Projects

Levitated micromagnets have been theoretically proposed as building blocks of a number of hybrid quantum systems and as ultralow-noise sensors of torque, force or magnetic field, capable of largely outperforming conventional quantum-limited devices in terms of energy resolution. This outstanding potential is supported by recent pioneering experiments, yet it is far from being fully demonstrated. In this project we aim at a systematic investigation of sensors based on levitated micromagnets. We will explore various levitation platforms, such as superconducting traps based on the Meissner effect, on-chip circuits for magnetic trapping, free-fall, and Paul traps for charged micromagnets, and exploit coupling to different quantum systems or devices, such as diamond Nitrogen-Vacancy (NV) centers and SQUIDs.

We will primarily investigate the librational and rotational motion of the levitated magnets and focus on the detection of ultralow torque and magnetic fields, with the goal of demonstrating unprecedented energy resolution. In particular, we aim at demonstrating the most sensitive regime corresponding to atom-like Larmor precession of a mesoscopic magnet, a peculiar effect arising from the quantum nature of intrinsic spin. Theoretically we expect to achieve outstanding improvement over the state of the art in terms of energy resolution. For instance, we expect to overcome by orders of magnitude the Energy Resolution Limit, a semiempirical bound which appears to be satisfied by all existing quantum magnetometers. We will then exploit the potential of the developed sensors in specific science cases of great interest in fundamental physics, such as probing exotic interactions arising from new physics beyond the standard model. As a long-term vision, we will investigate the potential for future tests of General Relativity with quantum spin systems (specifically, levitated magnets) in space. We aim at an initial proof-of-principle demonstration in the Einstein-Elevator, where the Larmor precession of a free-falling magnet can be observed in the cleanest conceivable way.

The project benefits from the highly transversal and interdisciplinary nature of the consortium, which gathers scientists converging to levitated quantum magnetomechanics from the diverse fields of diamond NV centers, ion traps, cold atoms, hybrid quantum systems, SQUIDs, optomechanics and optical magnetometry.

Superconducting traps

We will levitate hard micromagnets by means of Meissner effect in superconducting traps made of bulk type I superconductors. Micromagnets are typically made of NdFeB, trap materials can be for insatnce Pb, Ta or Al. Preliminary experiments have shown very promising mechanical properties, such as mechanical damping for translational and rotational modes in the order of 1E-5 Hz. This opens the way to the realization of mechanical systems with low temperature and very low damping, implying extremely low thermomechanical noise.

In LEMAQUME we plan to explore the potential of this platform by optimizing the setup for sensitive rotational mechanical measurements. Experiments will be performed in a vibrationally isolated cryostat. We will study the dynamics of levitated micromagnets by exploring the interplay of librational and precession regimes. The main goal is to operate the system close to the thermal noise level, which will enable extreme torque resolution. In particular, we expect a levitated micromagnet to be operated as magnetometer with resolution well beyond the Energy Resolution Limit.  We also plan to exploit this extreme sensitivity to perform fifth-force measurements.

Paul traps and magnetic traps

Paul trap can in principle trap charged magnetic particles without significant heating of the particle even under ultra-high vacuum. Our goal is to trap micro-magnets in a Paul trap and in a magnetic chip-traps and employ NV centers to sense the motion and to enter the regime where the precession and angular momentum transfer between the spin and the motion can be observed.

The librational motion of micron-size soft-magnets has been observed and the motion was detected both optically and using NV centers in the regime where libration dominates over the precessional motion.  Trapping hard nanomagnets will enable entering a regime where the spin angular momentum dominates over the classical dynamics.  The first experiments will be realized in a Paul trap. We will attach diamonds containing NV centers to the particle, which will enable detection of the precession motion.

Just like it is not possible to trap a charged particle with static electric fields, it is in not possible to trap a magnet with static magnetic fields only.  As part of the LEMAQUME project, we succeeded in trapping a magnet using an alternating magnetic field, realizing a so-called magnetic Paul trap… We are currently characterizing the trap with the goal of miniaturize it on a chip for  trapping smaller magnetic particles.

Free-fall

To make a ferromagnetic particle achieve ultimate sensitivity in magnetic and inertial sensing, the best way is to make the particle experience zero gravity, such that the limitations from the levitation methods can be avoided. Many demanding applications in the near future, such as navigation of space stations and aircraft, can benefit from such sensors if its performance in free-fall experiment is well demonstrated.

The first stage of such experiment is a terrestrial free-fall experiment. The Einstein Elevator facility in Hannover can support a 4 s free fall experiment with a repetition rate as high as 100 times per day. We will design an experiment that can freely launch the micrometer scale particle in the Einstein Elevator and use optical and magnetic methods to observe the dynamics of the particle. By optimizing the zero-gravity time and relaxing the vibrational dynamics of the launch, a quantum regime that is dominated by the internal angular momentum of the particle due to intrinsic electron spin will be realized. In this regime, Larmor precession of a microscopic particle can be observed. 

Theory

Ferromagnetic gyroscopes (FG), i.e., ferromagnets whose angular momentum is dominated by electron spin polarization and that precess under the action of an external torque, have been proposed for precision tests of fundamental physics. Such tests could include exploration of Lorentz symmetry, searches for exotic spin-dependent interactions, dark matter experiments, and measurements of electric dipole moments and gravitational dipole moments. In order to realize the potential sensitivity of a FG, it is essential to decouple the ferromagnet from the environment, e.g., from gravity, by requiring either microgravity or some method of frictionless suspension. A freely floating ferromagnet zero-gravity magnet system in a satellite was recently proposed to measure general-relativistic frame dragging and geodetic precession, i.e., the Lense-Thirring (frame dragging) and de Sitter (geodetic precession) effects due to the Earth's gravity.

Another scheme for developing FG magnetometers is levitation of magnets over diamagnets, e.g., type-II superconductors. An alternative platform for developing FG magnetometers is afforded by attaching a FG to a cantilever, or using a ferromagnetic cantilever. The FG-cantilever would not be free to translate since it is anchored to the surface supporting the cantilever, but a torque would be applied to the cantilever if it is displaced from its equilibrium position. Such a system has the potential advantage that the translational degrees of freedom are frozen out and therefore the effects of spatial variations in the magnetic field are eliminated.

Theoretical methods, including development of analytical techniques and numerical calculations, are pursued within LEMAQUME to support experimental studies for all the above directions.