Miniaturizing Delay Lines
Information traveling near the speed of light always sounds a little like science fiction. But this is what we get whenever we connect to the internet or watch cable television. Small packets of light called photons travel kilometers over networks of optical fiber, bringing information into our homes.
If fiber optic cable is ideal for carrying information, why haven’t photons replaced electrons entirely? Largely because miniaturizing photonic equipment all the way down micrometer scales often degrades their performance. 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.
Scientists are proposing a novel method for forcing photons to act like electrons. 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.
The proposed delay line, which harnesses sophisticated quantum effects, would help to protect signals from degradation and maybe lead to more complex photonic devices. The new work is described this week in Nature Physics (Advanced online publication August 21) in an article titled “Robust optical delay lines via topological protection.” **
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.
**“Robust optical delay lines via topological protection,” Mohammad Hafezi, Eugene A. Demler, Mikhail D. Lukin, and Jacob M. Taylor, Nature Physics (Advance Online Publication 10.1038/NPHYS2063)
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