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June 26, 2012

Rewriting Quantum Chips with a Beam of Light

The promise of ultrafast quantum computing has moved a step closer to reality with a technique to create rewritable computer chips using a beam of light. Researchers from The City College of New York (CCNY) and the University of California Berkeley (UCB) used light to control the spin of an atom’s nucleus in order to encode information.

Researchers have developed a technique to use laser light to pattern the alignment of “spin” within atoms so that the pattern can be rewritten on the fly. Such a technique may one day lead to rewritable spintronic circuits.

Digital electronics and conventional computing rely on translating electrical charges into the zeros and ones of binary code. A “spintronics” computer, on the other hand, would use the quantum property of electron spin, which enables the electron to store any number between zero and one.


Stray-field apparatus for imaging nuclear magnetism. The strong gradient in the stray field of a superconducting magnet provides the means for NMR imaging in one-dimension. Circularly polarized illumination is directed to the sample through an optical window on the sapphire sample support

Nature Communications - Optically rewritable patterns of nuclear magnetization in gallium arsenide



The control of nuclear spin polarization is important to the design of materials and algorithms for spin-based quantum computing and spintronics. Towards that end, it would be convenient to control the sign and magnitude of nuclear polarization as a function of position within the host lattice. Here we show that, by exploiting different mechanisms for electron–nuclear interaction in the optical pumping process, we are able to control and image the sign of the nuclear polarization as a function of distance from an irradiated GaAs surface. This control is achieved using a crafted combination of light helicity, intensity and wavelength, and is further tuned via use of NMR pulse sequences. These results demonstrate all-optical creation of micron scale, rewritable patterns of positive and negative nuclear polarization in a bulk semiconductor without the need for ferromagnets, lithographic patterning techniques, or quantum-confined structures.


The probe head used to send radio-frequency pulses onto the coil used for pulsed spin manipulation of a gallium arsenide (semiconductor) sample. (Credit: Yunpu Li)

Imagine this as if the electron were a “yin-yang” symbol in which the proportions of the dark and light areas—representing values from zero to one—could vary at will. This would mean that multiple computations could be done simultaneously, which would amp up processing power.

Attempts at using electrons for quantum computing have been plagued, however, by the fact that electron spins switch back and forth rapidly. Thus, they make very unstable vehicles to hold information. To suppress the random switching back and forth of electrons, the UCB and CCNY researchers used laser light to produce long-lasting nuclear spin “magnets” that can pull, push, or stabilize the spins of the electrons.

They did this by illuminating a sample of gallium arsenide – the same semiconductor used in cell phone chips – with a pattern of light, much as lithography etches a physical pattern onto a traditional integrated circuit. The illuminated pattern aligned the spins of all the atomic nuclei, and, thus, their electrons, at once, creating a spintronic circuit.

“What you could have is a chip you can erase and rewrite on the fly with just the use of a light beam,” said Professor Meriles. Changing the pattern of light altered the layout of the circuit instantly.

“If you can actually rewrite with a beam of light and alter this pattern, you can make the circuit morph to adapt to different requirements,” he added. “Imagine what you can make a system like that do for you!”


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