The wave function in quantum mechanics does not describe a quantum system's properties before measurement, contrasting with classical physics where properties are well-defined. This distinction, termed a 'cut' by Werner Heisenberg, has historically separated classical and quantum worlds. While early interpretations, like the Copenhagen interpretation, suggested a choice in where this cut is placed, modern capabilities to probe various length scales, including the mesoscale, reveal quantum behavior in macroscopic objects, necessitating an explanation for the quantum-to-classical transition.
The Measurement Problem
Quantum mechanics has not directly explained the measurement process, in which quantum probabilities 'collapse' into a single observed value.
Diverse interpretations have attempted to address this fundamental problem.
- Copenhagen Interpretation: Collapse is figurative, reflecting the classical world we experience.
- Spontaneous Collapse: A real, spontaneous physical event selects a unique outcome from possibilities.
- Pilot-Wave Theory (de Broglie-Bohm): Particles possess well-defined properties, guided by a 'pilot' wave.
- Many-Worlds Interpretation (Everett): No collapse occurs; all measurement outcomes are realized in parallel universes.
Zurek's Work on Decoherence and Entanglement
Physicist Wojciech Zurek, in collaboration with H. Dieter Zeh, investigated what quantum theory itself suggests about measurements, starting in the 1970s. Their approach centers on quantum entanglement, a phenomenon where interconnected quantum particles share a single wave function, causing measurements on one to appear to influence the other.
Entanglement is an inherent aspect of particle interactions and occurs when quantum objects interact with their environment, including measuring instruments. Zurek and Zeh demonstrated that this entanglement 'dilutes' the quantum properties of the object, as its quantumness becomes a shared property with the entangled environment.
This process, termed decoherence, renders quantum effects unobservable in the object itself, effectively spreading superpositions across a rapidly multiplying web of entangled entities.
Decoherence occurs very rapidly; for a dust grain in air, it happens in approximately 10^-31 seconds.
Quantum Darwinism and Pointer States
Decoherence theory further explains that entanglement with the environment imprints information about the object onto that environment. Zurek's work over the past two decades focuses on how certain quantum states, known as 'pointer states,' can generate multiple imprints on the environment without being obscured by decoherence.
These pointer states correspond to classically observable properties, such as position or charge, and are robust because the interactions generating their imprints do not alter the quantum system's original state. Zurek describes these robust pointer states as 'fittest,' likening their survival and amplification in the classical world to Darwinian evolution. This process is called 'quantum Darwinism,' where information about these states multiplies efficiently and rapidly within the environment.
Calculations by Zurek and Jess Riedel in 2010 indicated that the location of a dust grain could be imprinted approximately 10 million times by photons from the sun within a microsecond.
Emergence of a Unique Classical Reality
A key prediction of decoherence theory is that all imprints of a quantum system in its environment must be identical. This implies that quantum Darwinism necessitates the emergence of a unique classical world from quantum probabilities, offering an alternative to the concept of wave function collapse. The observed object, surrounded by a multitude of identical imprints in its macroscopic environment, forms an 'extanton' or element of 'relatively objective existence,' as described by Zurek.
Zurek's theory proposes a reconciliation between the Copenhagen (epistemic view of wave function) and many-worlds (ontic view of wave function) interpretations. He suggests that wave functions are 'epiontic,' meaning that before decoherence, all quantum possibilities are present, but decoherence and quantum Darwinism select only one as an observable reality. Other states remain in an abstract space of possibilities without developing into observable realities through entanglement.
Ongoing Questions and Research
While Zurek's framework provides an elegant explanation for classicality's emergence from quantum theory, some questions persist. These include the specific cause for a particular outcome's selection, the precise moment of irreversible commitment to a measurement outcome, and the need for more rigorous experimental testing.
Experts like Sally Shrapnel and Renato Renner acknowledge the elegance but point to remaining challenges, such as defining the 'quantum substrate' and addressing scenarios where observer agreement on outcomes might be difficult.