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Coherent-state convergence

Statement

Assume the Axiom of Countable Choice (The Axiom of Countable Choice (ACω)). Let H=L2(R,dy) with inner product linear in the first argument, let (gn)n∈N be the Hermite family of Hermite recurrence, and let a,a†,N be the closed ladder operators and number operator of Closed ladder domains with domains D(a),D(a†),D(N) and N=a†a on D(N). For z∈C put

ψz:=e−∣z∣2/2∑n=0∞znn! gn,

the series being taken in L2. Then:

(a) the series converges in L2, ∥ψz∥=1, and ψz=e−∣z∣2/2∑n=0∞znn!gn is the unique element of L2 with coefficients cn=e−∣z∣2/2zn/n! in the basis (gn);

(b) ψz∈D(Nk) for every k∈N, where Nk is the k-fold iterate of N with domain D(Nk)={x:∑nn2k∣⟨x,gn⟩∣2<∞}; in particular

∑nn2k∣cn∣2=e−∣z∣2∑n=0∞n2k∣z∣2nn!<∞;

(c) aψz=zψz (so ψz is a unit eigenvector of the annihilation operator with eigenvalue z), and ∥aψz∥2=∣z∣2;

(d) ⟨ψz,ψw⟩=e−(∣z∣2+∣w∣2)/2+zw‾=e−∣z−w∣2/2eiIm⁡(zw‾) for all z,w∈C, so ∥ψz−ψw∥2=2(1−e−∣z−w∣2/2cos⁡(Im⁡(zw‾))) and the map z↦ψz is injective;

(e) ⟨Nψz,ψz⟩=∣z∣2 and ∥a†ψz∥2=∣z∣2+1, and ψ0=g0.

The completeness of (gn) (Hermite completeness) is used for uniqueness from the prescribed coefficients in (a) and to invoke the closed ladder operators of Closed ladder domains on a complete orthonormal family. Series convergence and normalization use orthonormality and Hilbert-space completeness; the operator and moment conclusions use the supplied closed ladder domains.

Facts & Assumptions

Given: the Hermite family (gn) with its ladder operators a,a†,N; a complex number z; the numbers cn=e−∣z∣2/2zn/n!.

[F1]

⟨gn,gm⟩=δnm (Hermite orthogonality(a)), and L2 is a complex Hilbert space under Countable Choice (L2 with the integral pairing is a Hilbert space). Finite orthogonal sums satisfy ∥∑n∈Eangn∥2=∑n∈E∣an∣2. Thus a series ∑nangn converges exactly when ∑n∣an∣2<∞: finite tail norms give the Cauchy criterion and Hilbert-space completeness gives the limit; conversely convergence bounds the partial norms. Continuity of the norm and inner product gives the sum's squared norm and its coefficients.

[F2]

On D(N) one has a†a=N, aa†=N+I; D(aa†)=D(a†a)=D(N); and a is closed with a∗=a† (Closed ladder domains(b),(d)).

[F3]

et=∑n≥0tn/n! for complex t, the series converging absolutely (The complex exponential by its power series, The complex exponential series converges absolutely for every complex argument); powers and integer powers of complex numbers are those of the library's power conventions. The addition law ev+w=evew and agreement with the real exponential are exp⁡(z+w)=exp⁡z exp⁡w, and the complex exponential extends the real exponential; for real x,y, ex+iy=ex(cos⁡y+isin⁡y) and ∣ex+iy∣=ex are exp⁡(x+iy)=ex(cos⁡y+isin⁡y), ∣exp⁡(x+iy)∣=ex, and eiπ+1=0. Positivity and e−x=1/ex are The exponential is positive and satisfies exp⁡(−x)=1/exp⁡(x), and strict increase is The exponential function is strictly increasing.

[F4]

If 0≤an+1≤12an for all n≥N, then ∑n≥Nan≤2aN, by comparison with the geometric series ∑k2−k=2 (For ∣r∣<1, ∑k≥0rk=1/(1−r), and for ∣r∣≥1 the series diverges).

[F5]

agn=n gn−1 for n≥1 with ag0=0; a†gn=n+1gn+1 (Hermite recurrence(e)).

