Researchers develop optical technique for controlling electron spins in quantum dot ensembles

Nov 15, 2007

Scientists are closer to developing novel devices for optics-based quantum computing and quantum information processing, as a result of a breakthrough in understanding how to make all the spins in an ensemble of quantum dots identical.

This understanding, based upon a new optical technique and announced recently by researchers at the Naval Research Laboratory (NRL), the University of Dortmund, and the University of Bochum, is an important step toward realization of such quantum devices based on solid-state technology. The complete findings of the study are published in the September 28, 2007, issue of the journal Science.

An electron spin localized in a quantum dot is the quantum bit, which is the basic unit for solid-state based quantum computing and quantum information processing. The spin replaces a classical digital bit, which can take on two values, usually labeled 0 and 1. The electron spin can also take on two values.

However, since it is a quantum object, it can also take all values in between. Obviously, such a quantum unit can hold much more information than a classical one and, even more importantly the use of such quantum bits makes certain computer calculations exponentially more efficient than those using a standard computer. That is why, scientists around the world are trying to find an efficient way to control and manipulate the electron spin in a quantum dot in order to enable new quantum devises using magnetic and electric fields.

Until now, the major problem with using charged quantum dots in such devices is that the electron spins in different quantum dots are never identical. The electron spin precession frequencies in an external magnetic field are different from each other due to small variations of the quantum dot shape and size. In addition, the electron spin precession frequency has a contribution of a random hyperfine field of the nuclear spins in the quantum dot volume. This makes a coherent control and manipulation of electron spins in an ensemble of quantum dots impossible and pushes researchers to work with individual spins and to develop single spin manipulation techniques, which are much more complicated than an ensemble manipulation technique.

The team of researchers at the University of Dortmund, NRL and the University of Bochum has taken a significant step toward solving this problem by suggesting a new technique that would allow coherent manipulations of an ensemble of electron spins. Last year in a Science publication (Science, vol. 313, 341 (2006)), the same research team demonstrated a method, whereby a tailored periodic illumination with a pulsed laser can drive a large fraction of electron spins (up to 30%) in an ensemble of quantum dots into a synchronized motion.

In the new Science publication, the team shows that almost the whole ensemble of electron spins (90%) precesses coherently under periodic resonant excitation. It turns out that the nuclear contribution to the electron spin precession acts constructively by focusing the electron spin precession in different quantum dots to a few precession modes controlled by the laser excitation protocol, instead of acting as a random perturbation of electron spins, as it was thought previously. The modification of the laser protocol should allow scientists to reach a situation in which all electron spins have the same precession frequency, in other words to make all spins identical.

Future efforts involving the use of these identical electron spins will focus on demonstrating all coherent single q-bit operations using an ensemble of charged quantum dots. Another important use of such ensembles for quantum computing will be the demonstration of a quantum-dot gate operation. The macroscopic coherent precession of the electron spin ensemble will allow scientists to study several optical coherent phenomena, such as electromagnetically induced transparency and slow light, for example.

Source: Naval Research Laboratory

Explore further: Finding faster-than-light particles by weighing them

add to favorites email to friend print save as pdf

Related Stories

Scientists resolve spin puzzle

Dec 10, 2014

Scientists at the University of York have helped to uncover the properties of defects in the atomic structure of magnetite, potentially opening the way for its use in producing more powerful electronic devices.

Physicists explain puzzling particle collisions

Dec 10, 2014

An anomaly spotted at the Large Hadron Collider has prompted scientists to reconsider a mathematical description of the underlying physics. By considering two forces that are distinct in everyday life but ...

Superconductivity without cooling

Dec 04, 2014

Superconductivity is a remarkable phenomenon: superconductors can transport electric current without any resistance and thus without any losses whatsoever. It is already in use in some niche areas, for example ...

Recommended for you

Finding faster-than-light particles by weighing them

Dec 26, 2014

In a new paper accepted by the journal Astroparticle Physics, Robert Ehrlich, a recently retired physicist from George Mason University, claims that the neutrino is very likely a tachyon or faster-than-light par ...

Controlling core switching in Pac-man disks

Dec 24, 2014

Magnetic vortices in thin films can encode information in the perpendicular magnetization pointing up or down relative to the vortex core. These binary states could be useful for non-volatile data storage ...

Atoms queue up for quantum computer networks

Dec 24, 2014

In order to develop future quantum computer networks, it is necessary to hold a known number of atoms and read them without them disappearing. To do this, researchers from the Niels Bohr Institute have developed ...

New video supports radiation dosimetry audits

Dec 23, 2014

The National Physical Laboratory (NPL), working with the National Radiotherapy Trials Quality Assurance Group, has produced a video guide to support physicists participating in radiation dosimetry audits.

Acoustic tweezers manipulate cell-to-cell contact

Dec 22, 2014

Sound waves can precisely position groups of cells for study without the danger of changing or damaging the cells, according to a team of Penn State researchers who are using surface acoustic waves to manipulate ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

holoman
not rated yet Nov 16, 2007
Try rotating the object and using different plus / minus electric fields at sametime with laser to change electron spin. The effect will be enlightening.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.