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Harnessing Quantum Systems with Superconductivity and Magnetism

Final Report Summary - QSUPERMAG (Harnessing Quantum Systems with Superconductivity and Magnetism)

QSuperMag aims at using magnetic fields and superconductors to harness quantum degrees of freedom in order to make accessible an unprecedented parameter regime in quantum science. Laser light has been the ubiquitous tool in the last decades to control and manipulate quantum systems because it is fast, coherent, and can be focused to address individual degrees of freedom. However, the use of lasers poses fundamental limitations, such as heating and decoherence due to scattering and absorption of photons, and a minimum length-scale to achieve coherent control due to the diffraction limit. The main goal of QSuperMag is to circumvent these limitations by using magnetic fields and superconductors to harness quantum systems that are traditionally controlled and addressed by laser light. This is done by developing the underlying theory and proposing experiments which lie at the interplay between the fields of quantum science and superconductivity.
The outcomes of the project are so far the following:

* We have proposed and analyzed an all-magnetic scheme to perform matter-wave interference with a micron-sized superconducting sphere of mass 10^13 amu. Current optical matter-wave interferometers hold a mass record of 10^4 amu and prospects for increasing the mass while still using all-optical schemes require a space environment. QSuperMag has proposed an alternative approach for an all-magnetic, earth-based, on-chip scheme. Moreover we have shown that at such large scales the faint parameter-free gravitationally-induced decoherence collapse model, proposed by Diósi and Penrose in the 80’s, could be unambiguously falsified.

* We have developed a theoretical toolbox to describe the quantum dynamics of a magnetically levitated single-domain magnetic nanoparticle. The theoretical modeling of such rich system paves the path for proposing experiments in which the center-of-mass motion, the total angular momentum, and the macrospin degrees of freedom of the particle, can be brought and controlled in the quantum regime.

* We have shown that, despite Earnshaw’s theorem, a nonrotating single magnetic domain nanoparticle can be stably levitated in an external static magnetic field. The stabilization relies on the quantum spin origin of magnetization, namely, the gyromagnetic effect. Such a quantum spin stabilized levitation opens the door to experiments aiming not only at demonstrating the predicted phase diagram but also at bringing a nonrotating nanomagnet to the quantum regime, whose equilibrium states show nontrivial quantum correlations.

* We have proposed to use a diamagnetically levitated magnet as a force and inertial sensor with unprecedented sensitivities. The concept is rather general and can be applied to magnets with sizes ranging from nanometers to millimeters, spanning over 6 orders of magnitude. The use of a magnet with a strong magnetic moment gives rise to a simple passive trapping scheme, and provides direct ways to read and feedback cool its motion. Our analysis, including current technologies and realistic assumptions, indicates very promising sensitivities over a wide range of scales.

* We have theoretically shown that the inductive coupling between the quantum mechanical motion of a superconducting microcantilever and a flux-dependent microwave quantum circuit can attain the strong single-photon nanomechanical coupling regime with feasible experimental parameters. The strong magnetic response of a superconducting strip has been exploited to achieve the strong single-photon coupling regime, which is a very challenging parameter regime to be achieved in quantum nanomecahnics.

* We have proposed and analyzed a novel scheme for generating a high density lattice potential for ultracold atoms. This exploits the possibility to nanoengineer an array of superconducting vortices in a superconducting thin film, thereby generating a high density magnetic lattice. This offers an alternative approach to laser-based optical lattices for ultracold neutral atoms that could offer the possibility to implement a high-density lattice potential to speed up quantum simulation.

* We have shown how the static magnetic field of a finite source can be transferred and routed to arbitrary long distances using a superconducting-ferromagnet hybrid inspired by transformation optics. We have called this device a magnetic hose. These results have already been used in to locally address magnetically superconducting qubits.

* We have shown that a network of superconducting loops and magnetic particles can be used to implement magnonic crystals with tunable magnonic band structures. We have demonstrated that magnons can serve as a quantum bus for long-distance magnetic coupling of spin qubits.

* We have shown that a dipole externally driven by a pulse with a lower-bounded temporal width, and placed inside a cylindrical hollow waveguide, can generate a train of arbitrarily short and focused electromagnetic pulses. The temporal features of the generated EM fields could be used for precise sensing, as similarly done with frequency combs, and the spatial features for imaging and focusing of either magnetic or electric longitudinal fields

* We have theoretically and experimentally shown that a linear and isotropic electrically conductive material moving with constant velocity is able to circumvent the magnetostatic reciprocity principle and realize a diode for magnetic fields. We anticipate that this result will provide novel possibilities for applications and technologies based on magnetically coupled elements and might open fundamentally new avenues in artificial magnetic spin systems.