HYSPECQS · Hybrid spin-mechanical quantum systems

Hybrid spin-mechanical quantum systems

HYSPECQS aimed to create a new type of system combining nanomechanical devices and spins, a property inherent to quantum objects that is alike to a small magnet. By using nanomagnets to induce coupling between the mechanical oscillator and the spins, the project sought to study the interaction betwen the two systems and ultimately to generate unique states of motion through the quantum properties of the spin. These new states can be crucial for developing advanced quantum information and sensing technologies, as well as for testing the fundamental principles of quantum mechanics. In order to study the interaction between the two systems, three key parameters are crucial: the strength of the coupling between the two systems, which relies on small separation between spin and oscillator, and on how much the magnetic field generated by the nanomagnet varies over distance-known as magnetic field gradient; the quantum decoherence rate -i.e. the rate at which the two systems lose their quantum properties- of the spin and of the mechanical resonator need to be as small as possible. The project tackled the latter requirement by choosing appropriately the two platforms. An optically-active defect of the diamond crystal, known as the nitrogen-vacancy centre, which resembles an atom trapped in the solid state, provides a spin with naturally long coherence time, even at room temperature. The mechanical resonator consists of a 5mmx5mm silicon nitride membrane, a ceramic material, only few nanometers thick. The host group at the Niels Bohr Institute optimized the fabrication of such resonators by patterning the membrane with a periodic structure of holes, which confines the mechanical motion at the very centre of the membrane while isolating it from the environment. Using techniques known such as sideband cooling and cold damping, the vibrations can be almost completely removed, making the oscillator a quantum object. Most importantly, owing to its isolation from the environment, such quantum mechanical mode has been proven to survive for more than 100ms at few hundred milliKelvin. To ensure large coupling rate, the project aimed at growing nanomagnets at the centre of the silicon nitride membrane. The small separation was ensured by using a technique known as atomic force microscopy: a tip with a diameter of ~300nm, containing a single nitrogen-vacancy centre, is pushed against the membrane. By sensing the force building up between the tip and the membrane, their separation can be stabilized using precise motors, and thus allowing for tip-membrane separation below 1nm. Such small value place the spin contained in the tip in the region where the variation of magnetic field generated by the nanomagnet is the largest, thereby ensuring large coupling. By leveraging on the low decoherece rate of the two systems, combined with the maximization of the coupling rate, HYSPECQS aimed at generating strong spin-mechanical effects both in the classical regime, in the form of sensing of extremely small forces, and in the quantum regime, by generating new quantum states of motion, previously unobserved.

Data: CORDIS, © European Union