Periodic Reporting for period 1 - CLOSEtoQG (Cryogenic on-chip Levitated Optomechanics for a Spin Entanglement witness to Quantum Gravity)
Reporting period: 2022-06-01 to 2024-11-30
The interface between quantum mechanics and general relativity is a century old problem. On the one hand, the endeavour to combine the two into a grand unified theory provides a puzzle that has occupied theoretical physicists for decades and produced an astounding number of competing theories. Recent ideas pose that entanglement and quantum correlations may give rise to spacetime itself. On the other hand, there is a possibility that gravity may provide the means to restore a classical reality from the quantum one, for example through a gravitationally induced collapse of the wavefunction.
The first key question is then: is gravity quantum? Can spacetime itself be in a superposition? The renown physicist Richard Feynman came up with a thought-experiment that could answer this question and due to the tremendous progress in our control of quantum systems, what Feynman considered a thought-experiment, may now be within reach in a realistic future laboratory.
The key idea is this: by bringing two massive objects into a spatial quantum superpositions, their mutual gravitational interaction can bring them into an quantum entangled state. If we can demonstrate experimentally that the masses have become entangled, while they interact only through gravity, we arrive at the conclusion that, yes, the gravitational field must be considered a quantum system that supports superpositions of metrics. Only then can entanglement be created. Intuitively, because the only interaction between the masses was through gravity, and formally, because entanglement between systems cannot increase under local operations and classical communication. If, on the other hand, no entanglement is created, even though the experiment is carried out correctly (making sure the superposition states have sufficient coherence times, the masses and distances are sufficient to result in observable entanglement), we arrive at a perhaps even more profound conclusion: either quantum theory or general relativity is unsuitable to describe the experiment.
So far, no experimental platform exists that can meet the challenging central requirement for such a test: A picogram-scale mass in a micrometre-scale spatial superposition with a second-scale coherence time. The core objective of this ERC starting grant proposal is the development, characterization and demonstration of an experimental platform that is compatible with the requirements to generate entanglement through the gravitational field.
The first key question is then: is gravity quantum? Can spacetime itself be in a superposition? The renown physicist Richard Feynman came up with a thought-experiment that could answer this question and due to the tremendous progress in our control of quantum systems, what Feynman considered a thought-experiment, may now be within reach in a realistic future laboratory.
The key idea is this: by bringing two massive objects into a spatial quantum superpositions, their mutual gravitational interaction can bring them into an quantum entangled state. If we can demonstrate experimentally that the masses have become entangled, while they interact only through gravity, we arrive at the conclusion that, yes, the gravitational field must be considered a quantum system that supports superpositions of metrics. Only then can entanglement be created. Intuitively, because the only interaction between the masses was through gravity, and formally, because entanglement between systems cannot increase under local operations and classical communication. If, on the other hand, no entanglement is created, even though the experiment is carried out correctly (making sure the superposition states have sufficient coherence times, the masses and distances are sufficient to result in observable entanglement), we arrive at a perhaps even more profound conclusion: either quantum theory or general relativity is unsuitable to describe the experiment.
So far, no experimental platform exists that can meet the challenging central requirement for such a test: A picogram-scale mass in a micrometre-scale spatial superposition with a second-scale coherence time. The core objective of this ERC starting grant proposal is the development, characterization and demonstration of an experimental platform that is compatible with the requirements to generate entanglement through the gravitational field.
The work is divided into three main research lines: (i) magnetic levitation of microparticles, (ii) a superconducting circuit based readout of the particle motion though the magnetic flux, and (iii) a spin-based qubit interface to the particle motion. A final goal is the integration of the techniques developed in (i)-(iii) into a unified experimental platform.
At this stage of the project, the focus is on (i) and (ii), with the main activities and achievements:
(i) Both room temperature and cryogenic levitation of magnetic as well as superconduting particles, initially in the 50-100 um size, going to smaller particles at the later stage of the project.
(ii) Proof-of-principle version of the flux based superconducting readout, using a Superconducting quantum interference device (SQUID) embedded in a chip based microwave resonator, coupled to a magnetized cantilever mechanical resonators as a model system for the magnetically levitated microparticles.
At this stage of the project, the focus is on (i) and (ii), with the main activities and achievements:
(i) Both room temperature and cryogenic levitation of magnetic as well as superconduting particles, initially in the 50-100 um size, going to smaller particles at the later stage of the project.
(ii) Proof-of-principle version of the flux based superconducting readout, using a Superconducting quantum interference device (SQUID) embedded in a chip based microwave resonator, coupled to a magnetized cantilever mechanical resonators as a model system for the magnetically levitated microparticles.
The research performed in this project could result in the most macroscopic quantum superpositions to date, already allowing us to rule out a large range of modifications to quantum theory, such as physical collapse theories. In turn this would be a major step towards a spin-based entanglement witness of quantum gravity. At the same time, the techniques developed in the project can benefit applications in force sensing and magnetic resonance force microscopy.
A good example at this stage of the project is the development of a test setup, where we can levitate 0.25 mm sized magnetic particles at room temperature. Such a suspended object may function as a 3D integrated inertial sensor, detecting accelerations in three dimensions.
A good example at this stage of the project is the development of a test setup, where we can levitate 0.25 mm sized magnetic particles at room temperature. Such a suspended object may function as a 3D integrated inertial sensor, detecting accelerations in three dimensions.