Chips measure electron spin

By Eric Smalley, Technology Research News

Practical quantum computers are at least a decade away, and some researchers are betting that they will never be built.

This is because controlling individual particles like atoms, electrons and photons is extraordinarily challenging. Information carried in particles always comes in shades of gray and can be corrupted or wiped out by the slightest wisp of energy from the environment.

A pair of experiments has brightened prospects for quantum computing, however, by making it more likely that a practical means of reading electron-based quantum bits, or qubits, can be developed. Research teams from the University of California at Los Angeles and from Delft University of Technology in the Netherlands have developed electronic methods of detecting the spins of individual electrons.

Spin is a property of electrons that is akin to the rotation of a top. The two spin directions, spin up and spin down, are magnetically opposite, like the two poles of a kitchen magnet. The spins can represent the 1s and 0s and digital information.

Particles that are isolated from their environment are in the weird quantum state of superposition, meaning they are in some mix of the two spin directions. This means a qubit can be in some mix of 1 and 0, which allows a string of qubits to represent every binary number at once.

This gives a quantum computer the ability to check every possible answer to a problem with a single set of operations, promising speedy solutions to problems that classical computers have to churn through one answer at a time. These include factoring large numbers, a problem whose difficulty is the foundation of most of today's security codes.

Electronic equipment has become sensitive enough that it is no longer difficult to detect the presence of a single electron. But detecting an electron's spin orientation is another matter.

In recent years, researchers have succeeded in detecting electron spin optically using specialized laser setups. The key to using electron spin in quantum computers whose architecture is similar to today's computer chips is being able to detect the spin orientation electronically.

The UCLA team's method of electron spin detection uses devices that are already mass-produced. The researchers flipped a single electron spin in a commercial transistor chip, and detected the spin flip by measuring changes in current flowing through the device.

Several proposed quantum computer architectures call for circuits that can be manufactured using today's chipmaking techniques. "The transistor structure used for our experiment [closely] resembles some proposed spin-based qubit architectures," said Hong-Wen Jiang, a professor of physics at the University of California at Los Angeles. "We believe that our read-out scheme can be readily adapted in a scalable quantum information processor," he said.

Electrons travel through a transistor via a semiconductor channel that is electrically insulated. The transistor is controlled by a gate electrode, which produces an electric field that penetrates the insulator and increases the conductivity of the channel, allowing electrons to flow. Occasionally defects occur, producing one or more spots in the insulator that can draw individual electrons from the channel and trap them.

The researchers sought out transistors that contained single defect traps, set the gate voltage so that the trap had an equal chance of attracting an electron or not, and applied a large magnetic field to the trap.

A high magnetic field causes electrons in the spin-down state to have slightly more energy than spin-up electrons. The researchers flipped the electron's spin with a microwave pulse. An electron that is spin-up fills the trap but a higher-energy spin-down electron leaves room, electrically speaking, for a second, spin-up electron from the channel to join it in the trap.

The difference between having one and having two electrons in the trap is measurable as a change in the current flowing through the transistor. Two electrons decrease the amount of current. The researchers can observe a microwave pulse flipping the spin of an electron in the trap by measuring the current.

In its present form, the UCLA device uses a randomly-positioned defect as its electron trap, and electrons cycle through the trap rapidly enough that the spin measurement is an average of a few thousand electrons. The researchers are conducting similar experiments in specially designed semiconductor structures that promise greater control over electron spin, the ability to entangle two spins, and to eventually build a scalable quantum processor, said Jiang.

Properties of entangled particles, including spin, remain in lockstep regardless of the distance between them. Entanglement is a basic requirement of quantum algorithms, and entangled electrons would enable information to be teleported between circuits within a quantum computer.

Meanwhile, the Delft team devised a way to measure the spin of an electron trapped in a quantum dot -- a tiny semiconductor device that produces electric fields capable of confining one or a few electrons. "The technique works fully electrically, and is therefore... suitable for integration with existing solid-state technologies," said Jeroen Elzerman, a researcher at Delft University of Technology.

The researchers applied a large magnetic field to the trapped electron, which caused the spin-down state to have slightly more energy than the spin-up state. They tuned the quantum dot's electric field so that the energy of a spin-down electron was just high enough for it to escape, but the energy of a spin-up electron was below the threshold. Therefore, if an electron is present it is spin-up, and if the quantum dot is empty, the electron that escapes is spin-down.

The researchers next step is to to use pulsed microwaves to control the exact quantum superposition of the spin, said Elzerman. They then plan to entangle two spins. "When this is done, all the basic ingredients for a quantum computer are in place," he said.

Coupling many spins and controlling their interactions accurately enough to perform a quantum algorithm is a matter of improving control over the fabrication process, said Elzerman. "We need cleaner and purer materials and more reproducible electron beam lithography so that all dots on a single chip are really identical," he said.

Jiang's research colleagues were Ming Xiao and Eli Yablonovitch of UCLA, and Ivar Martin of Los Alamos National Laboratory. They published the research in the July 22, 2004 issue of Nature. The research was funded by the Defense Advanced Research Projects Agency (DARPA) and the Defense Microelectronics Activity (DMEA).

Elzerman's research colleagues were Ronald Hanson, Laurens Willems van Beveren, Benoit Witkamp, Lieven Vandersypen and Leo Kouwenhoven. They published the research in the July 22, 2004 issue of Nature. The research was funded by DARPA, the Office of Naval Research, the European Union and the Dutch Organization for Fundamental Research on Matter (FOM).

Timeline:   10 years; 10-20 years
Funding:   Government
TRN Categories:   Physics; Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical papers, "Electrical detection of the spin resonance of a single electron in a silicon field-effect transistor," Nature, July 22, 2004; "Single-shot read-out of an individual electron spin in a quantum dot," Nature, July 22, 2004


August 11/18, 2004

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