Quantum threesome

The linking of coupled 'nanomagnets' in a single molecule creates the possibility of an entangled three-qubit system for quantum computing

Credit: NPG © 2009

An important requirement for quantum computing is control of the 'entanglement' of quantum spin states — which means that the states cannot be described without reference to one another. Now a team led by Richard Winpenny at the University of Manchester and Marco Affronte at the University of Modena & Reggio Emilia have shown¹ that the spin of 'nanomagnets' can be coupled in a single molecule — providing a system with three bits of quantum information — and have proposed how they can be entangled.

The scientists studied magnetic rings of chromium and nickel ions (Cr₇Ni), which behave as individual qubits with a spin of one half. They connected two rings together using organic ligands that link through a central copper ion that acts as a third qubit. The advantages of using such a system — rather than conventional metals or semiconductors — are the longer spin-coherence preservation times and distances.

The maximal entanglement of more than two systems has only been achieved before with photons or trapped ions but the researchers propose that in this molecular arrangement the entangled states could be induced simply by applying microwave pulses. Additionally, the binding ligands and the central metal can be varied to change the way that the Cr₇Ni rings are entangled.