Behind the buzz and beyond the hype:
Our Nanowerk-exclusive feature articles
Posted: Oct 01, 2007
Towards spooky nanotechnology with dynamic control of nanocavity Q
(Nanowerk Spotlight) In our Spotlight from a few weeks ago - Nanopyramids - temporary resting places for light - we wrote about things like Q-factors, qubits, stopping light and other fascinating concepts and emerging techniques that could lead to quantum computing. Some of the feedback we received could be summarized with "Huh? Q-what...?" So today we'll take another look at the Quality (Q) factor of photonic-crystal nanocavities and the context it is relevant in. These 'nanocages' for light are currently the focus of much interest of nanotechnology research in photonics because they can strongly confine photons in a tiny space. Just as semiconductor crystals control the flow of electrons (the basis for all electronics), photonic crystals are a unique material used to construct photonic devices and circuits for manipulating light, i.e. photons. A prominent example of a photonic crystal is the naturally occurring gemstone opal. Photons (behaving as waves) propagate through it - or not - depending on their wavelength. Wavelengths of light (stream of photons) that are allowed to travel throught he crystal are known as "modes". Disallowed bands of wavelengths are called photonic band gaps. What's so interesting for researchers is that, once you are able to fully control and manipulate photons, you could not only vastly improve existing applications like optical data storage, high-precision sensing and telecommunications, but develop exotic technologies like quantum computing.
Photons move at the speed of light (close to 300,000 km per second), much faster than electrons. Photonic quantum computers would therefore be able to operate at unprecedented speed. However, trapping (slowing or stopping altogether) light is a necessary element in replacing electron storage for computer logic because only when light has been slowed down sufficiently can information be mapped onto it.
In a quantum computer, the fundamental unit of information (called a quantum bit or qubit), is not binary like in electronics as we know it. This qubit property arises as a direct consequence of its adherence to the laws of quantum mechanics which differ radically from the laws of classical physics. "A qubit can exist not only in a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states corresponding to a blend or superposition of these classical states. In other words, a qubit can exist as a zero, a one, or simultaneously as both 0 and 1, with a numerical coefficient representing the probability for each state. This may seem counterintuitive because everyday phenomenon are governed by classical physics, not quantum mechanics – which takes over at the atomic level." (quoted from Jacob West's excellent introduction to the quantum computer)
And that's where it gets decidedly unnerving: The weirdest idea in quantum mechanics is what scientists call "spooky action at a distance" – the notion that certain subatomic particles like photons can affect one another no matter how far apart they are. Imagine you had two identical marbles in your office in New York and you would ship one to Tokyo. Then you would spin the one in New York; the one in Tokyo would start spinning at exactly the same speed but in the reverse direction (Niels Bohr once remarked that a person who wasn't outraged on hearing about quantum theory didn't understand what had been said). This phenomenon is called quantum entanglement and it has led to Schrödinger's famous thought experiment now known as Schrödinger's Cat.
To map information onto light beams scientists would use the 'spin' of photons (all elementary particles have a natural orientation, like 'up' or 'down'). Flipping the spin has the same effect as switching a transistor on and off. Unfortunately, the entangled state of the photon represents all possible arguments and corresponding values of the function as a linear superposition, so this information is not accessible beforehand. The problem is that, thanks to the laws in the quantum world and what is known as quantum interference, the spin of a photon could be up or down simultaneously, until the photon is observed.
The processing power of a quantum computer is a direct result of quantum interference. Since a qubit can represent several states at the same time, a quantum computer is exponentially more powerful than its binary 20th century brethren.
There is lots of information about quantum computing available on the Web in case you want to delve deeper; so, after this introductory detour into quantum theory let's get back to photonic crystals and the Q-factor. Now that we know why researchers are so interested in being able to stop or slow down light, let's look at how they are trying to achieve this.
Modes, i.e. the wavelength of light allowed to travel through a photonic crystal, can be qualified according to their Q factor. This is a measure for the finite photon lifetime and indicates the amount of resistance to resonance in such a system. The average lifetime of a resonant photon in the cavity is proportional to the cavity's Q.
To take an analogy from acoustics: When you strike a bell or pluck a guitar string, it will vibrate within a small range of frequencies, centering on what is called the resonant frequency. Quality factor refers to how narrow that range will be. It is defined as the ratio of the resonant frequency to the range of frequencies over which resonance occurs. A radio receiver with high-Q circuitry, for example, will be more effective in separating one station from another.
"The next important issue is how to deliberately control storage and release photons from such a high Q nanocavity" Dr. Susumu Noda explains to Nanowerk. "When the cavity Q increases, the photon lifetime is increased, and the operation speed becomes slow. Thus, the important issue is as follows: When we introduce photons into the nanocavity, the Q factor should be small, and once the photons are introduced into the nanocavity, the Q factor should be increased rapidly before the photons leak out from the nanocavity. And, if necessary, the Q factor should be decreased again so that the photons can be released quickly from the nanocavity. In other words, the dynamic control of the Q factor of the nanocavity in a picosecond timescale is very important." (Don't even try to imagine how short a picosecond – one trillionth of a second – is.)
However, so far there has been no concept to achieve such a dynamic control of the nanocavity Q. Now, though, Noda and his collaborators propose and demonstrate for the first precisely such a concept.
Noda is a Professor in the Department of Electronic Science and Engineering and the Vice Director of the Photonics and Electronics Science and Engineering Center at Kyoto University in Japan. His latest research findings titled "Dynamic control of the Q factor in a photonic crystal nanocavity" have been published in the September 2, 2007 online edition of Nature Materials.
Method for dynamic control of the nanocavity Q factor. a) Schematic diagram of the typical configuration of a photonic-crystal nanocavity, where the nanocavity is accompanied by a waveguide which introduces (or releases) photons into (or from) the nanocavity. b) Schematic diagram of a system for dynamic control of the Q factor, consisting of a cavity, a waveguide with nonlinear optical response (the phase θ is controllable) and a perfect mirror. c) Concrete example of such a system. The nanocavity consists of three missing air holes, and air holes at the cavity edges are shifted. A line-defect waveguide is formed in the vicinity of the nanocavity. A hetero-interface, which acts as a perfect mirror, is located at one end of the waveguide, formed by slightly modifying the lattice constant. (Reprinted with permission from Nature Publishing Group)
The team in Kyoto has successfully demonstrated the dynamic change of the Q factor of a nanocavity from 3,000 to 12,000 on a picosecond timescale by using a system composed of a nanocavity, a waveguide with nonlinear response and a hetero-interface mirror.
"We believe that this demonstration is an important step towards realization of the slowing and/or stopping of light, and quantum information processing where nanocavities could be integrated on a chip and the transfer, storage, and exchange of photons would be possible through integrated waveguides" says Noda.
In the above mentioned paper, the scientists in Japan have employed a nanocavity with Q=55,000. In a next step, they will use a nanocavity with a much higher Q factor of 1,000,000 to 2,000,000. "Also" says Noda, "we would like to integrate such high Q nanocavities and investigate the exchange of photons among them."