Symbol rho_S
what an observer restricted to the system alone can predict with.
Read this term in its guide →Published equation contexts
Why this formula appears here
A quantum state in superposition carries relative phases between its components, and those phases are what produce interference — the double-slit fringes, the oscillating populations of a qubit. Measurement, in the everyday sense of “a detector clicks and now the system has a definite outcome,” requires that those phases stop mattering. The mechanism for that is decoherence: the measured system unavoidably becomes entangled with a much larger environment — stray photons, phonons in a substrate, the electromagnetic modes of a shielding can — and once the joint system-environment state is traced back down to the system alone, the interference terms are suppressed at a rate set by how strongly…
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Symbol t
t is an argument of the function-like quantity on the left; its role is set by that function’s stated inputs.
Read this term in its guide →Symbol E
E is an input to the expression that computes the quantity on the left.
Read this term in its guide →Symbol U
U is an input to the expression that computes the quantity on the left.
Read this term in its guide →Symbol rho_E
rh is an input to the expression that computes the quantity on the left.
Read this term in its guide →Symbol U^dagger
agger is an input to the expression that computes the quantity on the left.
Read this term in its guide →How to interpret it
Read it with the definitions, units, and assumptions supplied by the article.
Research cited beside this formula
Published contexts (1)
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Equation 1 · Physics
How Quantum Foundations and Measurement Actually Work
This equation states an equality: the expressions on both sides have the same value under the article’s assumptions.
A quantum state in superposition carries relative phases between its components, and those phases are what produce interference — the double-slit fringes, the oscillating populations of a qubit. Measurement, in the everyday sense of “a detector clicks and now the system has a definite outcome,” requires that those phases stop mattering. The mechanism for that is decoherence: the measured system unavoidably becomes entangled with a much larger environment — stray photons, phonons in a substrate, the electromagnetic modes of a shielding can — and once the joint system-environment state is traced back down to the system alone, the interference terms are suppressed at a rate set by how strongly…