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: Missing link in metal physics explains Earth's magnetic field

add to favorites email to friend print save as pdf

Related Stories

High-temperature superconductor 'fingerprint' found

Jan 07, 2015

(Phys.org) —Theorists and experimentalists working together at Cornell may have found the answer to a major challenge in condensed matter physics: identifying the smoking gun of why "unconventional" superconductivity occurs, ...

New light shed on electron spin flips

Jan 07, 2015

Researchers from Berlin Joint EPR Lab at Helmholtz-Zentrum Berlin (HZB) and University of Washington (UW) derived a new set of equations that allows for calculating electron paramagnetic resonance (EPR) transition ...

A qubit candidate shines brighter

Dec 29, 2014

In the race to design the world's first universal quantum computer, a special kind of diamond defect called a nitrogen vacancy (NV) center is playing a big role. NV centers consist of a nitrogen atom and ...

Recommended for you

Particle physicists discuss JUNO neutrino experiment

5 hours ago

The construction of the facilities for the JUNO neutrino experiment has been initiated with an official groundbreaking ceremony near the south Chinese city of Jiangmen. Involved in the Jiangmen Underground ...

New pathway to valleytronics

Jan 27, 2015

A potential avenue to quantum computing currently generating quite the buzz in the high-tech industry is "valleytronics," in which information is coded based on the wavelike motion of electrons moving through ...

New portable vacuum standard

Jan 26, 2015

A novel Portable Vacuum Standard (PVS) has been added to the roster of NIST's Standard Reference Instruments (SRI). It is now available for purchase as part of NIST's ongoing commitment to disseminate measurement ...

Prototype for first traceable PET-MR phantom

Jan 26, 2015

As cancer diagnostic tools, a new class of imagers – which combines positron-emission tomography (PET) with magnetic resonance imaging (MR or MRI) – has shown promise in the few years since these hybrid ...

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.