Weak value is a physical property of a quantum system which manifests itself through a weak measurement using different preand post-selected ensembles of the system. The weak values of an operator may differ significantly from its eigenvalues and can even lie outside the spectrum if it is bound: they can be ‘superweak’. The latter, originating due to a coherent superposition of waves, may appear as a ‘supershift’ on the measuring device. This property has potential application in the amplification and detection of extremely weak signals.Weak value is a physical property of a quantum system which manifests itself through a weak measurement using different preand post-selected ensembles of the system. The weak values of an operator may differ significantly from its eigenvalues and can even lie outside the spectrum if it is bound: they can be ‘superweak’. The latter, originating due to a coherent superposition of waves, may appear as a ‘supershift’ on the measuring device. This property has potential application in the amplification and detection of extremely weak signals.

Current ScienceDec 1, 20150 citations

The Coherent State Projection Eigenvalue Theorem

Peter D. Drummond, Margaret D. Reid

Recent papers have developed a cyclic symmetry projected extension for coherent state representations of bosonic Hilbert space. This extension can greatly reduce sampling errors in phase-space simulations of bosonic quantum systems that have symmetries. Here we prove an eigenvalue theorem for the continuum limit of the projected coherent states. Our intuitively appealing result is that in the continuum limit the projected coherent states are eigenstates of the symmetry generating operator. This result gives a differential identity in phase-space that includes the conserved eigenvalues. Detailed applications of these differential identities to obtaining improved Fokker-Planck and stochastic equations in many-body physics will be given elsewhere.Recent papers have developed a cyclic symmetry projected extension for coherent state representations of bosonic Hilbert space. This extension can greatly reduce sampling errors in phase-space simulations of bosonic quantum systems that have symmetries. Here we prove an eigenvalue theorem for the continuum limit of the projected coherent states. Our intuitively appealing result is that in the continuum limit the projected coherent states are eigenstates of the symmetry generating operator. This result gives a differential identity in phase-space that includes the conserved eigenvalues. Detailed applications of these differential identities to obtaining improved Fokker-Planck and stochastic equations in many-body physics will be given elsewhere.

quant-pharXiv:2610.33613Oct 27, 20260 citations

Isospectral majorization and isoperimetric inequalities for coherent states on the Bloch sphere

