A last word on quantum Darwinism

In the opening paragraphs of “Quantum Darwinism, classical reality, and the randomness of quantum jumps” (Physics Today, October 2014, page 44 ), Wojciech Zurek invokes an analogue to Schrödinger’s cat and describes how the environment-driven process of decoherence removes the interferences that would otherwise exist between the live and dead states. He thus explains how the environment effectively “measures” the state of the cat. The article leaves me uncertain whether Zurek judges those comments to be a resolution of the quantum measurement or “Schrödinger’s cat” problem, but some readers might conclude that they indeed offer such a solution. Those readers would be wrong. As Stephen Adler has shown, ¹ and as decoherence expert Maximilian Schlosshauer has stated, ² decoherence does not solve the measurement problem because it does not solve the so-called “problem of outcomes.”

Decoherence theory shows how interactions with environmental quanta (photons, air molecules, and so forth) convert a quantum system’s superposition state α|s₁⟩ + β|s₂⟩ into the entangled “measurement state” α|s₁⟩|e₁⟩ + β|s₂⟩|e₂⟩, where |e₁⟩ and |e₂⟩ are orthogonal many-body states of the environment. Thus the environment acts just like a “which-slit” detector in the double-slit experiment with photons. Such a which-slit measurement transforms each photon from its premeasurement superposition over both slits into a measured photon whose single-slit states |s₁⟩ and |s₂⟩ are entangled with the two orthogonal states |e₁⟩ and |e₂⟩ of the detector. That removes the interferences from—that is, it decoheres—the photon.

At that point, the theory runs into the problem of outcomes: The measurement state is still a superposition, albeit one in which the measured quantum no longer interferes with itself. By what logic does that imply definite (but indeterminate) individual outcomes? In other words, by what logic is Schrödinger’s cat converted into a cat that is either dead or alive rather than one that is both dead and alive? Zurek’s analysis does nothing to resolve that question.