May 10 2017
In the last six decades, computers have become faster, compact, and cheaper. However, for engineers, the options have almost saturated for how small the silicon transistors can be made and how fast they can transfer electricity via devices to form digital ones and zeros.
Such a restriction has led Jelena Vuckovic, a Stanford electrical engineering Professor, to turn towards quantum computing, which is dependent on light and not electricity. Quantum computers operate by distancing the spinning electrons inside an innovative kind of semiconductor material. Once the electron is struck by a laser, it emits one or more quanta (or particles) of light to display the manner in which the electron spins. These spin states replace the ones and zeros of conventional computing.
Vuckovic, one of the leading scientists in the field, stated that quantum computing is perfect for analyzing biological systems, performing cryptography, or data mining — or even solving any challenge with a number of variables.
When people talk about finding a needle in a haystack, that’s where quantum computing comes in.
Vuckovic.
According to Marina Radulaski, a postdoctoral fellow in Vuckovic’s laboratory, the problem-solving ability of quantum computers arises from the complexity of the interactions between laser and electron fundamental to the concept.
With electronics you have zeros and ones. But when the laser hits the electron in a quantum system, it creates many possible spin states, and that greater range of possibilities forms the basis for more complex computing.
Radulaski.
Capturing electrons
Acquiring information related to the interactions between electrons and light is difficult. Few of the major technology companies around the world are endeavoring to construct massive quantum computers that are dependent on materials that are super-cooled to near absolute zero, which is the theoretical temperature at which the movement of atoms is restricted.
Vuckovic’s two decades of own research has focused on one facet of the problem, namely, developing innovative quantum computer chips that will be the building blocks of prospective systems.
To fully realize the promise of quantum computing we will have to develop technologies that can operate in normal environments. The materials we are exploring bring us closer toward finding tomorrow’s quantum processor.
Vuckovic.
The obstacle to be overcome by Vuckovic and her colleagues is to create materials with the ability to trap a single, isolated electron. The research team has worked alongside international collaborators and has recently investigated three disparate ways to overcome the problem. One way is to enable operations at room temperature, which is a crucial step if quantum computing is to be developed into a practical tool.
For all three approaches, the researchers began by using semiconductor crystals, which are materials that have a regular atomic lattice similar to the girders of a skyscraper. When the lattice is slightly modified, a structure can be developed in which the atomic forces applied by the material have the ability to trap a spinning electron.
We are trying to develop the basic working unit of a quantum chip, the equivalent of the transistor on a silicon chip.
Vuckovic.
Quantum dots
One method of developing such a laser-electron interaction chamber is by means of a structure called as a quantum dot. In physical terms, the quantum dot is a small quantity of indium arsenide enclosed inside a gallium arsenide crystal. The atomic characteristics of the two materials are known to confine a spinning electron.
In a latest paper published in the journal Nature Physics, Kevin Fischer (who is a graduate student at Vuckovic’s laboratory) has reported the ways in which laser-electron processes can be used within such a quantum dot to regulate the input and output of light. When more laser power is applied to the quantum dot, it can be forced to emit precisely two photons in the place of one. According to the researchers, the quantum dot has practical benefits when compared to other major quantum computing platforms. Yet, it mandates cryogenic cooling, and hence might not prove handy for general-purpose computing. However, it can be used for developing tamper-proof communication networks.
Color centers
Vuckovic employed a varied approach to electron capture in two other papers, where she modified a single crystal to confine light in the so-called color center.
In a latest paper in the journal NanoLetters, Vuckovic and her colleagues have analyzed color centers in diamond. Naturally, the crystalline lattice in diamond is made of carbon atoms. Jingyuan Linda Zhang (who is a graduate student in Vuckovic’s laboratory) reported the manner in which a 16-member research group substituted certain carbon atoms with silicon atoms. The single modification led to the formation of color centers that could efficiently confine spinning electrons inside the crystalline lattice in diamond.
However, similar to the quantum dot, most of the diamond color center experiments mandate cryogenic cooling. Despite the fact that it is an enhancement over other techniques that mandated an elaborate cooling, Vuckovic aspired to achieve more.
Therefore, she collaborated with another international team of researchers to analyze a third material, namely, silicon carbide. Silicon carbide is generally called as carborundum, and is a hard, transparent crystal used for manufacturing brake pads, clutch plates, and bulletproof vests. Earlier studies have demonstrated that silicon carbide can be altered to form color centers at ambient temperature. However, this potential has not been made adequately efficacious to synthesize a quantum chip.
Vuckovic and her colleagues removed specific silicon atoms from the silicon carbide lattice to form highly efficacious color centers. They further produced nanowire structures around the color centers to enhance photon extraction. Radulaski was the first author of that study, which was reported in another paper published in NanoLetters. According to Radulaski, the net outcomes, such as efficacious color center, working at ambient temperature, in a material well known in the industry, were highly advantageous.
We think we’ve demonstrated a practical approach to making a quantum chip.
Radulaski.
However, this field is just emerging and electron confinement is not so easy. Not even the scientists are confident on the technique, or techniques, that will be effective.
We don’t know yet which approach is best, so we continue to experiment.
Vuckovic.