A molecular multi-qubit model system for quantum computing

Molecules could represent useful systems for quantum computing, but they must contain individually addressable, interacting quantum bit centers. In the diary applied Chemistrya research team has now presented a molecular model with three different coupled qubit centers. Since each center is spectroscopically addressable, quantum information processing (QIP) algorithms could be developed for this multi-qubit molecular system for the first time, the team says.

Computers calculate with 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 once. These superimposed states allow a quantum computer to perform computations in parallel, and when using multiple qubits it has the potential to be much faster than a standard computer.

However, for the quantum computer to be able to perform these calculations, it must be able to evaluate and manipulate the multi-qubit 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 that can be detected spectroscopically and whose states can be switched by interacting with each other.

“In our proposed molecular system, unpaired electrons form the basis of the qubit centers instead of atoms or photons,” explains Bowen. “Electrons have a property known as spin. Since the spin takes on two superimposable quantum states, molecules containing multiple electron spin systems could be useful as potential multi-qubit systems for quantum computing.”

For their molecule (which contains a copper ion complex, a ring of seven chromium ions and one nickel ion, and a nitroxide moiety), the team observed characteristic signals for each qubit center in the electron paramagnetic resonance (EPR) spectrum. “The presented results prove that individual qubit units could be addressed independently and controllably with EPR – an important prerequisite for the use of multi-qubit systems in quantum computing,” says Bowen.

Compared to the systems currently used, such molecular multi-qubit systems could offer some advantages. So far, qubit systems have mainly been produced by superconducting circuits or from individual atoms or photons that have to be cooled in a complex manner. Molecular systems could offer the advantage of containing multiple qubit units that can be easily altered and reconfigured through chemical synthesis. They could also be operated at higher temperatures. This holds the opportunity to make quantum computing cheaper.