New nanoscale device reveals behavior of individual electrons
Laptop computers can generate enough heat that in rare cases, they actually catch fire. The fact is that while engineers have a great grasp of how to control electrical charge in circuits, getting rid of the heat created by flowing electrons is another matter. What's missing is a fundamental understanding of how individual electrons generate heat. A new device developed by University of Wisconsin-Madison Electrical and Computer Engineering Associate Professor Robert Blick not only promises to change that, but also will provide insights into harnessing quantum forces for communication and computing.
Blick, his graduate student Eva Hoehberger and his colleague Werner Wegscheider, developed something like an incredibly small, tunable trampoline for bouncing individual electrons. The device is just 100 nanometers wide or about one ten-millionth of an inch. Featured on the cover of the June 9 issue of Applied Physics Letters, the tool operates as an artificial atom suspended over a semiconductor cavity. It will allow researchers to study for the first time, in detail, the influence of heat dissipation on single electron transport in these transistors.
It looks and acts, in a way, like an incredibly small guitar. Blick says the effects of heat dissipation will show up as vibrations of the suspended artificial atoms. A conventional guitar string vibrates at several thousand cycles per second, but if you reduced the size to several hundred nanometers, the string would vibrate at speeds in the gigahertz regime or around a billion cycles per second. On that scale, the movement in the string, or suspended membrane in this case, is incredibly small. But the motion of the membrane, relative to a nearby conductor or gate causes a change in voltage that researchers can measure.
“Our system is comprised with many gates so that we can study the full variety of electronic systems starting with two-dimensional electron flows, which is common in many transistors these days, ” Blick says. “We can then reduce that to a channel where electrons flow in only one dimension like a string of electrons, and finally we can tune the device to a zero-dimensional state, which is the so-called single-electron transistor. We can bounce around single electrons, very controlled, and see how they spread energy in these very thin membranes.”
Blick says understanding energy transfer at these levels offers very practical, near-term benefits for chip manufacturers. The device itself is constructed of semiconductor materials and at 100 nanometers; its size and fabrication represent the future of the industry. Lessons learned from this tool could allow engineers to optimize existing technology currently limited by heat dissipation.
But more importantly, in the longer term, the tool could reveal secrets that allow researchers to exploit the power of quantum computing and communication. In a conventional computer, the presence of a group of electrons shows up as a negative charge and represents the “zero state” in binary logic, called a bit. When that charge is missing, the “one state” is represented. But a quantum computer deals with the quantum mechanics of electrons, which can be used to define so-called quantum bits or qubits. These qubits can exist in more than one state at once. This frees quantum computers to calculate all the possible solutions to a complex problem simultaneously, rather than running through them one-by-one like their slower, serial counterparts. Key to developing a practical quantum computer will be understanding exactly what represents information and how to get it out of the device.
Blick's system, when tuned to the zero-dimension state, will allow researchers to observe an individual electron near the qubit level as it approaches what's known as the Heisenberg uncertainty principle. This law of nature holds that as soon as you try to exactly determine the whereabouts of a quantum mechanical particle, you can no longer be certain of where it is going, since any action to measure the particle changes the particle's condition.
“An electron spread out as a wave, as a fermionic particle, has a scale of some five nanometers and this is exactly what we can address with our device.” Blick says. “We can study information processing on the quantum level and see whether the Heisenberg principle gives us a real obstacle, or whether we can find ways around it by using quantum-nondemolition techniques.”