[F6]

The Hermite family is complete: any h∈L2 with ⟨h,gn⟩=0 for every n is zero (Hermite completeness(b)).

Proof

1.1givenF1F3F6

[Convergence and normalization] For N≥M, the partial sums SN:=e−∣z∣2/2∑n≤Nznn!gn satisfy ∥SN−SM∥2=e−∣z∣2∑M<n≤N∣z∣2nn! by [F1]. As N,M→∞ the right side tends to 0 because ∑n∣z∣2nn!=e∣z∣2<∞ by [F3]; hence (SN) is Cauchy and ψz:=lim⁡NSN∈L2 exists, and taking limits in the same identity gives ∥ψz∥2=e−∣z∣2∑n∣z∣2nn!=1. For fixed n, inner-product continuity gives ⟨ψz,gn⟩=lim⁡M→∞⟨SM,gn⟩=cn. If x∈L2 has these same coefficients, then ⟨x−ψz,gn⟩=0 for every n, so x=ψz by completeness [F6].

2.1step 1.1F1F2F4algebra

[Number domains] For k=0 use N0=I, D(N0)=H and n0=1, including n=0; step 1.1 proves the assertion. Now fix k≥1 and let an:=n2k∣z∣2n/n!≥0. If z=0, ψ0=g0, which lies in every D(Nk) and all terms an with n≥1 vanish. If z≠0, for n≥1, an+1an=(n+1n)2k∣z∣2n+1→0, so there is N with an+1≤12an for n≥N; by [F4], ∑n≥Nan≤2aN<∞, and the finitely many initial terms are finite. Hence ∑nn2k∣cn∣2=e−∣z∣2∑nan<∞. Since D(Nk)={x:∑nn2k∣⟨x,gn⟩∣2<∞} (inducting the domain formula of Closed ladder domains(a): Njx has coefficients nj⟨x,gn⟩, and the weight n2k dominates every lower weight for n≥1), this says ψz∈D(Nk).

2.2step 1.1F2F5algebra

[aψz=zψz] Each partial sum SN is a finite combination of the gn, so SN∈D(N)⊆D(a); by [F5], aSN=e−∣z∣2/2∑n=1Nznn gn−1/n!=ze−∣z∣2/2∑m=0N−1zmm!gm=zSN−1. As N→∞, SN→ψz and aSN→zψz by step 1.1; since a is closed [F2], ψz∈D(a) and aψz=zψz. Taking norms, ∥aψz∥=∣z∣ by (a).

2.3step 1.1F1F3algebra

[Inner products] For z,w∈C, ⟨ψz,ψw⟩=e−(∣z∣2+∣w∣2)/2∑nznw‾ nn! by [F1] applied to the two convergent series, and ∑n(zw‾)nn!=ezw‾ by [F3] applied to the series of the complex number zw‾; hence ⟨ψz,ψw⟩=e−(∣z∣2+∣w∣2)/2+zw‾=e−∣z−w∣2/2eiIm⁡(zw‾). Its modulus is e−∣z−w∣2/2. Then ∥ψz−ψw∥2=∥ψz∥2+∥ψw∥2−2Re⁡⟨ψz,ψw⟩=2(1−e−∣z−w∣2/2cos⁡(Im⁡(zw‾))). When z≠w the overlap modulus is strictly less than 1, so this distance is positive; when z=w it is zero; the map is injective.

3.1step 1.1step 2.1F1F2F3algebra∎

[Number and creation moments] By step 2.1 with k=1, ψz∈D(N) and Nψz has coefficients ncn; hence ⟨Nψz,ψz⟩=∑nn∣cn∣2=e−∣z∣2∑nn∣z∣2nn!. The series reindexes as ∣z∣2∑m≥0∣z∣2mm!=∣z∣2e∣z∣2 by [F3], so the value is ∣z∣2. Next, ∥a†ψz∥2=⟨a†ψz,a†ψz⟩=⟨ψz,aa†ψz⟩=⟨ψz,(N+I)ψz⟩=∣z∣2+1 using [F2] and ψz∈D(N). Finally ψ0=g0 since all coefficients with n≥1 vanish and ∥ψ0∥=1.

Depends on

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