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Research

Context

Quantum computing

The main objective of the research in the group is to design, synthesize and characterize new quantum bit candidates in the form of magnetic complexes that allow for an electrical access to the spin states. Why quantum bits? To make a quantum computer, or course. Such systems have been the object of much theoretical interest in the past two decades, following the recognition that they have the potential to become a technology as transformative to our society as classical computing already has been. A quantum computer, by taking advantage of the fundamental quantum mechanical properties of superposition and entanglement expected to permit calculations at speed that will always be unaccessible to classical computer, thereby making possible the solution of several fundamental problems. Applications in the fields of cryptography, engineering, drug discovery etc. abound.

Quantum bits

However, despite the numerous efforts inspired by these possibilities, actual operational quantum computers of more than a handful of quantum bits still remain far out or reach. None of the various systems investigated as quantum bits (Josephson junctions,1 nitrogen vacancies in diamond,2 31P atoms trapped in silicon,3 ultra-cold ions trapped in an electromagnetic field4 etc.) can easily match the five DiVincenzo criteria5 deemed critical for computing. Those criteria are

  • scalability of the system used to make the quantum bits
  • easiness of writing the initial quantum state of the quantum bit
  • easiness of reading-out the final quantum state
  • sufficient coherence time for the calculation to proceed between write-in and read-out without the information being lost
  • capacity to assemble the quantum bits into logic gates.

Magnetic molecules as quantum bits

Magnetic molecules have been proposed as spin quantum bits,6 but most of the very little experimental work done so far concentrated on obtaining long coherence times, and the mechanism of writing and reading of the spin states of a single molecule remains a significant challenge. In a classical computer's CPU, writing and reading are accomplished by electrical fields and conductivity measurements. Our goal is to make magnetic molecules that could function as quantum bits and utilize similar concerts (and the existing technology!) to write and read a quantum spin state.

Attachment of magnetic molecules to carbon nanotubes

Functionalized carbon nanotube
Schematic representation of a carbon nanotube-complex assembly. C: black, H: white, O: red, N: blue, Mn: yellowCarbon nanotube properties

Carbon nanotubes are ideal points of contact between the molecular, nanoscale world and the macroscopic. They present metallic or semi-conductive quasi one-dimensional conductivity along the tube, and can at low temperature demonstrate ballistic transport over several hundred of nanometers, thus allowing conservation of the electron spin over the same distance. Modification of the nanotube affects its conductivity. Conversely, it has been recently demonstrated that charge transport along the nanotube can be used to detect the state of a molecule absorbed onto it.7 Our target in this area is the synthesis of complexes presenting interesting magnetic properties tailored to be covalently attached to a carbon nanotube. The covalent links are design to constrain as much as possible the relative positions of molecule and nanotube, thus enhancing the reproducibility of device fabrication. Various other functionalities facilitating the investigation of the system (spectroscopic handles for EDX, Raman, etc.) or enhancing the properties of the complex (e.g. chirality, electric dipole) can also be introduced on the complex. After a thorough investigation of the properties of the complex and its assembly on carbon nanotubes, further investigations of the transport properties of single-carbon nanotube devices are conducted in the group of Dr. Carola Meyer at the University of Osnabrück (Germany).

Lanthanoid complexes with aromatic ligands

NdPc2 on Cu(100)
Schematic representation of a bis(phthalocyaninato)neodymium(III) complex investigated by STM on a Cu(100) surface. C: black, H: white, N: blue, Nd: red, Cu: orange, Cr: grey.

Another area of research we are interested in is lanthanoid chemistry. Lanthanoid complexes usually present some extremely interesting magnetic properties, in particular large magnetic anisotropies. One of the limitation of those species, however, is that the magnetically competent f orbitals do not interact much with the O- and N- donating ligands most commonly used, thus restricting the ability of the chemist to tune the magnetic properties of the center. In the case of aromatic ligands, however, the f-π interaction is much stronger, which opens many avenues for fine-tuning the properties of the metal center. Additionally, this interaction results in some degree of spatial expansion of the electronic density of the f orbitals. Combined with a judicious choice of metal to bring the corresponding energy levels as close as possible to the Fermi level, this enables a direct tunneling of electrons through the magnetic f orbitals in a scanning tunneling microscope setting.8 The STM and spin-polarized STM investigations are conducted by Dr. Daniel Bürgler and Dr. Frank Matthes at the Peter Grünberg Institute (PGI-6) of the Forschungszentrum Jülich (Germany).

References

  1. Schnirman, A.; Schön, G.; Phys. Rev. Lett. 1997, 2371-2374, doi: 10.1103/PhysRevLett.79.2371 ↩
  2. Gurudev Dutt, M. V.; Childress, L.; Jiang, L.; Togan, E.; Maze, J.; Jelezko, F.; Zibrov, A. S.; Hemmer, P. R.; Likin, M. D. Science 2007, 316(5829), 1312-1316, doi: 10.1126/science.1139831 ↩
  3. Morton, J. J. L.; Tyryshkin, A. M.; Brown, R. M.; Shankar, S.; Lovett, B. W.; Ardavan, A.; Schenkel, T.; Haller, E. E.; Ager, J. W.; Lyon, S. A. Nature 2008, 455, 1085-1088, doi: 10.1038/nature07295 ↩
  4. Cirac, J. I.; Zoller, P. Phys. Rev. Lett. 1995, 74, 4091-4094, doi: 10.1103/PhysRevLett.74.4091 ↩
  5. DiVincenzo, D. P. Fortschr. Phys. 2000, 48, 9-11, 771-783. arXiv:quant-ph/0002077 ↩
  6. (a) Leuenberger, M. N.; Loss, D. Nature 2001, 410, 789-793, doi:10.1038/nmat2133 (b) Lehmann, J; Gaita-Ariño, Coronado, E.; Loss, D. Nature Nanotechnology 2007, 2, 312-317, doi:10.1038/nnano.2007.110 ↩
  7. Urdempilleta, M.; Klyatskaya, S.; Cleuziou, J.-P.; Ruben, M.; Wernsdörfer, W. Nature 2011, 10, 502-506, doi: 10.1038/nmat3050 ↩
  8. Fahrendorf, S.; Atodiresei, N.; Besson, C.; Caciuc, V.; Matthes, F.; Blügel, S.; Kögerler, P.; Bürgler, D. E.; Schneider, C. M. Accessing 4f-states in single-molecule spintronics. Nature Commun. 2013, 4, 2425-2430, doi: 10.1038/ncomms3425. ↩
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