Oct 15, 2022 
Spectroscopically controlled quantum bits
(Nanowerk News) Molecules could make useful systems for quantum computers, but they must contain individually addressable, interacting quantum bit centers. In the journal Angewandte Chemie ("Modelling Conformational Flexibility in a Spectrally Addressable Molecular MultiQubit Model System"), a team of researchers has now presented a molecular model with three different, coupled qubit centers.

As each center is spectroscopically addressable, quantum information processing (QIP) algorithms could be developed for this molecular multiqubit system for the first time, the team says.


DFT optimised model of the single crystal XRD (XRay Diffraction) structure a multispin CuII porphyrinCr7Ni ringnitroxide hybrid [2]rotaxane (H atoms have been removed for clarity). (© Wiley)

Computers compute using bits, while quantum computers use quantum bits (or qubits for short). While a conventional bit can only represent 0 or 1, a qubit can store two states at the same time. These superimposed states mean that a quantum computer can carry out parallel calculations, and if it uses a number of qubits, it has the potential to be much faster than a standard computer.

However, in order for the quantum computer to perform these calculations, it must be able to evaluate and manipulate the multiqubit information. The research teams of Alice Bowen and Richard Winpenny, University of Manchester, UK, and their colleagues, have now produced a molecular model system with several separate qubit units, which can be spectroscopically detected and the states of which can be switched by interacting with one another.

“In our proposed molecular system, unpaired electrons instead of atoms or photons form the basis of the qubit centers,” Bowen explains. “Electrons have a property known as spin. As the spin assumes two superimposable quantum states, molecules containing several electron spin systems may be of use as potential multiqubit systems for quantum computing.”

For their molecule (containing a copper ion complex, a ring formed of seven chromium ions and a nickel ion, and a nitroxide unit), the team observed characteristic signals for each qubit center in the electron paramagnetic resonance (EPR) spectrum. “The results presented prove that individual qubit units could be addressed independently and controllably—a vital prerequisite for using multiqubit systems in quantum computing—using EPR,” Bowen says.

Compared to the systems currently used, such molecular multiqubit systems could offer some advantages. To date, qubit systems have mainly been produced by superconducting circuits or from individual atoms or photons, which need extensive cooling. Molecular systems could afford the advantage of containing multiple qubit units, which can be easily changed and reconfigured by chemical synthesis. They could also be run at higher temperatures. This poses the chance to make quantum computing cheaper.
