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A two-dimensional normalization and phase computation

Example

Let H=C2 carry the standard inner product ⟨z,w⟩=z1w1‾+z2w2‾ (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 e1=(1,0), e2=(0,1). Take ψ=(1,i),z=1+i,φ:=zψ, 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 φ is ∣z∣ times the norm of ψ, the normalized representative of φ is the phase multiple (z/∣z∣) of the normalized representative of ψ, the rays of ψ and φ coincide, and the normalized rank-one operators of ψ, φ and iψ are the same pure state The state space: density operators, pure and mixed states, and purity.

Facts & Assumptions

Given: The Hilbert space H=C2 with the standard inner product and basis (e1,e2); the vectors ψ=(1,i), φ=zψ with z=1+i, and iψ; 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 (ACω)), the choice hypothesis of the cited rank-one and state-space suppliers.

[L1]

Standard computations in C: i2=−1, (1+i)2=2i, (1+i)(1+i)−1 expansions below are termwise multiplications with 1+i‾=1−i, ∣1+i∣2=2, ∣i∣=1 (The complex numbers as R[x]/(x2+1), with the real embedding and imaginary unit i, Conjugation is an involutive real-field automorphism, zz‾=∣z∣2, and modulus is definite, multiplicative, and subadditive).

[L2]

Norm and pairing: ∥v∥=⟨v,v⟩ with ⟨z,w⟩=z1w1‾+z2w2‾, so ∥(a,b)∥2=∣a∣2+∣b∣2, and ∥λv∥=∣λ∣ ∥v∥ (The induced length is a norm, Conjugation is an involutive real-field automorphism, zz‾=∣z∣2, and modulus is definite, multiplicative, and subadditive).

[L3]

Normalization and phases (Normalization, ray equivalence and the phase orbits of the unit sphere): ν(v)=∥v∥−1v, [v]={w:w=tv, t≠0}, and ν(tv)=(t/∣t∣)ν(v) for t≠0; [v]=[w] if and only if ν(w)=ζν(v) for some ζ∈U(1).

[L4]

Rank-one operators (Rays, unit vectors and rank-one Hilbert projections, The state space: density operators, pure and mixed states, and purity): Tv,vx=⟨x,v⟩v, Ttv,tv=∣t∣2Tv,v, Tv,v/∥v∥2=Tν(v),ν(v) is a pure state, and for unit u,w one has Tu,u=Tw,w exactly when w=ζu with ∣ζ∣=1.

Verification

technique · direct; compute the displayed vectors, their normalizations and the associated rank-one operators
1.1givenL1L2algebra

Compute φ=zψ=(1+i)(1,i)=((1+i)⋅1, (1+i)⋅i)=(1+i, i−1). By [L2] and [L1], ∥ψ∥2=∣1∣2+∣i∣2=2 and ∥φ∥2=∣1+i∣2+∣i−1∣2=2+2=4, so ∥ψ∥=2, ∥φ∥=2, and indeed ∥φ∥=∣z∣ ∥ψ∥=2⋅2=2, verifying the homogeneity used in the normalization lemma.

2.1givenstep 1.1L1L2L3algebra

The normalizations are ν(ψ)=(1/2, i/2) and ν(φ)=((1+i)/2, (i−1)/2). With ζ:=(1+i)/2 one has ∣ζ∣2=∣1+i∣2/2=1, so ζ∈U(1), and ζ ν(ψ)=1+i2(12,i2)=(1+i2,i(1+i)2)=(1+i2,i−12)=ν(φ), which is the identity ν(zψ)=(z/∣z∣)ν(ψ); by [L3] this also identifies [φ]=[ψ], so ψ, φ and iψ lie in one ray.

3.1givenstep 2.1L1L4algebra

The rank-one operator of the unit vector u=ν(ψ)=(1/2, i/2) has matrix in the basis (e1,e2) with columns Tu,ue1=⟨e1,u⟩u=u1‾u=12u=(12,i2) and Tu,ue2=⟨e2,u⟩u=u2‾u=−i2u=(−i2,12), that is Tu,u=12(1−ii1). This matrix is self-adjoint, has trace 1, and satisfies Tu,u2=Tu,u because (1−ii1)2=2(1−ii1) by [L1]; hence Tu,u is a pure state by [L4].

4.1givenstep 1.1step 3.1L1L2L4algebra∎

For the second representative, Tφ,φ=∣z∣2Tψ,ψ=2Tψ,ψ by [L4], and dividing by ∥φ∥2=4 from step 1.1 gives Tφ,φ/∥φ∥2=Tψ,ψ/2=Tψ,ψ/∥ψ∥2=Tu,u; likewise Tiψ,iψ=∣i∣2Tψ,ψ=Tψ,ψ, so the phase multiple iψ 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 iψ and the rescaled representative φ 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

Dependency tree · 2 levels

Sources