Quantum bits, unlike their classical counterparts, can assume an infinite number of states, rather than just 0 or 1. Electron spin is one example of a quantum bit — a ‘qubit’ — and like a magnet, it can be oriented ‘up’ or ‘down’ in a magnetic field, but it can also be both slightly ‘up’ and ‘down’ at the same time.

To perform logical operations using qubits, it is necessary to preserve a given bit in its state — that is, the particular combination of ‘up’ and ‘down’ needed to define the state. If the spin is associated with an electron in a solid, vibrations or the spins of the surrounding nuclei and other electrons can act to destroy this state.

This is why Jiangeng Du and his colleagues at the University of Science and Technology of China in Hefei1 are exploring a technique called ‘dynamical decoupling’ to reduce the effects of the surrounding noise. The idea is to flip the spin several times using a magnetic field so that its interaction with the noisy environment averages to zero. The challenge, however, is to do this the minimum number of times, since the flips themselves introduce some error.

Fig. 1: When irradiated with gamma rays, the malonic acid molecule shown here contributes one spin (blue arrow) that could potentially act as a quantum bit.

The team showed how to optimize this series of flips on the electron spins in an organic crystal of malonic acid, which consists of two (CH2)(COOH)2 molecules (Fig. 1). The crystals were irradiated with gamma rays, which left one of the carbon atoms in each acid without a bond and therefore possessing a free electron spin.;

Normally, if the spins are polarized with a magnetic field to point ‘up’, they would lose their polarization with respect to one another within about 40 nanoseconds because of the noise from the surrounding solid. However, by applying a sequence of seven magnetic field pulses, Du and his team were able to extend this time by nearly a factor of one thousand.

Team member Renbau Liu emphasizes that “this work shows that the method can solve an outstanding roadblock in realizing quantum computing, even for spin qubits in solids that are at room temperature.”