Quantum Optics seminar by Jan Gieseler, ICFO


Coherent coupling between a single spin degree of freedom and a massive mechanical mode is still an outstanding challenge. Such a system is highly desirable, as it allows to prepare macroscopic quantum states of motion and it could be used to mediate an effective spin-spin interaction between distant spins. This principle is in close analogy to trapped ions that interact via collective mechanical modes, which have already demonstrated high fidelity quantum gates. One promising approach to engineer a strong spin-mechanical coupling is via magnetic field gradients. In this scheme, maximizing the spin-resonator coupling requires to employ a compliant, high quality mechanical resonator, strong magnetic field gradients, and spin qubits with very long spin coherence times. In addition, they have to be combined while preserving the excellent properties of the individual components. To address this formidable challenge, we propose a new platform based on levitated nano-magnets and report on our experimental progress towards integrating a micro-magnet levitated above a YBCO superconductor with nitrogen-vacancy (NV) centres in diamond. The absence of any support structure results in low mechanical damping. Working in the field-cooled regime, we measure kHz centre-of-mass mode frequencies with Q factors approaching 1 million. The radius of the magnets is of order ~10μm. Thus, scaling down the size of the magnets below μm, we expect frequencies of the order of 100 kHz, competitive with optical levitation and higher than what has been achieved with Paul traps. Furthermore, we integrate the levitated magnet with nitrogen-vacancy (NV) centre defects in diamond, which serve as the spin-qubits. The large magnetic moment to mass ratio enables strong coupling between the micro-magnet and nearby spins. The NV features optical spin read-out and initialization, microwave control, and weak coupling to the environment, resulting in long spin coherence times. When positioned nearby the levitated magnet, the NV centre experiences a magnetic field that depends on the motion of the magnet. Conversely, spin flips of the electronic spin of the NV centre exert forces and torques on the magnet. This leads to coupled dynamics of the magnet motion and the NV centre spin. Our goal is to harness this coupling for quantum applications such as sensing, creation of macroscopic quantum states and long-range spin-spin interactions.