In Search Of The Quantum-Classical Boundary

Physicists have long-sought to understand the process of wave function collapse: the transitional phase on a quantum objects evolutionary timeline that marks a state change from superposition to macroscopic definiteness, famously outlined in 1935 by Schrödinger’s cat-in-a-box thought experiment.

As a sceptic of Bohr’s Copenhagen interpretation, Schrödinger’s cat scenario was designed to highlight clear contradictions between quantum mechanics (QM) and everyday macroscopic reality. This long standing conundrum has lead researchers to search for alternative descriptions. For instance: can the evolution of a particle, from creation to measurement, be explained by a more coherent theory that eliminates both quantum probabilities and collapsing wave functions?

In a recent paper, researchers from Bonn University describe a novel set of experiments based on the concept of macroscopic realism.  ‘Macrorealism’ is based on two postulates that we associate with everyday experience: 1) a system is always in a well-defined state and not in a superposition of all states, and 2) non-evasive measurability. Utilising Leggett-Garg inequalities, if any of these definitions are violated then macrorealism is falsified and cannot be applied.

By manipulating/measuring caesium atoms within a novel one dimensional optical lattice, the team report a 6σ (standard deviation) violation of the LG inequalities. Far from falsifying QM—and the concept of superposition, as described by Schrödinger’s wave equations—experimental correlations show that particles, of caesium macroscopicity, exist simultaneously in all possible states and do not follow well-defined classical trajectories. Our experiment gives a rigorous, quantitative demonstration of the nonclassicality of a massive-particle quantum walk’, report the team.

Historically, interference patterns within double slit experiments have provided qualitative confirmation of superpositional behaviour—however, loopholes may exist. By ruling out macrorealism, researchers have provided an alternative verification of QM and shown that the quantum-classical boundary must reside at higher particle masses.

To date, the largest particles to show requisite diffraction behaviour has been carbon-60 molecules (‘Buckyballs’) therefore the next step will be to verify these results with the technique. The team highlight: ‘The interaction-free detection method of the atoms position can be adapted well to other systems like matter wave interferometers with large spatial splitting’ therefore further experiments utilising larger quantum objects are planned for the future.

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Paul Hattle

As a strong advocate for science and learning, I am a passionate supporter of the 'Campaign for Science and Engineering' (CaSE) Fellow of the 'Royal Astronomical Society' (RAS) Associate Member of the 'Institute of Physics' (IOP) & 'The Institute of Scientific and Technical Communicators' (ISTC)

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