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A two-dimensional normalization and phase computation
Example
Let carry the standard inner product (Real and complex inner product spaces, with the inner product linear in the first argument, Hilbert space; completeness follows from Every finite-dimensional normed space is Banach), and let , . Take and verify on this explicit data the normalization and phase statements of Normalization, ray equivalence and the phase orbits of the unit sphere: the norm of is times the norm of , the normalized representative of is the phase multiple of the normalized representative of , the rays of and coincide, and the normalized rank-one operators of , and are the same pure state The state space: density operators, pure and mixed states, and purity.
Facts & Assumptions
Given: The Hilbert space with the standard inner product and basis ; the vectors , with , and ; the ray, normalization and phase notions of State vectors, unit vectors, rays, normalization and phases of a Hilbert space; and the Axiom of Countable Choice (The Axiom of Countable Choice ()), the choice hypothesis of the cited rank-one and state-space suppliers.
Standard computations in : , , expansions below are termwise multiplications with , , (The complex numbers as , with the real embedding and imaginary unit , Conjugation is an involutive real-field automorphism, , and modulus is definite, multiplicative, and subadditive).
Norm and pairing: with , so , and (The induced length is a norm, Conjugation is an involutive real-field automorphism, , and modulus is definite, multiplicative, and subadditive).
Normalization and phases (Normalization, ray equivalence and the phase orbits of the unit sphere): , , and for ; if and only if for some .
Rank-one operators (Rays, unit vectors and rank-one Hilbert projections, The state space: density operators, pure and mixed states, and purity): , , is a pure state, and for unit one has exactly when with .
Verification
Compute . By [L2] and [L1], and , so , , and indeed , verifying the homogeneity used in the normalization lemma.
The normalizations are and . With one has , so , and which is the identity ; by [L3] this also identifies , so , and lie in one ray.
The rank-one operator of the unit vector has matrix in the basis with columns and , that is This matrix is self-adjoint, has trace , and satisfies because by [L1]; hence is a pure state by [L4].
For the second representative, by [L4], and dividing by from step 1.1 gives ; likewise , so the phase multiple determines the same pure state. This verifies on explicit data the representative and phase independence of the normalized rank-one operator.
Remarks
- The computation checks the definitions and the normalization lemma on a fixed two-dimensional example. It asserts nothing about any physical system, preparation or apparatus, and it records no measured value.
- The phase multiple and the rescaled representative give the same pure state, while the relative phase between the two components of a vector is not removed by any of these operations; the effect of relative phase belongs to the framework's probability law on the observable pages and is not examined here.
Depends on
State vectors, unit vectors, rays, normalization and phases of a Hilbert space
Normalization, ray equivalence and the phase orbits of the unit sphere
Rays, unit vectors and rank-one Hilbert projections
The state space: density operators, pure and mixed states, and purity
Real and complex inner product spaces, with the inner product linear in the first argument
Hilbert space
The induced length is a norm
The complex numbers as , with the real embedding and imaginary unit
Conjugation is an involutive real-field automorphism, , and modulus is definite, multiplicative, and subadditive
Every finite-dimensional normed space is Banach
The Axiom of Countable Choice ()
Dependency tree · 2 levels
Sources
- J. Preskill, Lecture Notes for Ph219/CS219: Quantum Information, Chapter 2 (updated July 2015), Sections 2.1 and 2.2.1, printed pages 4 and 8-9: normalized representatives of rays, the phase redundancy of |psi> and the qubit state |psi(phi,theta)> = (e^{-i phi/2} cos(theta/2), e^{i phi/2} sin(theta/2))
- J. Binney and D. Skinner, The Physics of Quantum Mechanics, Section 1.3.4 and 1.3.5, printed pages 12-13: normalization of a two-component state and the role of relative phase