Scientific Method / Science & Exploration

A single molecule magnet may enable quantum computing

Quantum device manipulates and measures four spin states of a single nucleus.

The terbium atom (red) is sandwiched between two organic molecules (grey and blue) to form a single-molecule magnet.

Spin is one of the intrinsic quantum properties of particles. The spin of electrons orbiting an atom has significant consequences, such as determining the magnetic properties of materials. Atomic nuclei also have spin, but that is harder to manipulate: it interacts less with other spins and nuclei are much more massive, so they aren't as easily moved. However, those very properties could make nuclear spin a good option for for quantum computing, since the spin state of a nucleus is less subject to environmental influences that might alter its state. But reading out the nuclear spin state is notoriously difficult.

A new proof-of-principle experiment by Romain Vincent, Svetlana Klyatskaya, Mario Ruben, Wolfgang Werndorfer, and Franck Balestro measured the nuclear spin of a single atom. The nucleus belonged to a terbium (Tb) ion inside a larger molecule, which the researchers linked to a gold nanowire to construct a transistor-like device. They measured the four possible nuclear spin states, and observed them to be stable for tens of seconds—long enough to perform entanglement and other quantum-information processes.

Spin is integral to particles: all electrons (for example) have the same amount of spin. The spin quantum state is the relative orientation of the spin with respect to some other spin, or to an external magnetic field. Electrons are low mass particles and relatively lightly bound to atoms, so their spins are fairly easy to manipulate. As a result, the spins of atoms are typically determined by their electrons—including the magnetic properties. However, because electrons' spins are subject to strong environmental influences, they are somewhat unreliable from a quantum information perspective. If you write information to an electron's spin, it won't stay written for long.

Atomic nuclear spin is less strongly linked to the environment, which makes it both harder to manipulate and to measure. However, if those problems can be overcome, nuclear spin could potentially be better for quantum computing, since it's relatively stable.

The technique described in a recent Nature paper involves creating a single-molecule magnet (SMM). While they are not "permanent magnets" (ferromagnets), which possess magnetic properties in isolation, SMMs exhibit significant magnetism when exposed to an external magnetic field. In this case, the SMM was a triply ionized terbium atom (Tb3+) sandwiched between two complex organic molecules (called aromatic phthalocyanine (Pc) ligands, for the curious). The missing electrons in the Tb3+ and its coupling to the organic molecules made the nuclear spin far more accessible to external stimuli. The particular spin configuration had four possible quantum states (making it a spin-3/2 system, in contrast to the two-state, spin-1/2 electrons).

The SMM was deposited on a gold nanowire with three connection points, so that it acted as a transistor. While under ordinary (equilibrium) conditions, all four spin states would be equally probable, the researchers could induce transitions between them by adjusting the voltage at the connection points. The particular transition used a phenomenon known as quantum tunneling of magnetization (QTM)—the Tb normally wouldn't be able to switch magnetic states, but by applying that external current, these transitions could occur.

They also were able to measure how the quantum state of the Tb3+ changed over time, showing that the induced spin state was stable over a period of several seconds, after which thermal and other fluctuations damped out the particular chosen spin state until all four states occurred with equal likelihood. While this is a short period in everyday terms, it's quite stable by quantum standards.

Stable, coherent spin states that can be measured non-destructively are extremely useful for quantum computing. Two or more such states could be entangled, allowing for the implementation of algorithms and transfer of information. The terbium ion also possesses four quantum states, meaning the SMM transistor could potentially realize algorithms that currently only lie in the realm of theory. While this experiment only involved one SMM at a time and therefore didn't implement entangled states, it's a significant step toward studying quantum devices based on nuclear spin.

Nature, 2012. DOI: 10.1038/nature11341  (About DOIs).

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