While classical computers store and process information as "bits" that can have one of two logic states – "0" or "1" – quantum computers will 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”. The 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 because the fragile nature of these quantum states means that they are easily destroyed and are difficult to control. For a qubit to work, it should be well isolated from its environment to preserve its quantum properties, and prevent “decoherence”, and at the same time 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 because it fulfills all of these criteria.

"Double decker" magnet

The magnetic moment of a nuclear spin is 10 billion times smaller than the moment of one bit of a modern hard drive, explains team leader Wolfgang Wernsdorfer of the Institut Néel in Grenoble. To detect such a tiny signal we have developed a very sensitive magnetic field sensor – the single-molecule magnetic spin-transistor (see figure).

“The heart of this device is a terbium ‘double decker’ single-molecule magnet that both amplifies and detects terbium’s nuclear spin. To amplify the nuclear spin signal, we exploit the so-called hyperfine interaction between the terbium’s nuclear and electronic spin,” he told nanotechweb.org. “By mapping the nuclear spin state onto the electronic spin state, we can amplify the signal by over 1000 times.”

The researchers are then able to read out the nuclear spin-dependent magnetic moment of the terbium via the tunnel current through the molecule, since terbium’s electronic spin strongly interacts with the tunnelling electrons via a phenomenon called exchange coupling.

The Stark effect and manipulating the nuclear spin state

The team, which includes scientists from the Université de Grenoble, the Institut Universitaire de France in Paris, the Institute of Nanotechnology at the Karlsruhe Institute of Technology and the Institut de Physique et de Chimie des Materiaux in Strasbourg, says that it can manipulate terbium’s nuclear spin using just an oscillating electric field. “That we are able to do this at all is remarkable in itself, since the magnetic moment of the nuclear spin is, by its very nature, insensitive to external electric fields,” says team member Stefan Thiele in Grenoble.

The experiments work thanks to the "Stark effect", he explains. An applied electric field modifies the electronic wave functions of the terbium. These modifications, in turn, change the hyperfine interactions and these altered interactions are seen as a change of the effective magnetic field at the nucleus. “An oscillating electric field is thus transformed into an oscillating effective magnetic field, and if the frequency of this oscillation resonates with the nuclear spin level spacing, we are able to manipulate the nuclear spin state.”

And that is not all: since the hyperfine Stark effect also exists in other nuclear spin qubits, like those in bismuth or phosphorus impurities in silicon, the researchers reckon that the result should apply to these systems too. “Our technique could thus be a very general way to electrically manipulate nuclear spin-based devices,” said Thiele.

The team says that it would now like to entangle two different single-nuclear spins by using a macromolecule composed of a couple of terbium single molecule magnets. “Doing this would be proof that scaling nuclear spin qubits from the bottom up is indeed possible,” added Thiele.

Full details of the work are detailed in Science DOI: 10.1126/science.1249802.