a single channel. One cavity playing both roles can’t meet both metrics, but two cavities achieves both simultaneously.
Hyeongrak “Chuck” Choi, Study First Author and Graduate Student, MIT Research Laboratory of Electronics
Other contributors to the paper are Dirk Englund, an associate professor of electrical engineering and computer science, a researcher in RLE, and head of the Quantum Photonics Laboratory; Di Zhu, a graduate student in RLE; and Yoseob Yoon, a graduate student in the Department of Chemistry.
The comparatively new quantum emitters, called “single-photon emitters,” are brought about by flaws in otherwise pure materials, like doped carbon nanotubes, diamonds, or quantum dots. Light generated from such “artificial atoms” is captured by a small optical cavity in photonic crystal—a nanostructure that acts as a mirror. A few of the photons escape, whereas others bounce around the cavity, forcing the photons to possess the same quantum properties—mainly, a number of frequency properties. Upon measuring them to match, they leave the cavity via a waveguide.
However, single-photon emitters also undergo immense environmental noise—for instance, electric charge fluctuation or lattice vibrations—that generate different phase or wavelength. It is not possible to “interfere” photons with distinct properties, such that their waves overlap, leading to interference patterns. Fundamentally, that interference pattern is the one observed and measured by a quantum computer to perform computational tasks.
Photon indistinguishability is a measure of the potential of the photons to interfere. Thereby, it is a useful metric to mimic their use for practical quantum computing.
Even before photon interference, with indistinguishability, we can specify the ability for the photons to interfere. If we know that ability, we can calculate what’s going to happen if they are using it for quantum technologies, such as quantum computers, communications, or repeaters.
Hyeongrak “Chuck” Choi, Study First Author and Graduate Student, MIT Research Laboratory of Electronics
In the system developed by the researchers, a small cavity is positioned to be attached to an emitter, which in their analyses was an optical flaw in a diamond, known as a “silicon-vacancy center”—a silicon atom that replaces two carbon atoms in a diamond lattice. Light generated by the defect is gathered into the first cavity. Due to its light-focusing structure, photons are extracted at extremely high rates. Subsequently, the photons are channeled by the nanocavity into a second, larger cavity. In the second cavity, the photons bounce back and forth for a specific period of time. Upon reaching high indistinguishability, the photons leave through a partial mirror formed by holes that connects the cavity to a waveguide.
Significantly, Choi stated that it is not necessary for either of the cavities to fulfill strict design requirements for indistinguishability or efficiency as conventional cavities, known as the “quality factor (Q-factor).” The Q-factor is inversely proportional to the energy loss in optical cavities. However, cavities that have high Q-factors are technologically difficult to develop.
In the research, the coupled cavity of the team generated photons of higher quality compared to any possible single-cavity system. Although its Q factor was approximately one-hundredth the quality of the single-cavity system, they could realize the same indistinguishability with an efficiency that was three times higher.
It is possible to tune the cavities to optimize for efficiency against indistinguishability—as well as to take any constraints on the Q factor into account—based on the application. Choi further stated that it is significant since existing emitters that function at room temperature can differ to a large extent in terms of properties and quality.
As the next step, the scientists are testing the maximum possible theoretical limit of multiple cavities. One more cavity would still be able to efficiently perform the initial extraction; however, it would be linked to a number of cavities that extract photons for different sizes to realize some optimal indistinguishability. Yet, there will most possibly be a limit, stated Choi: “With two cavities, there is just one connection, so it can be efficient. But if there are multiple cavities, the multiple connections could make it inefficient. We’re now studying the fundamental limit for cavities for use in quantum computing.”