Molecular engineers record an electron’s quantum behavior

These images show a diamond sample with a hemispherical lens (right and lower left), and the location of a single electron spin/quantum state visible through its light emission (upper left). The scale bar on the image at upper left measures five microns, the approximate diameter of a red blood cell. Credit: Courtesy of Awschalom Lab/University of Chicago

A team of researchers led by the University of Chicago has developed a technique to record the quantum mechanical behavior of an individual electron contained within a nanoscale defect in diamond. Their technique uses ultrafast pulses of laser light both to control the defect’s entire quantum state and observe how that single electron state changes over time. The work appears in this week’s online Science Express and will be published in print later this month in Science.

This research contributes to the emerging science of quantum information processing, which demands that science leave behind the unambiguous universe of traditional binary logic—0 or 1—and embrace the counterintuitive quantum world, where behavior is radically different from what humans experience every day. While people are generally content being in one place at a time, electrons can be in many states at once.

The team researches a quantum mechanical property of the electron known as spin. Much like conventional computers use the charge state of electrons to constitute bits of information, a quantum computer would use the spin state of a single electron as its quantum bit, or qubit. The work could accelerate development of quantum computing devices, and the extra computing power that would come with them, because it will be easier to identify materials that have appropriate quantum properties.

Scientists at the University of Chicago and the University of California, Santa Barbara, used this optical apparatus to direct ultrafast pulses of light to manipulate the quantum state of a single electron spin in diamond. Credit: Courtesy of Christopher Yale and Joseph Heremans/University of Chicago

The spin system studied is known as the nitrogen-vacancy (NV) center, an atom-sized defect that occurs naturally in diamond, consisting of a nitrogen atom next to a vacant spot in the crystal lattice. “These defects have garnered great interest over the past decade, providing a test-bed system for developing semiconductor quantum bits as well as nanoscale sensors,” said team leader David Awschalom, the Liew Family Professor of Molecular Engineering at UChicago. “Here, we were able to harness light to completely control the quantum state of this defect at extremely high speeds.”

Quantum snapshots

In this new technique, the researchers locate a single NV center and then illuminate it with a pair of extremely short pulses of laser light. Each pulse lasts less than a picosecond (or a millionth of a millionth of a second). The first pulse excites the quantum states of the defect-bound electron, which then change or evolve in characteristic ways. The second pulse stops that evolution, capturing a picture of the quantum state at that elapsed time.

Scientists at the University of Chicago and the University of California, Santa Barbara, used this optical apparatus to direct ultrafast pulses of light to manipulate the quantum state of a single electron spin in diamond. Credit: Courtesy of …more

By progressively extending the elapsed time between the two pulses, the team creates a sequence of quantum-state snapshots—a movie of how the quantum state changes in time. The elapsed time can be as short as femtoseconds (a billionth of a millionth of a second) or as long as nanoseconds. On the human scale, this range of time is like the difference between an hour and a century.

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