PORTLAND, Ore. -- Ionic rain irrigates forests of nanotubes, while ionic winds blow cool breezes over chips. Now, the ionic solid-state harbors the flag-ship of quantum computing--quantum bits. Q-bits can now be encoded on the spin of the electron that makes a quantum dot ionic, promising cheap, ultra-low-power and easy-to-fabricate quantum computers, according to researchers at the University of Michigan (Ann Arbor), the University of California (San Diego) and the Naval Research Laboratory (Washington, D.C.).
Quantum computing could enable uncrackable encryption codes, accounting for the funding of such researcher by the National Security Agency (NSA). Quantum-computing milestones are no new thing these researchers who had already demonstrated the world's first quantum gate in a semiconductor.
"We had already demonstrated that a solid-state system could exhibit the qualities needed by quantum computers," said Duncan Steel, an EE professor at the University of Michigan. "But now we have an architecture that is scalable."
Steel's group is one of about a half a dozen worldwide that are using a photonic material--gallium arsenide--to house quantum dots that are activated by a laser. The indium arsenide quantum dots are 10- to 30-nanometers in diameter, surrounded by gallium arsenide. For the current demonstration, Steel's group had individual quantum dots grown at the Naval Research Laboratory. The original demonstration used lasers to toggle the indium arsenide atoms between two different quantum states of excitation, whereas the current demonstration encodes that quantum state onto a single electron inside the dot.
"Each quantum dot has about a billion electrons, but it is electrically neutral. We add one more electron to give a charge of minus one," said Steel.
According to Steel, encoding quantum bits on the spin of this single electron enables much longer quantum calculations to be performed--more than 100-fold longer than the coherence time of an atom. Unfortunately, all the designs of quantum computers, so far, have used the excitation of atoms to store quantum states. By moving to electrons in semiconductors these researchers claim that quantum computers can perform much longer calculations. What's more, all the manufacturing techniques for gallium-arsenide chips can be used to fabricating the quantum computer chips.
"The advantage of spin excitation is that its quantum memory is orders of magnitude longer--that's why so many groups are interested in it today," said Steel.
About six groups worldwide are working on storing q-bits on the spin of electrons in gallium arsenide, but Steel claims his group has consistently stayed on step ahead of the pack. Steel also claims that by encoding the q-bits onto a single electron, with a pulsed laser, his new design sets the standard for using ultra-low power to process information--over a millions times less than the nano-joules needed to switch a semiconductor memory bit.
"It only takes about ten to the minus 18 joules [18 billionths of a nano-joule] of power to switch from one quantum state to the other," said Steel.
Next, Steel's group will fabricate quantum dots so close together that their wavelengths overlap with the adjacent quantum dots in the array. The dual nature of matter enables the energy of an electron to be measured by its wavelength. By making the quantum dots close enough together, the wavelength of the electrons, the spin of which holds the q-bit, can be made to overlap adjacent quantum dots in the array, thereby enabling quantum computations to be performed by virtue of "entanglement."
"Right now we plan to demonstrate quantum entanglement by making our quantum dots about 20 nanometers in diameter and spacing them about 30 nanometers center to center," said Steel.