Molecular magnets closer to application in quantum computing

In a Nature Communications publication, the results of the collaboration between scientists of the Institut Laue-Langevin (ILL), the University of Parma, ISIS and the University of Manchester, the (Cr7Ni)2 dimer has been used as a benchmark system to demonstrate the capability of 'four-dimensional' inelastic neutron scattering to investigate entanglement between molecular qubits. By utilising high-quality single crystals and the full capabilities of the time-of-flight spectrometer IN5, the team was able to demonstrate and quantify the entanglement through the huge amount of data they were able to extract from the 4D phase space (Qx,Qy,Qz,E), where Q is the momentum-transfer vector and E the energy transfer. Indeed, the neutron cross-section directly reflects dynamical correlations between individual atomic spins in the molecule. Hence, the corresponding pattern of maxima and minima in the measured neutron scattering intensity as a function of Q is a sort of portrayal of the entanglement between the molecular qubits. Furthermore, the team has also developed a method to quantify entanglement from INS data.

Such a measurement opens up remarkable perspectives in understanding entanglement in complex spin systems. "The research on molecular nanomagnets has been an attractive topic on the IN5 time-of-flight spectrometer since many years. In this recent work the top class chemistry and theoretical work meet the advanced neutron scattering methods to highlight the intricate physics of quantum entanglement, guiding further research towards a better understanding of the practical challenges in quantum information technology," said Dr Hannu Mutka and Dr Jacques Ollivier, ILL scientists.

With this benchmark measurement it looks as though neutrons will continue to be an essential tool in helping molecular nanomagnets realise their potential for quantum technologies of the future.

Nextbigfuture interviewed the researchers.

1. What are the next steps in this research?
By exploiting the (Cr7Ni)2 supramolecular dimer as a benchmark, we have shown that the four-dimensional inelastic neutron scattering technique (4D-INS) enables one to portray and quantify entanglement between weakly coupled molecular nanomagnets, which provide ideal test beds for investigating entanglement in spin systems. The next steps will be the application of 4D-INS to dimers of more complex molecular qubits, like those containing 4f or 5f magnetic ions or to supramolecular compounds with more than two qubits.

2. Can the timing be seen for possible commercialization?
The use of molecular nanomagnets for quantum information processing (QIP) is a relatively unexplored field. Therefore, as in other approaches to implement qubits, commercialisation is certainly not immediate. However, molecular magnetism constitutes an alternative route to QIP that uses low-cost, yet powerful, chemical methods to fabricate basic components and integrate them in future devices.

3. Is there an effort to enable qubits via this approach?
Neutron scattering is a very powerful technique and enables one to achieve a sound characterisation of both molecular qubits and their supramolecular assemblies. Therefore, we plan to apply it to new interesting systems in the near future. In addition, we believe that our work will stimulate similar studies by other research groups. In this way, promising molecules with improved characteristics for QIP will be identified.

4. How does this work fit into a larger area of research? I.e. broad advances are happening and this is just a part.

This work provides an important tool for molecular qubits, which in turn fit the broad quest for quantum information technologies. The latter constitutes one of the most important current research areas. Indeed, some of the most important private companies and international institutions are investing a huge amount of money on this subject. For instance, the European Commission will launch a a1 billion quantum technologies flagship in 2018.

5. What do the researchers see as highlights for how this work advances the state of the art?

Experimentally measuring entanglement in complex systems is generally very difficult. In this work, we have put forward a method to demonstrate and quantify entanglement between molecular qubits, by measuring the dependence of the neutron cross-section on the three components of the momentum transfer Q. Such measurements are challenging, but we have demonstrated this with the spectrometer IN5 at the Institut Laue-Langevin, indicating that they can now be performed exploiting state-of-the-art neutron spectrometers.

6. Do the researchers have a context or vision they can share?
Quantum computers will be powerful devices able to solve problems that are impossible even on the best traditional computers. Molecular nanomagnets might provide a relatively cheap route to reach this extremely ambitious goal and 4D-INS can be an important tool in the understanding and engineering of molecules with the right characteristics for efficiently encoding and processing quantum information.

7. Anything else that the researchers think is relevant in understanding this work and its importance?
In our opinion, this work represents a very good example of how the interplay between theory, experiments and chemical synthesis can be very fruitful and can enable us to make a significant step toward an ambitious objective.