The quantum world and our everyday world are very different places. In a
publication that appeared as the "Editor's Suggestion" in Physical Review A
this week, UvA physicists Jasper van Wezel and Lotte Mertens and their
colleagues investigate how the act of measuring a quantum particle
transforms it into an everyday object.

Quantum mechanics is the theory that describes the tiniest objects in the
world around us, ranging from the constituents of single atoms to small dust
particles. This microscopic realm behaves remarkably differently from our
everyday experience—despite the fact that all objects in our human-scale
world are made of quantum particles themselves. This leads to intriguing
physical questions: why are the quantum world and the macroscopic world so
different, where is the dividing line between them, and what exactly happens
there?

### Measurement problem

One particular area where the distinction between quantum and classical
becomes essential is when we use an everyday object to measure a quantum
system. The division between the quantum and everyday worlds then amounts to
asking how 'big' the measurement device should be to be able to show quantum
properties using a display in our everyday world. Finding out the details of
measurement, such as how many quantum particles it takes to create a
measurement device, is called the quantum measurement problem.

As experiments probing the world of quantum mechanics become ever more
advanced and involve ever larger quantum objects, the invisible line where
pure quantum behavior crosses over into classical measurement outcomes is
rapidly being approached. In an article, UvA physicists Jasper van Wezel and
Lotte Mertens and their colleagues take stock of current models that attempt
to solve the measurement problem, and particularly those that do so by
proposing slight modifications to the one equation that rules all quantum
behavior: SchrÃ¶dinger's equation.

### Born's rule

The researchers show that such amendments can in principle lead to
consistent proposals for solving the measurement problem. However, it turns
out to be difficult to create models that satisfy Born's rule, which tells
us how to use SchrÃ¶dinger's equation for predicting measurement outcomes.
The researchers show that only models with sufficient mathematical
complexity (in technical terms: models that are non-linear and non-unitary)
can give rise to Born's rule and therefore have a chance of solving the
measurement problem and teaching us about the elusive crossover between
quantum physics and the everyday world.

## Reference:

Lotte Mertens et al, Inconsistency of linear dynamics and Born's rule,
Physical Review A (2021).
DOI: 10.1103/PhysRevA.104.052224

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