Quantum mechanics is a branch of physics that describes the behavior of very small particles, such as atoms and photons. One of the most fascinating and counterintuitive aspects of quantum mechanics is the phenomenon of superposition, which means that a quantum object can exist in two or more states at the same time, until an observation collapses it into one definite state.
For example, a photon can be in a superposition of being both horizontally and vertically polarized, until it passes through a polarizer that reveals its actual polarization. This is the basis of many quantum technologies, such as quantum cryptography and quantum computing.
However, creating and maintaining quantum superpositions of large objects, such as molecules or nanoparticles, is much more challenging, because they interact with their environment and lose their quantum coherence. This is known as decoherence, and it is the main obstacle for realizing quantum experiments with macroscopic objects.
A team of theoretical physicists, led by Oriol Romero-Isart from the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW) and the Department of Theoretical Physics at the University of Innsbruck, has proposed a novel method to overcome this challenge and generate quantum superpositions of large objects in a fast and reliable way.
How to make a nanoparticle quantum
The researchers propose to use an optically levitated nanoparticle, such as a nanoscale-sized glass bead, as the quantum object. By shining a laser beam on the nanoparticle, they can cool it down to its motional ground state, which means that it has the lowest possible energy and is ready to enter the quantum regime.
However, as soon as the laser is switched off, the nanoparticle is exposed to the effects of air molecules and scattered light, which quickly heat it up and destroy its quantum state. To avoid this, the researchers suggest to let the nanoparticle evolve in the dark, guided only by non-uniform electrostatic or magnetic forces.
These forces create a potential, or a landscape of energy, for the nanoparticle to move in. By choosing a suitable shape for the potential, such as a curved ramp, the researchers can make the nanoparticle roll down the ramp and split into two paths, creating a quantum superposition of being in both paths at the same time.
This process is very fast, taking only a few milliseconds, and can be repeated many times with the same nanoparticle. The researchers also show how to verify the quantum nature of the superposition by measuring the interference pattern of the nanoparticle after recombining the two paths.
Why this is important
This proposal is a significant advancement for the field of quantum mechanics, as it demonstrates a feasible way to create and observe quantum superpositions of large objects, which are usually considered to be classical and obey the laws of Newtonian physics.
This could have profound implications for our understanding of the quantum-classical boundary, the nature of reality, and the role of observation in quantum physics.
Moreover, this proposal could also enable new applications of quantum technologies, such as quantum metrology, quantum sensing, and quantum information processing, with macroscopic objects.
The researchers have discussed their proposal with experimental partners in Q-Xtreme, an ERC Synergy Grant project financially supported by the European Union. They expect that their protocol could be tested soon with thermal particles in the classical regime, which would help to measure and minimize the sources of noise and errors in the experiment.
They are confident that, with some technical improvements, their proposal could be realized with quantum particles in the near future, opening a new window to the quantum world.
