|Hybrid Rotaxanes: Interlocked Structures for Quantum Computing?|
While conventional computers store information in binary bits, quantum computers would use a more complex equivalent—a qubit—which not only represents 0 and 1, but all possible superpositions of the quantum states representing 0 and 1 simultaneously. The complexity introduces the possibility of some calculations being performed much more quickly than could be achieved with conventional computers. Previous work by the group at Manchester had looked at molecular magnets as possible qubits, but how to create suitable nanostructures in which to place them remained.
The “bottom-up” approach to fabrication of nanostructures requires chemical methods to link functional building blocks into larger structures. A problem arises when the electronic structure of the building block is to be retained in the material constructed. Traditional chemistry would link blocks through covalent bonds, but these inevitably involve a strong interaction between the electronic structure of one block and of its neighbor, and hence the individual character of building blocks could be lost. Fortunately, the Edinburgh group had shown it could make very complex interlocked structures using organic supramolecular chemistry, which focuses on chemical systems made up of a discrete number of assembled molecular subunits or components. The supramolecular chemistry is such that many related structures can be made from similar basic building blocks.
Bringing together the work of these two groups has led to interlocked assemblies where the potential qubits are brought into close proximity without strong interactions between the electronic structures of the qubits. The structures formed are rotaxanes that feature the inorganic molecular magnet acting as the ring about an organic axle, with bulky stoppers attached to the end of the axle to prevent the ring sliding off the end. The combination of organic and inorganic chemistry has allowed synthesis of -, - and rotaxanes in good yields. The rotaxane, in which two threads pass through two rings, has only a single precedent in the literature.
X-ray single crystal diffraction at ALS Beamline 11.3.1 was used to verify the structures of the rotaxane produced and of several related compounds. The next stage was to show molecular motion by NMR spectroscopy. Rotation of the ring about the axle is very fast, but motion of the ring along the axle in the rotaxane (a molecular shuttle) occurs about once per second. The very different time-scales for the two types of motion is unusual.
Future steps in the project are to introduce methods for switching interactions on and off between the qubits on the axle and to look for means for controlling the speed of molecular shuttling. The threaded architecture ensures that the electronic, magnetic, and paramagnetic characteristics of the inorganic rings could be influenced by the organic portion of the rotaxane. A photo-active organic component could allow use of light as a means to switch on and off the interactions between qubits threaded onto a single axle during computation. It is also possible to imagine much more complex interlocked structures through further modifications of the chemistry.
Research conducted by C.-F. Lee, D.A. Leigh, and D. Schultz, (University of Edinburgh, UK); R.G. Pritchard, G.A. Timco, and R.E.P. Winpenny (University of Manchester, UK); and S.J. Teat (ALS).
Research funding: the Engineering and Physical Sciences Research Council (UK), the European Commission Network of Excellence “MAGMANet,” and The Royal Society (UK). Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publication about this work: C.-F. Lee, D.A. Leigh, R.G. Pritchard, D. Schultz, S.J. Teat, G.A. Timco and R.E.P. Winpenny, “Hybrid organic-inorganic rotaxanes and molecular shuttles,” Nature 458, 314 (2009).