E-Fields, Nuclear Spin, Qubits, Silicon...

Photograph of a sapphire wafer that is patterned with the photonic bandgap resonators used in this work. It shows two full devices and parts of four others. Before using them in experiments they are cut out of the array and wired up. The devices themselves are about 1 cm long. The serpentine structures are microwave Bragg mirrors and the straight lines of varying width at the center of each device are the microwave cavities. Courtesy: A Sigillito

Topics: Electromagnetism, Solid State Physics, Quantum Dots, Quantum Mechanics

Researchers at Princeton University and the Lawrence Berkeley National Laboratory have succeeded in controlling nuclear spins in silicon by purely electrical means. Until now, electronic or nuclear spins could only be manipulated through radio-frequency magnetic fields. The feat could help in the development of quantum processors based on nuclear spin qubits.

Classical computers store and process information as "bits" that can have one of two logic states ("0" or "1"), but quantum computers work on the principle that a quantum particle (such as an electron or atomic nucleus) can be in two states at the same time – "spin up" or "spin down". These two spin states represent a logical "1") or a "0", so N such particles –or quantum bits (qubits) – could be combined or "entangled" to represent 2N values simultaneously. This would lead to the parallel processing of information on a massive scale not possible with conventional computers.

In practice, it is difficult to make even the simplest quantum computer, however, because these quantum states are fragile and are easily destroyed. They are also difficult to control. For a qubit to work, it should thus be well isolated from its environment to preserve its quantum properties, and prevent "decoherence". At the same time it should be robust enough so that its state can be read out and manipulated. The intrinsic magnetic moment of an atomic core, or nuclear spin, is a good qubit candidate in this respect because it fulfills all of these criteria.

There is a problem, however, in that the magnetic moment of a nuclear spin is 10 billion times smaller than the moment of one bit of a modern hard drive, and it is almost impossible to detect, let alone manipulate, such a tiny signal.

Electric fields control nuclear spin qubits in silicon, Belle Dumé, Nanotechweb.org