Power/Performance Bits: July 9

All-optical transistors; reusable biosensors.


All-optical transistor
Optical computing uses light rather than electricity to perform calculations and is expected to potentially pay dividends for both conventional computers and quantum computers, which are largely hypothetical devices that could perform some types of computations exponentially faster than classical computers.

One drawback is that optical computing requires light particles — photons — to modify each other’s behavior, which is something they are naturally averse to doing. Two photons that collide in a vacuum simply pass through each other.

However, researchers at MIT’s Research Laboratory of Electronics together with colleagues at Harvard University and the Vienna University of Technology have described an experimental realization of an optical switch that’s controlled by a single photon, allowing light to govern the transmission of light. As such, it’s the optical analog of a transistor, the fundamental component of a computing circuit.

And since the counterintuitive effects of quantum physics are easier to see in individual particles than in clusters of particles, the ability to use a single photon to flip the switch could make it useful for quantum computing, the researchers asserted.

The heart of the switch is a pair of highly reflective mirrors. When the switch is on, an optical signal — a beam of light — can pass through both mirrors. When the switch is off, only about 20% of the light in the signal can get through.

The paired mirrors constitute what’s known as an optical resonator. If there was just one mirror, all the light would come back but when there are two mirrors, something very strange happens.

Light can be thought of as particles — photons — but it can also be thought of as a wave, i.e., an electromagnetic field. Even though on the particle description, photons are stopped by the first mirror, on the wave description, the electromagnetic field laps into the space between the mirrors. If the distance between the mirrors is precisely calibrated to the wavelength of the light, a very large field builds up inside the cavity that cancels the field coming back and goes in the forward direction. In other words, the mirrors become transparent to light of the right wavelength.

In the researchers’ experiment, the cavity between the mirrors was filled with a gas of supercooled cesium atoms. Ordinarily, these atoms don’t interfere with the light passing through the mirrors but if a single “gate photon” is fired into their midst at a different angle, kicking just one electron of one atom into a higher energy state, it changes the physics of the cavity enough that light can no longer pass through it.

For conventional computers, the chief advantage of optical computing would be in power management: As computer chips have more and more transistors crammed onto them, they draw more power and run hotter. Computing with light instead of electricity would address both problems.

But clouds of supercooled atoms are not a practical design for the transistors in, say, a Web server. The researchers said for the classical implementation, this is more of a proof-of-principle experiment showing how it could be done, and one could imagine implementing a similar device in solid state — for example, using impurity atoms inside an optical fiber or piece of solid. The quantum-computing applications may be more compelling.

Reusable biosensors
Imagine a swarm of tiny devices only a few hundred nanometers in size that can detect trace amounts of toxins in a water supply or the very earliest signs of cancer in the blood. Now imagine that these tiny sensors can reset themselves, allowing for repeated use over time inside a body of water…or a human body. Improving nanodevice biosensors is the goal of researchers at the Yale School of Engineering & Applied Science who have reported a recent breakthrough in designing electronic biosensors that can be regenerated and reused repeatedly.

Biosensors are used to detect and measure toxins in the environment; in the body they can identify chemical biomarkers that signal cancer and disease states by detecting changes at the molecular level. The researchers have created biosensors using silicon nanowires configured as tiny transistors that are exponentially more sensitive than current sensing technology, in addition to being cheaper and easier to use.

The method adds a layer of molecules to the surface of the biosensor that can be chemically regenerated, allowing for reuse. The ability to recharge nanodevice biosensors makes them more useful for applications like the remote monitoring of toxins or biothreats.

In addition to pollution and toxin detection, nanodevice biosensor technology has the potential to transform health care, enabling the diagnosis of diseases long before they can be detected by current methods and allowing for much earlier intervention and treatment.

~Ann Steffora Mutschler