In today’s Science Express, the advance online
publication of the journal Science, researchers report a series of
experiments that mark an important step toward understanding a
longstanding fundamental physics problem of quantum mechanics. The
scientists presented their findings at the annual meeting of the
American Physical Society here this week.
The problem the physicists addressed is how a fundamental
particle in matter loses track of its quantum mechanical properties
through interactions with its environment.
The research was performed by scientists at the California
NanoSystems Institute at the University of California, Santa Barbara
and the U. S. Department of Energy Ames Laboratory in Iowa.
At the quantum level things like particles or light waves
behave in ways very different from what scientists expect in a
human-scale world. In the quantum world, for example, an electron can
exist in two places at the same time, what is called a "superposition"
of states, or spin up and down at the same time.
Quantum mechanics in computing could lead to communication
with no possible eavesdropping, lightning-fast database searches, and
The answer to the problem the researchers have tackled is key
to unraveling how the classical world in which we live emerges from all
the interacting quantum particles in matter. This scientific query
surrounds the basic quantum dynamics of a single particle spin coupled
to a collection, or bath, of random spins. This scenario describes the
underlying behavior of a broad class of materials around us, ranging
from quantum spin tunneling in magnetic molecules to nuclear magnetic
resonance in semiconductors.
“We were stunned by these unexpected experimental
results, and extremely excited by the ability to control and monitor
single quantum states, especially at room temperature,” said
author David Awschalom, a professor of physics at UC Santa Barbara.
Awschalom is affiliated with the California NanoSystems Institute at
UCSB and is the Director of the Center for Spintronics &
Quantum Computation, also at the university.
Recently the issue of how fundamental particles lose track of
quantum mechanical properties through interaction with the environment
has gained crucial importance in the field of quantum information. In
this area, robust manipulation of quantum states promises enormous
speedups over classical computation. Keeping track of the quantum phase
is essential for keeping the quantum information, and insight into loss
of the phase will greatly help to mitigate this process.
Experimental work on this subject has thus far been hindered
by the lack of high-fidelity coherent control of a single spin in
nature and our inability to directly influence the bath dynamics.
In a collaboration between physicists in Awschalom’s
research group at UCSB and Slava Dobrovitski, a visiting scientist from
Ames Laboratory in Iowa, a series of experiments were undertaken that
utilized electron spins in diamond to investigate different regimes of
spin-bath interactions, and provide much information about the
The scientists use diamond crystals to study a single electron
spin tied to an adjustable collection of nearby spins. Two features of
diamond that make this system viable for unprecedented investigations
into the coherent dynamics are the precise optical control of a single
spin that is unique to diamond, and the magnetic tunability of the
spin-bath and intrabath dynamics with small permanent magnets. Their
team’s observations contain a number of extraordinary
discoveries, such as the time-dependent disappearance and reappearance
of quantum oscillations of the spins in the diamond lattice.
“To our surprise, when looking at longer times, the
oscillations disappeared then re-appeared,” said co-author
Ronald Hanson, a postdoctoral student at UCSB during this period who is
now a professor at the Kavli Institute of Nanoscience Delft, at Delft
University of Technology, in the Netherlands. “At first it
looked like an artifact, but repeated measurements reproduced this
The problem of a single spin coupled to a bath of spins has
been the subject of an intense international research effort, as this
conceptual framework describes the physical behavior of a number of
real systems. Among others, these include atomic and electronic spins
that are prime candidates for implementing quantum information
processors and coherent spintronics devices.
A series of direct experiments coupled to theoretical
simulations demonstrate that spins in diamond serve as a nearly ideal,
adjustable, model of central spin.
“This work demonstrates a rare level of synergy
between experiment, analytical theory, and computer
simulations,” said Dobrovitski. “These three
constituents all agree, support, and complement each other. Together,
they give a lucid qualitative picture of what happens with spin centers
in diamond, and, at the same time, provide a quantitatively accurate
description. This agreement is hard to anticipate in advance for such
complex systems, where many nuclear and electron quantum spins interact
with each other.”
Studies of the quantum dynamics of spins in diamond is an
emerging topic involving several leading research groups worldwide. It
may also be important in the context of recent interest in possible
carbon-based electronic devices employing carbon nanotubes and/or