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Dipartimento di Fisica e Scienze della Terra


Highlights


 

Molecular Qubits

Spin coupling among molecular nanomagnets allow the production of three qubit entangled states.

Nature Nanotechnology 4, 173 - 178 (2008)

 

Qubits are like the usual bits (1,0) of computers, but they can be manipulated in parallel, thanks to their quantum nature. This is the essence of quantum computing, stemming from an original idea of Richard Feynman, back in 1982. Quantum computers could be much faster that present day computers for certain categories of very complex numerical problems.

It was shown that magnetic molecules behave as an ensemble of interacting qubits and open a new perspective towards quantum computing. In the past few years the molecular nanomagnet theory group of our Physics Department, in collaboration with Modena-Reggio Emilia University and Manchester University, proposed to use ring-shaped magnetic molecules as qubits, to implement quantum computing algorithms. Theoretical modeling showed that we understand how to alter the links among these rings, a relatively easy task for chemists, in order to control the effective interaction among qubits. These supermolecular complexes can thus be designed and produced to yield fully entangled states of more than two qubits, Thus the building blocks of a future quantum computer can be realized in a solid state system.

MnSi: a weak non centrosymmetric helical magnet

This material is a weak metal and manganese moments order along a helix. The interest lies in the rich physics that emerge when  MnSi is perturbed by pressure and magnetic field.  The material can give rise to magnetic vortex phases (skyrmions) and also to superconductivity.

Phys. Rev. B 89 184425 (2014) Editor's suggestion

Recently it was proposed however that muons are not suited to investigate these fascinating properties. A recent paper, Phys Rev B 83 40404 (2011), suggested that, due to the poor metallic properties of MnSi, the implanted  muon would easily perturb its surrounding producing a local disturbance, a polaron, with bound unpaired electrons. Thus the muon would measure the properties of this new entity instead of those of the bulk material.

Our investigation proves that the new supposed entity is not there. The demonstration has two aspects. A very careful experimental work on a single crystal identifies the muon stopping site, its coupling to Mn moments and verifies that the data agree with bulk MnSi susceptibility with extremely high accuracy. A parallel DFT work, performed in Parma, finds the same site by ab-initio calculations with precision of better that one hundredth of an Angstrom.

 

Left: the muon siite identified by DFT. The site is represented by a  yellow isoenergy surface. The muon energy at any point in  this isosurface  coincides with the Zero Point Energy of the quantum muon oscillator, i.e. a muon can be thought as a wave function that has significant probability density within the depicted isosurface.