System Bits: July 9

New quantum algorithm; flat transistors.


New quantum computing algorithm
Researchers at the University of California, San Diego, have proposed a new algorithm for quantum computing that they believe will speed a particular type of problem…but swifter calculations would come at the cost of greater physical resources devoted to precise timekeeping.

The algorithm would be used to conduct a task called an unstructured search. The goal is to locate a particular item within an unsorted pile of data. Solving this problem on a classical computer, which uses 1s and 0s stored on magnetic media, is akin to flipping through a deck of cards, one by one, and searching through a large data set could take a very long time.

Quantum computing, based on matter held in a quantum state, often for quite brief periods of time, takes advantage of an oddity of the quantum world in which a particle, like a photon or a boson, can exist in more than one state at once, a property called superposition. This would allow multiple possibilities to be considered simultaneously, though once measured, quantum objects will yield a single answer.

The trick then, the researchers explained, is to design algorithms so that wrong answers cancel out and correct answers accumulate. The nature of those algorithms depends on the medium in which information is stored.

The UCSD researchers considered a computer based on a state of matter called a Bose-Einstein condensate, which are atoms caught in an electromagnetic trap and chilled so cold that they “fall” into a shared lowest quantum state and act as one. The equation usually used to describe quantum systems is linear, but the one that approximates the state of a Bose-Einstein condensate has a term that is cubed.

The researchers propose computing with this cubic equation which will more rapidly converge on the answer but because the search is so sudden, timekeeping, which uses an atomic clock, would have to be very precise. This requirement sets a lower limit on the number of ions that make up the atomic clock.

Next gen green electronics
Researchers at UC Santa Barbara, in collaboration with University of Notre Dame, have recently demonstrated the highest reported drive current on a transistor made of a monolayer of tungsten diselenide (WSe2), a 2-dimensional atomic crystal categorized as a transition metal dichalcogenide (TMD). The discovery is also the first demonstration of an “n-type” WSe2 field-effect-transistor (FET), showing the potential of this material for future low-power and high-performance integrated circuits.

Monolayer WSe2 is similar to graphene in that it has a hexagonal atomic structure and derives from its layered bulk form in which adjacent layers are held together by relatively weak Van der Waals forces. However, WSe2 has a key advantage over graphene in that in addition to its atomically smooth surfaces, it has a considerable band gap of 1.6 eV.

Understanding the nature of the metal-TMD interfaces was key to the successful transistor design and demonstration, the researchers said, who pioneered a methodology using ab-initio Density Functional Theory (DFT) that established the key criteria needed to evaluate such interfaces leading to the best possible contacts to the monolayer TMDs. The DFT technique was pioneered by UCSB professor emeritus of physics Dr. Walter Kohn, for which he was awarded the Nobel Prize in Chemistry in 1998.

Guided by the contact evaluation methodology, the transistors achieved ON currents as high as 210 uA/um, which are the highest reported value of drive current on any monolayer TMD based FET to date. They were also able to achieve mobility of 142 cm2/V.s, which is the highest reported value for any back-gated monolayer TMD FET.

~Ann Steffora Mutschler