Luis Daniel Abreu

Let $\mathcal{P}_{N}$ be the $(N+1)$ -dimensional Hilbert space of analytic polynomials of degree at most $N$ . This is the natural environment to define $SU(2)$ (Bloch) coherent states. Let $Q_{ρ}$ be the Husimi function of a density operator $ρ$ on $% \mathcal{P}_{N}$ . We prove an isospectral version of Lieb-Solovej inequality: if $ρ^{\downarrow }$ is obtained by placing the eigenvalues of $ρ$ in decreasing order along the monomial basis, then \begin{equation*} \int_{\mathbb{C}}Φ(Q_{ρ}(z))\,dm(z)\leq \int_{\mathbb{C}}Φ(Q_{ρ^{\downarrow }}(z))\,dm(z) \end{equation*}% for every convex function $Φ$ on $[0,1]$ . Applying the corresponding reversed inequality to the concave function $Φ(t)=-t\log t$ gives the Wehrl entropy. In the process it is show that the output state of $ρ$ under Lieb-Solovej's channel is majorized by the output of the state $ρ^{\downarrow }$ . As an application, among all measurable subsets of the sphere having a fixed area, spherical caps maximize every partial sum of the eigenvalues of Toeplitz operators with symbol $1_Ω$ . Equivalently, caps maximize all Ky Fan norms, leading to the sharp inequalities, previously known for $r=1$ : \begin{equation*} \sum_{j=1}^{r}λ_{j}(Ω) \leq r-\sum_{k=0}^{r-1}(r-k)\binom{N+1}{k} m(Ω)^{k}\bigl(1-m(Ω)\bigr)^{N+1-k}. \end{equation*} This implies isoperimetric inequalities for all Schatten sums of $T_Ω$ , obtained without using the spherical isoperimetric inequality.Let $\mathcal{P}_{N}$ be the $(N+1)$-dimensional Hilbert space of analytic polynomials of degree at most $N$. This is the natural environment to define $SU(2)$ (Bloch) coherent states. Let $Q_{ρ}$ be the Husimi function of a density operator $ρ$ on $% \mathcal{P}_{N}$. We prove an isospectral version of Lieb-Solovej inequality: if $ρ^{\downarrow }$ is obtained by placing the eigenvalues of $ρ$ in decreasing order along the monomial basis, then \begin{equation*} \int_{\mathbb{C}}Φ(Q_{ρ}(z))\,dm(z)\leq \int_{\mathbb{C}}Φ(Q_{ρ^{\downarrow }}(z))\,dm(z) \end{equation*}% for every convex function $Φ$ on $[0,1]$. Applying the corresponding reversed inequality to the concave function $Φ(t)=-t\log t$ gives the Wehrl entropy. In the process it is show that the output state of $ρ$ under Lieb-Solovej's channel is majorized by the output of the state $ρ^{\downarrow }$. As an application, among all measurable subsets of the sphere having a fixed area, spherical caps maximize every partial sum of the eigenvalues of Toeplitz operators with symbol $1_Ω$. Equivalently, caps maximize all Ky Fan norms, leading to the sharp inequalities, previously known for $r=1$: \begin{equation*} \sum_{j=1}^{r}λ_{j}(Ω) \leq r-\sum_{k=0}^{r-1}(r-k)\binom{N+1}{k} m(Ω)^{k}\bigl(1-m(Ω)\bigr)^{N+1-k}. \end{equation*} This implies isoperimetric inequalities for all Schatten sums of $T_Ω$, obtained without using the spherical isoperimetric inequality.

quant-pharXiv:2608.12248Aug 13, 20260 citations

Coherence by measurement

Gregor Weihs

Nature PhysicsOct 1, 20071 citation

Higher-order coherence functions

Ravinder Nath

Lettere al nuovo cimento della societa italiana di fisica/Lettere al nuovo cimentoNov 1, 19784 citations

Let

\mathcal{P}_{N}

be the

(N+1)

-dimensional Hilbert space of analytic polynomials of degree at most

N

. This is the natural environment to define

SU(2)

(Bloch) coherent states. Let

Q_{ρ}

be the Husimi function of a density operator

ρ

% \mathcal{P}_{N}

. We prove an isospectral version of Lieb-Solovej inequality: if

ρ^{\downarrow }

is obtained by placing the eigenvalues of

ρ

in decreasing order along the monomial basis, then \begin{equation*} \int_{\mathbb{C}}Φ(Q_{ρ}(z))\,dm(z)\leq \int_{\mathbb{C}}Φ(Q_{ρ^{\downarrow }}(z))\,dm(z) \end{equation*}% for every convex function

Φ

[0,1]

. Applying the corresponding reversed inequality to the concave function

Φ(t)=-t\log t

gives the Wehrl entropy. In the process it is show that the output state of

ρ

under Lieb-Solovej's channel is majorized by the output of the state

ρ^{\downarrow }

. As an application, among all measurable subsets of the sphere having a fixed area, spherical caps maximize every partial sum of the eigenvalues of Toeplitz operators with symbol

1_Ω

. Equivalently, caps maximize all Ky Fan norms, leading to the sharp inequalities, previously known for

r=1

: \begin{equation*} \sum_{j=1}^{r}λ_{j}(Ω) \leq r-\sum_{k=0}^{r-1}(r-k)\binom{N+1}{k} m(Ω)^{k}\bigl(1-m(Ω)\bigr)^{N+1-k}. \end{equation*} This implies isoperimetric inequalities for all Schatten sums of

T_Ω

, obtained without using the spherical isoperimetric inequality.