Quantum foundations
This detailed orientation connects core ideas, mathematical formalism, practical applications and worked reasoning. Explore the original complete site for its chapters, references and activities.
Physical meaning
Quantum states are mathematical descriptions of physical systems, not ordinary classical labels. A pure state is represented by a normalized vector in a complex Hilbert space. For a two-level system, |ψ⟩ = α|0⟩ + β|1⟩ with |α|² + |β|² = 1. The coefficients are probability amplitudes; probabilities arise from their squared magnitudes.
Mathematical framework
The distinction between amplitudes and probabilities explains interference. Two alternatives may have amplitudes a and b; their combined probability is |a+b|², which contains a cross term absent from the classical expression |a|²+|b|². Relative phase therefore matters even when individual outcome probabilities appear unchanged.
From theory to technology
An observable is represented by a Hermitian operator A. Measurement outcomes are its eigenvalues, while the Born rule gives p(a)=⟨ψ|Pₐ|ψ⟩ for the projector Pₐ. After a selective ideal projective measurement, the state becomes Pₐ|ψ⟩/√p(a). Not all real measurements are projective: POVMs describe more general measurement statistics.
Limits and experimental realities
Time evolution of an isolated system follows iℏ d|ψ⟩/dt=H|ψ⟩. A time-independent Hamiltonian yields U(t)=exp(−iHt/ℏ). Open systems require density operators and often master equations because environmental coupling creates decoherence.
Worked thought experiment
For |ψ⟩=(|0⟩+|1⟩)/√2, a Z-basis measurement yields 0 or 1 with equal probability. Applying a Hadamard before measurement changes the result to 0 with certainty. This illustrates how phase-sensitive transformations expose coherence.
Check your understanding
1. Which assumptions are essential to the equations above? 2. What measurement would reveal the phenomenon? 3. Which errors or approximations matter in an experiment? 4. How does the idea connect to another QubitSemantics domain?
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