Laser Cooling Of Semiconductor Membranes Opens Doors For Quantum Computing

(CORDIS) — By innovatively combining two research fields – quantum physics and nanophysics – EU-funded Danish researchers have discovered a new method for laser cooling semiconductor membranes. Semiconductors are vital components in many electronic goods such as solar cells and light-emitting diodes (LEDs), and it is important to be able to cool these components for the future development of quantum computers and ultrasensitive sensors.

However, despite being called a cooling method, the technique the physicists employ works by doing exactly the opposite – heating up the material. Writing in the journal Nature Physics, the team, from the Niels Bohr Institute at the University of Copenhagen in Denmark, explains how they developed the use of lasers that were able to cool the membrane fluctuations to minus 269 degrees Celsius.

Their study received a EUR 4 700 000 funding boost as part of the Q-ESSENCE (‘Quantum interfaces, sensors and communication based on entanglement’) project, funded under the ‘ICT’ (Information and Communication Technologies) Theme of the EU’s Seventh Framework Programme (FP7).

Lead author of the study, Koji Usami, explains: ‘In experiments, we have succeeded in achieving a new and efficient cooling of a solid material by using lasers. We have produced a semiconductor membrane with a thickness of 160 nanometres and an unprecedented surface area of 1 by 1 millimetre. In the experiments, we let the membrane interact with the laser light in such a way that its mechanical movements affected the light that hit it. We carefully examined the physics and discovered that a certain oscillation mode of the membrane cooled from room temperature down to minus 269 degrees Celsius, which was a result of the complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances.’

The Danish team have long been perfecting their laser cooling of atoms technique, and have previously managed to cool gas clouds of caesium atoms down to near absolute zero, minus 273 degrees Celsius, using focused lasers. They managed to create entanglement between two atomic systems. This occurs when the atomic spin becomes entangled and the two gas clouds forge a link, due to quantum mechanics. Using quantum optical techniques, they measured the quantum fluctuations of the atomic spin.
‘For some time we have wanted to examine how far you can extend the limits of quantum mechanics – does it also apply to macroscopic materials? It would mean entirely new possibilities for what is called optomechanics, which is the interaction between optical radiation, i.e. light, and a mechanical motion,’ explains another study author, Professor Eugene Polzik.

But before they could see if their theories worked in practice, they needed to check they had the right materials for the job.

It all began back in 2009 when one member of the study team, Peter Lodahl, gave a lecture at the Niels Bohr Institute: he showcased a special photonic crystal membrane that was made of the semiconducting material gallium arsenide (GaAs). After hearing the lecture, Professor Polzik immediately thought that this nanomembrane would have many advantageous electronic and optical properties. He suggested that they use this kind of membrane for experiments with optomechanics, and after a year of experimenting with different dimensions, the team succeeded in making a suitable one.

The researchers managed to produce a nanomembrane that is only 160 nanometres thick, and with an area of more than 1 square millimetre.

In the experiment, they shone the laser light onto the nanomembrane in a vacuum chamber. When the laser light hit the semiconductor membrane, some of the light was reflected, and the light was reflected back again via a mirror in the experiment, so that the light flew back and forth in this space, forming an optical resonator. Some of the light was absorbed by the membrane and released free electrons. The electrons decayed and thereby heated the membrane, producing a thermal expansion. In this way, the distance between the membrane and the mirror was constantly changed in the form of a fluctuation.

Koji Usami, explains further: ‘Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances, and you can control the system so as to cool the temperature of the membrane fluctuations. This is a new optomechanical mechanism, which is central to the new discovery. The paradox is that even though the membrane as a whole is getting a little bit warmer, the membrane is cooled at a certain oscillation and the cooling can be controlled with laser light. So it is cooling by warming! We managed to cool the membrane fluctuations to minus 269 degrees Celsius.’

These findings could lead to the development of cooling components for quantum computers. A quantum computer is a device for computation that makes direct use of quantum mechanical phenomena such as superposition and entanglement to perform operations on data.

The main objectives of the Q-ESSENCE project are to develop quantum interfaces capable of high-fidelity mapping of quantum information between different quantum systems, the generation of quantum entanglement at new scales and distances as a resource to carry out quantum information tasks, and the engineering of multipartite entanglement in specific topologies of elementary systems.

This project also supports researchers from Australia, Austria, Germany, Italy, Poland, Slovakia, Spain, Switzerland, the Netherlands and the United Kingdom. Due to run until 2013, it will create opportunities in quantum information technologies that can be developed into realistic and complete schemes for executing ICT tasks.