A step towards quantum computation with ultracold molecules

One of the fundamental principles of quantum mechanics is that a particle such as an atom or molecule can be prepared in a coherent quantum superposition of internal states. This is one of the key ingredients that makes a quantum computation so powerful: the quantum bit (or qubit) can be in an arbitrary superposition of the states 0 and 1, unlike a classical bit which is constrained to be in either 0 or 1. However, such coherent superpositions of states are often extremely fragile. Experimental noise and coupling to the environment can lead to rapid dephasing or decoherence, resulting in an incoherent mixture of 0s and 1s.

Phil Gregory and members of the research team led by Prof. Simon Cornish have recently showed how the problems of decoherence can be eliminated for ultracold molecules, leading to coherence times of over 5 seconds. They used a gas of ultracold RbCs molecules in which each molecule was prepared in a coherent superposition of hyperfine states. After years of studying the molecules, they were able to identify the perfect conditions to eliminate dephasing due to both magnetic field noise and light shifts in an optical trap. The coherence time was quantified by studying decay in the contrast of the fringes in a Ramsey interferometry sequence. Remarkably, they found coherence times much longer than the lifetime of the gas. These results are a key step towards realising a quantum computation platform using ultracold molecules.

The results of these experiments have been published in Nature Physics.

“Robust storage qubits in ultracold polar molecules”, P.D. Gregory, J.A. Blackmore, S.L. Bromley, J.M. Hutson and S.L. Cornish, Nature Physics 17, 1149-1152 (2021).

https://doi.org/10.1038/s41567-021-01328-7

They have also been highlighted in Phys.org https://phys.org/news/2021-09-robust-storage-qubits-ultracold-polar.html