Tjerk Oosterkamp Lab (Microscopy and Quantum Mechanics at milliKelvin temperatures)

# Quantum to Classical

In Quantum mechanics, particles can be in multiple positions simultaneously. Yet, when a measurement is made, the particle is found only in one place. Technology has come to a point where we may design experiments that will tell us how.

## Breaking of Unitarity in Quantum Mechanics for Heavy Objects

Our setup, which we also use for nano-MRI, is well-suited to study this problem due to its low temperature and force noise. The core of the setup is a very cold cantilever (diving-board shaped mechanical resonator) with a micron-size magnet attached at the tip.

Is wavefunction collapse a real physical process? There are two possible experiments to study this question: an experiment showing that it is not, by creating a very large and heavy superposition. That is the experiment we explain here. The other experiment, which aims to measure the effects of a hypothesized process of spontaneous wavefunction collapse, is explained on this page.

Let’s assume quantum mechanics applies just as well to large and heavy objects, and that the collapse of the wavefunction is merely a result of our inability to experience a superposition. In that case, we would want to show this by demonstrating a massive object can be in a large superposition, by showing an interference effect.

Previous attempts to create a massive superposition have dealt with only a single or a few phonons. Our setup, however, could potentially be used to create a very large superposition in terms of phonon numbers. The idea is to couple the resonator’s motion magnetically to a single spin. This spin could be a diamond color center with a high coherence time. If we then put this spin into a superposition state (this is a proven technique) then the resonator will follow a superposition of paths. An interference experiment could then prove that the massive magnet was in a superposition.

This requires a much lower damping and a lower temperature of the resonator in order to work. The relevant projects to make this experiment possible are:

• Creating a strong static magnetic field to suppress magnetic damping when near the diamond. The challenge is that the noise and heating due to the current in the coil must be very low. [hyperlink to Nano-MRI#setup-B0-and-pcs]
• Coherently manipulating a single spin, or a small ensemble of spins. This is necessary to achieve single-spin resolution (coupling to a single spin).
• A resonator with lower intrinsic damping. We are currently investigating whether we could use a superconducting trap with a floating microparticle inside, instead of the cantilever.

These projects are currently ongoing in the Oosterkamp group.