Quantum Spin Hall Effect for Light
Mohammad Hafezi, Eugene A. Demler, Mikhail D. Lukin, and Jacob M. Taylor [photos courtesy of Joint Quantum Institute (JQI) and Harvard University]
The advent of optical fibers a few decades ago made it possible for dozens of independent phone conversations to travel long distances along a single glass cable by, essentially, assigning each conversation to a different color—each narrow strand of glass carrying dramatic amounts of information with little interference.
Surprisingly, transmitting information-rich photons thousands of miles through fiber-optic cable is far easier than reliably sending them just a few nanometers through a computer circuit -- and this makes it difficult to employ photons as information carriers inside computer chips.. However, it may soon be possible to steer these particles of light accurately through microchips because of research  performed at the Joint Quantum Institute of the National Institute of Standards and Technology (NIST) and the University of Maryland, together with Harvard University.
The scientists behind the effort say the work not only may lead to more efficient information processors on our desktops, but also could offer a way to explore a particularly strange effect of the quantum world known as the quantum Hall effect in which electrons can interfere with themselves as they travel in a magnetic field. Manipulating photons such that they behave like their electrical counterparts- the electron- is a rich area of research with applications extending into quantum information and condensed matter. The corresponding physics is rich enough that its investigation has already resulted in three Nobel Prizes, but many intriguing theoretical predictions about it have yet to be observed.
Two researchers at the Joint Quantum Institute (JQI), Mohammad Hafezi and Jacob M. Taylor, and two researchers at Harvard, Eugene A. Demler and Mikhail D. Lukin, propose an optical delay line that could fit onto a computer chip. Delay lines, added to postpone a photon’s arrival, are passive, but critical in processing signals. Kilometers of glass fiber are easily obtained, but fabricating optical elements that can fit on a single chip creates defects that can lead to reduced transmission of information.
"We run into problems when trying to use photons in microcircuits because of slight defects in the materials chips are made from," says Jacob Taylor, a theoretical physicist at JQI. "Defects crop up a lot, and they deflect photons in ways that mess up the signal."
These defects are particularly problematic when they occur in photon delay devices, which slow the photons down to store them briefly until the chip needs the information they contain. Delay devices are usually constructed from a single row of tiny resonators, so a defect among them can ruin the information in the photon stream. But the research team perceived that using multiple rows of resonators would build alternate pathways into the delay devices, allowing the photons to find their way around defects easily.
Artist's rendering of the proposed JQI fault-tolerant photon delay device for a future photon-based microchip. The devices ordinarily have a single row of resonators; using multiple rows like this provides alternative pathways for the photons to travel around any physical defects. Transmission of light is protected from defects because the system exhibits a photonic version of the quantum spin Hall effect. [Image credit: E. Edwards/JQI]
As delay devices are a vital part of computer circuits, the alternate-pathway technique may help overcome obstacles blocking the development of photon-based chips, which are still a dream of computer manufacturers. While that application would be exciting, lead author Mohammad Hafezi says the prospect of investigating the quantum Hall effect with the same technology also has great scientific appeal.
"The photons in these devices exhibit the same type of interference as electrons subjected to the quantum Hall effect," says Hafezi. "We hope these devices will allow us to sidestep some of the problems with observing the physics directly, instead allowing us to explore them by analogy."
Quantum Hall physics is the remarkable phenomenon at the heart of this new approach. The quantum Hall effect occurs in a two-dimensional sea of electrons under the influence of a large magnetic field. The electrons are allowed to travel along the edges of the material but do not have enough energy to permeate throughout the bulk or central regions. It is as if there are conduction highways along the edge of the material. Even if there are defects in the material, like potholes in the road, electrons still make it to their destination.
These highways, called “edge states” are open for transit only at specific values of the externally applied magnetic field. Because the routes are so robust against disorder and reliably allow for electron traffic, this effect provides a standard for electrical resistance.
In recent years, scientists have discovered that some materials can exhibit what is known as the quantum spin Hall effect (QSHE), which depends on the “spin” attributes of the electron. Electrons not only carry charge, but also “spin.” Electrons can be thought of as tiny spinning tops that can rotate clockwise (in which case they are in a “spin-up” condition) or counter clockwise (“spin-down”). Notably, the robust edge states are present in the QSHE even without externally applied magnetic fields, making them amenable for developing new types of electronics.
In the Nature Physics article, the JQI-Harvard team is proposing a device supporting these “edge states” that are a hallmark of the QSHE, where light replaces the electrons. This device can be operated at room temperature and does not require any external magnetic field, not even the use of magnetic materials. They show that the resilience of the edge states can be used to engineer novel optical delay lines at the micrometer scale.
Hafezi explains that a key step is confining the photon pathways to two-dimensions: “In the QSHE, electrons move in a two-dimensional plane. Analogously, one can imagine a gas of photons moving in a two dimensional lattice of tiny glass racetracks called resonators.”
Resonators are circular light traps. Currently one-dimensional lines of these micro-racetracks can be used for miniaturized delay lines. Light, having particular colors (in other words, frequencies), can enter the array and become trapped in the racetracks. After a few swings around, the photons can hop to neighboring resonators. The researchers propose to extend this technology and construct a two-dimensional array of these resonators (see Figure).
Once light is in the array, how can it enter the quantum edge highway? The secret is in the design of the lattice of resonators and waveguides, which will determine the criteria for light hopping along the edge of the array rather than through the bulk. The photons will pile into the edge state only when the light has a particular color.
The fabrication process for these micro-resonators is susceptible to defects. This is true for both one- and two-dimensional resonator arrays, but it is the presence of quantum edge states that reduces loss in signal transmission.
When photons are in an edge state created by the 2D structure their transmission through the delay line is protected. Only along these highways will they will skirt around defects, unimpeded. They cannot do a U-turn upon encountering a defect because they do not have the appropriate light frequency, which is their ticket to enter the backwards-moving path.
Taylor explains an advantage of their proposal: “Right around the point where other [1D] technologies become operational, this same 2D technology also becomes operational. But thereafter, the transmitted signal will be much more robust for this approach to delay lines compared to the 1D approach.”
For example, the length of delay is given by the size of the array or the length of the photon’s path, whether 1D or 2D. However, as the number of resonators and optical features increases to accommodate longer delays, the inherent defects will eventually cause a roadblock for the photons, while the transmission using quantum pathways remains unobstructed.
The researchers hope that building these simple passive devices will lay the foundation for creating robust active circuit elements with photons, such as a transistor.
 Mohammad Hafezi, Eugene A. Demler, Mikhail D. Lukin, and Jacob M. Taylor, "Robust optical delay lines via topological protection", Nature Physics, doi:10.1038/nphys2063 (Published online August 21, 2011). Abstract.
[We thank Joint Quantum Institute of the National Institute of Standards and Technology (NIST) and the University of Maryland for materials used in this report]