Coherence in a crowd of molecular-spin qubits

The electron and nuclear spin states of a magnetic molecule are among the many quantum systems being explored as building blocks for a quantum computer. All such systems are challenged by decoherence—the destructive interaction of the quantum unit of information, or qubit, with its environment. Some sources of decoherence, such as unwanted interactions with nuclei or other qubits, show up as fluctuations in the magnetic field felt by each qubit. The main strategy to reduce decoherence in molecular magnetic qubits has been to work with extremely dilute samples. There is now another approach, as shown by a collaboration between teams led by Stephen Hill of the National High Magnetic Field Laboratory and Florida State University and by Eugenio Coronado of the University of Valencia. They reduced coherence times even in relatively dense samples by using a trick that was first introduced to make atomic clocks immune to magnetic field fluctuations and that has subsequently been applied to some other qubit systems. The gist of the approach is to engineer the molecular system to have a “sweet spot,” where transitions between the information-carrying spin states are insensitive to the troublesome fluctuations. Hill and his team applied this technique to a polyoxometalate molecule containing holmium, tungsten, and oxygen atoms. They measured coherence times as long as 8 µs, on par with those previously found only in samples in which the concentration of magnetic holmium ions was at least one hundred times more dilute. The demonstration should encourage chemists to find other molecular systems amenable to the same technique in the hopes of further improving those coherence times. (M. Shiddiq et al., Nature 531, 348, 2016.)