Levitated Optomechanical Technologies In Space
Optomechanics is an emerging platform for quantum science and technology which controls and measures mechanical modes of nanoscale objects with light. Levitated optomechanics uses individual, optically levitated nanoparticles as the mechanical mode and promises applications from precision force measurement to quantum information processing and tests of fundamental physics.
Optomechanics, more generally, includes micro-cantilevers, toroidal resonators, and silicon nitride nanobeams, offering a wealth of technologies which exploit the coherent interaction of light with the mechanical motion of nanoscale objects. Some of these systems have been cooled to their quantum mechanical ground state and theoretical and experimental work is driving fast towards demonstrating uniquely quantum properties such as entanglement and Schrödinger-cat states [Kanari-Naish 2022]. Levitated Optomechanics offers additional opportunities because the mechanical mode is exceptionally well isolated from the environment. These mesoscopic systems are far larger than what we would normally consider the quantum domain but can nevertheless be driven into quantum mechanical states [Delić 2020], manipulated into squeezed states [Rashid 2016], and there is a clear strategy towards generating and observing stationary optomechanical entanglement [Magrini 2022].
Cold atom technologies have demonstrated in recent years that these quantum effects are not just laboratory curiosities but can become a core sensing technology for terrestrial [Phillips 2022] and spaceborne applications [Carraz 2014]. These devices use matter-wave interferometers which are sensitive to inertial forces and gravity and offer a performance which, even at this early stage, is competitive with the best traditional gravimeters [Devani 2020]. The CASPA-ADM Mission [Siemes 2022] will bring this technology to the specific challenge of measuring the thermospheric mass density in a free-flying 6U CubeSat mission.
Matter-wave interferometric sensitivity, such as to inertial and gravitational forces, depends on the mass and spatial superposition separation. Larger mass quantum objects are possible with levitated optomechanics than with cold atom technologies, by a factor ~\(10^6\). Possible schemes include near-field matter-wave interferometry [Bateman 2014] and non-linear cavity optomechanical coupling [Qvarfort 2018]. Levitated Optomechanics offers complementary measurement modalities and hybrid devices are feasible. The MAQRO Collaboration, which includes around 50 scientists and engineers from across Europe, the USA, and Australia, identified the challenges to realising quantum levitated optomechanics in space. Recent studies of this proposed mission include the European Space Agency's Concurrent Design Facility study of the MAQRO Mission [MAQRO CDF 2018] and a summary is available in the NASA BPS 2023 Decadal Survey [Kaltenbaek 2022].
The challenges identified by the ESA CDF Study of MAQRO include the stability of free-space optics, drag- free requirements, and cryogenics, many of which are not required for terrestrial levitated optomechanics and have been solved in principle by other missions, notably LISA Pathfinder and JWST. There remain challenges specific to levitated optomechanics and these are primarily the sourcing and conditioning of nanoparticles for loading into a focused laser beam at the centre of levitated optomechanical apparatus.
LOTIS will address the specific levitated optomechanics challenges: Storage & dispensing (SD), Capture & conditioning (CC), and Optomechanical trapping (OT).
The underpinning technologies of current levitated optomechanics studies have been developed for terrestrial, ambient-pressure, laboratory environments, where ample space, power, and heat dissipation is available. Much like the evolution seen in cold atom systems, it is by developing these technologies that we will enable a transition of levitated optomechanics from laboratory-based demonstrations into a useful space- based sensing platform.
The principal challenge of the nanoparticle source is that the intermolecular forces between surfaces are very strong compared with the inertia of a typical nanoparticle. Consequently extremely high surface accelerations ~\(10^7\)g are needed to liberate nanoparticles which are adhered to a substrate. LOTIS will explore adapting several distinct lab-based technologies which routinely achieve this goal so that they are suitable for inclusion on a small (CubeSat scale) satellite mission.
Nanoparticles liberated from a surface, particularly if done so under high vacuum conditions, may require some pre-capture or conditioning before loading into the optomechanical laser trap. LOTIS will explore technologies to achieve this including adapting ion traps, and using several unique features of levitated optomechanics including that time-scales for their motion permit closed-loop feedback using fast electronics.
Laboratory based levitated optomechanical experiments often employ highly mature, robust and laser technologies developed for fibre optical telecommunications. Historically, these devices have been optimised for extremely low noise which is advantageous but not strictly necessary for many optomechanical demonstrations. LOTIS will produce the minimum viable levitated optomechanical trap to fit within the size, weight, and power (SWaP) requirements of a typical small satellite.
Furthermore, LOTIS will explore the extent to which novel protocols can be implemented with the hardware we design. This interaction will be two-way, with requirements of the sensing protocols informing the technology development where critical features are identified, and vice-versa. This is crucial as many existing theoretical studies consider idealisations without reference to the practicalities and imperfections inherent in real experiments.
Finally, LOTIS will analyse the system integration requirements so that the hardware demonstrated can be understood in context and the next steps on the technology development roadmap can be clearly recognised.
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