Final Report Summary - MOLNANOSPIN (Molecular spintronics using single-molecule magnets)
The realization of an operational quantum computer is one of the most ambitious technologically goals of today’s scientists. In this regard, the basic building block is generally composed of a two-level quantum system, namely a quantum bit (or qubit). Such quantum system must be fully controllable and measurable, which requires a connection to the macroscopic world. In this context, solid state devices, which establish electrical interconnections to the qubit, are of high interest, mainly due to the variety of methods available for fabrication of complex and scalable architectures. Moreover, outstanding improvements in the control of the qubit dynamics have been achieved in the last years.
Among the different solid state concepts, spin based devices are very attractive since they already exhibit relatively long coherence times. For this reason, electrons possessing a spin 1/2 are conventionally thought as the natural carriers of quantum information. However, the strong coupling to the environment makes it extremely difficult to maintain a stable entanglement. Alternative concepts propose the use of nuclear spins as building blocks for quantum computing, since they benefit from longer coherence times compared to electronic spins, because of a better isolation from the environment. But weak coupling comes at a price: the detection and manipulation of individual nuclear spins remain difficult tasks.
In this context, the objective of the project MolNanoSpin was to lay the foundation of a new field, combining the disciplines, spintronics, molecular electronics, and quantum information processing. In particular, the objective was to fabricate, characterize and study molecular devices (molecular spin-transistor, molecular spin-valve and spin filter, molecular double-dot devices, carbon nanotube nano-SQUIDs, etc.) in order to read and manipulate the spin states of the molecule and to perform basic quantum operations. The project MolNanoSpin was designed to play a role of pathfinder in this –still largely unexplored - field. The main target of the 5 years concerned fundamental science, but applications in quantum electronics are expected in the long run.
Among the most important results, we showed the possibility of magnetic molecules to act as building blocks for the design of quantum spintronic devices and demonstrated the first important results in this new research area. For example, we have built a novel spin-valve device in which a non-magnetic molecular quantum dot, consisting of a single-wall carbon nanotube contacted with non-magnetic electrodes, is laterally coupled via supramolecular interactions to a TbPc2 molecular magnet. The localized magnetic moment of the SMM led to a magnetic field-dependent modulation of the conductance in the nanotube with magnetoresistance ratios of up to 300% at low temperatures. We also provided the first experimental evidence for a strong spin–phonon coupling between a single molecule spin and a carbon nanotube resonator. Using a molecular spin-transistor, we achieved the electronic read-out of the nuclear spin of an individual metal atom embedded in a single-molecule magnet (SMM). We could show very long spin lifetimes (several tens of seconds). Finally, we proposed and demonstrated the possibility to perform quantum manipulation of a single nuclear spin by using an electrical field only. This has the advantage of reduced interferences with the device and less Joule heating of the sample. Since an electric field is not able to interact with the spin directly, we used an intermediate quantum mechanical process, the so called hyperfine Stark effect, to transform the electric field into an effective magnetic field.
The ERC support was essential to start and succeed this ambitious project. As initially planned, additional funding was received, which supported the ERC project, but the ERC gave the vital kick-off. Scientifically, the project was very ambitious but all objectives and technical goals for the project were achieved. We could even go beyond the objectives, that is, we succeeded to perform, for the first time, nuclear spin manipulations using electric fields instead of magnetic fields.
Among the different solid state concepts, spin based devices are very attractive since they already exhibit relatively long coherence times. For this reason, electrons possessing a spin 1/2 are conventionally thought as the natural carriers of quantum information. However, the strong coupling to the environment makes it extremely difficult to maintain a stable entanglement. Alternative concepts propose the use of nuclear spins as building blocks for quantum computing, since they benefit from longer coherence times compared to electronic spins, because of a better isolation from the environment. But weak coupling comes at a price: the detection and manipulation of individual nuclear spins remain difficult tasks.
In this context, the objective of the project MolNanoSpin was to lay the foundation of a new field, combining the disciplines, spintronics, molecular electronics, and quantum information processing. In particular, the objective was to fabricate, characterize and study molecular devices (molecular spin-transistor, molecular spin-valve and spin filter, molecular double-dot devices, carbon nanotube nano-SQUIDs, etc.) in order to read and manipulate the spin states of the molecule and to perform basic quantum operations. The project MolNanoSpin was designed to play a role of pathfinder in this –still largely unexplored - field. The main target of the 5 years concerned fundamental science, but applications in quantum electronics are expected in the long run.
Among the most important results, we showed the possibility of magnetic molecules to act as building blocks for the design of quantum spintronic devices and demonstrated the first important results in this new research area. For example, we have built a novel spin-valve device in which a non-magnetic molecular quantum dot, consisting of a single-wall carbon nanotube contacted with non-magnetic electrodes, is laterally coupled via supramolecular interactions to a TbPc2 molecular magnet. The localized magnetic moment of the SMM led to a magnetic field-dependent modulation of the conductance in the nanotube with magnetoresistance ratios of up to 300% at low temperatures. We also provided the first experimental evidence for a strong spin–phonon coupling between a single molecule spin and a carbon nanotube resonator. Using a molecular spin-transistor, we achieved the electronic read-out of the nuclear spin of an individual metal atom embedded in a single-molecule magnet (SMM). We could show very long spin lifetimes (several tens of seconds). Finally, we proposed and demonstrated the possibility to perform quantum manipulation of a single nuclear spin by using an electrical field only. This has the advantage of reduced interferences with the device and less Joule heating of the sample. Since an electric field is not able to interact with the spin directly, we used an intermediate quantum mechanical process, the so called hyperfine Stark effect, to transform the electric field into an effective magnetic field.
The ERC support was essential to start and succeed this ambitious project. As initially planned, additional funding was received, which supported the ERC project, but the ERC gave the vital kick-off. Scientifically, the project was very ambitious but all objectives and technical goals for the project were achieved. We could even go beyond the objectives, that is, we succeeded to perform, for the first time, nuclear spin manipulations using electric fields instead of magnetic fields.