Theory goes for a spin

Researchers from the US, The Netherlands and Japan have shown that a device which consists of a resonator coupled to a nanoscale object is entirely feasible and could act as a potential detector for small charges or magnetic moments [Kovalev et al., Phys  Rev Lett (2011) 106, 147203]. [We recently reported on a quantum interface between electrical and mechanical oscillations; click here for more information].

 

The theoretical model proposed by Alexey Kovalev and his co-workers is based upon a torsional nanomechanical resonator which consists of a load attached to a beam in a magnetic gradient. The load can be a nanoparticle or a single molecule magnet connected to a paddle, while the beam is a carbon nanotube or a chemical bond. The load or spin transfers momentum to the beam, resulting in torsion, thus conserving the laws of angular momentum.

 

“The first direct observation of quantum behavior of a macroscopic mechanical resonator constituting a nanoelectromechanical system (NEMS) has been reported recently,” says Kovalev. “However, such devices usually employ a superconducting qubit that couples to the mechanical mode. We suggest a novel realization that involves a single molecule magnet which might have advantages compared to the conventional setup.”

 The spin and mechanical modes, also called magnetopolaritons, are a combination of magnetic excitations and lattice deformations and they allow energy to be exchanged coherently within the system. The molecules can interact by exchanging phonons and the result is that spin-spin interactions become possible and quantum information can be transferred from one spin to the next.

 

“By testing our predictions, such as formation of magnetopolaritons, one can test the laws of quantum mechanics on macroscopic objects, e.g., nanomechanical resonators,” Kovalev tells Materials Today. “For example the magnetopolaritons represent a quantum entangled state of a mechanical system and spin, i.e., it is a superposition of a spin up state with n phonons and spin down state with n-1 phonons. We further demonstrate the suppression of the tunneling between opposite magnetizations and destruction of magnetopolaritons (coherent magnetomechanical oscillations) by nanomechanical interference. The predictions can be verified experimentally by a molecular magnet attached to a nanomechanical bridge.”

 

Although the experiment has yet to be carried out in the laboratory, the authors are confident that quantum control of magnetization may be achieved at single-phonon level using current state-of-the-art equipment.