The collapse of the quantum wave function – in which quantum objects go from existing within many possible states at once to settling into a concrete one understood by classical physics – does not happen instantaneously, directly challenging a long-standing textbook understanding of quantum physics.
The most notorious example of this phenomenon is Erwin Schrödinger’s thought experiment where a cat in a box is in a quantum state of both “dead” and “alive” at the same time. Opening the box and looking at the cat – measuring the system – “collapses” the cat’s quantum state into a single definitive way of being.
Historically, physicists have found it challenging to study exactly what happens when a measurement makes a quantum object lose its quantumness, because “seeing” this process would require more measurements. The assumption that it happens instantaneously partly reflects some of that lack of knowledge.
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at the Joint Quantum Institute in Maryland and his colleagues set out to derive a formula for how long you must measure the state of a quantum object to achieve wave function collapse.
In their mathematical model, the researchers used the idea of decoherence. That is, they assumed that the state of a quantum object is altered because it interacts with something non-quantum – for example, an instrument touching a quantum object, or light hitting it before bouncing into your eye so that you can see it, leads to “non-quantumness” of everything around the object spreading to the object itself.
The researchers applied their mathematical method to two models similar to past experiments concerning decoherence that measured the quantum states of giant or charged atoms. The minimum duration for a measurement to trigger wave function collapse ranged from 0.1 billionth to 0.1 sextillionth of a second – less time than it takes a single particle of light to cross a molecule comprising two hydrogen atoms.
This time is inversely proportional to how strongly the measured object and the measurement device interact, so a very strong interaction collapses the quantum state very quickly. The team’s calculations also showed that objects that start off looking less quantum – that is, they embody fewer possible quantum states at once – require a shorter measurement process to collapse into a classical state.
at the Autonomous University of Barcelona in Spain says that while these measurement times are incredibly short, they still challenge the historical view that even instantaneous measurements, lasting no time at all, can collapse wave functions, and they could, in principle, be tested in future experiments. He says that some chemistry experiments can already study processes as quick as a quadrillionth of a second.
at the National University of Singapore, who was part of the research team, says that if any such experiments contradicted their results – if they happened faster than the minimum time found here – this could usher in a new kind of physics that goes beyond what quantum physics can explain right now.
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