From 41a4191aeead222cfe89de8ab595617fdea3524a Mon Sep 17 00:00:00 2001
From: Ian Jauslin <ian@jauslin.org>
Date: Sun, 20 Feb 2022 19:24:31 -0500
Subject: As presented at the University of Milan on 2022-02-21

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+\documentclass{ian-presentation}
+
+\usepackage[hidelinks]{hyperref}
+\usepackage{graphicx}
+\usepackage{array}
+\usepackage{xcolor}
+
+
+\definecolor{ipurple}{HTML}{4B0082}
+\definecolor{iyellow}{HTML}{DAA520}
+\definecolor{igreen}{HTML}{32CD32}
+\definecolor{iblue}{HTML}{4169E1}
+\definecolor{ired}{HTML}{DC143C}
+
+\definecolor{highlight}{HTML}{328932}
+\definecolor{highlight}{HTML}{981414}
+
+\begin{document}
+\pagestyle{empty}
+\hbox{}\vfil
+\bf\Large
+\hfil An effective equation to study Bose gases\par
+\smallskip
+\hfil at all densities\par
+\vfil
+\large
+\hfil Ian Jauslin
+\normalsize
+\vfil
+\hfil\rm joint with {\bf Eric A. Carlen}, {\bf Markus Holzmann}, {\bf Elliott H. Lieb}\par
+\vfil
+arXiv:{\tt\ \parbox[b]{3cm}{
+  \href{https://arxiv.org/abs/1912.04987}{1912.04987}\par
+  \href{https://arxiv.org/abs/2010.13882}{2010.13882}\par
+  \href{https://arxiv.org/abs/2011.10869}{2011.10869}\par
+  \href{https://arxiv.org/abs/2202.07637}{2202.07637}
+}}
+\hfill{\tt \href{http://ian.jauslin.org}{http://ian.jauslin.org}}
+\eject
+
+\setcounter{page}1
+\pagestyle{plain}
+
+\title{Bose-Einstein condensation}
+\begin{itemize}
+\item System of many Bosons, e.g. {\color{highlight}Helium} atoms, {\color{highlight}Rubidium} atoms, etc...
+\item {\color{highlight}Bose-Einstein condensate}: most particles are in the same quantum state.
+\item Related to the phenomena of {\color{highlight}superfluidity} (flow with zero viscocity) and {\color{highlight}superconductivity} (currents with zero resistance).
+\item Predicted theoretically in {\color{highlight}1924-1925}, experimentally observed in {\color{highlight}1995}.
+\item Mathematical understanding: still {\color{highlight}no proof} of the existence of a condensate (at finite density, in the presence of interactions and in the continuum).
+\end{itemize}
+\vfill
+\eject
+
+\title{Repulsive Bose gas}
+\begin{itemize}
+  \item Potential: {\color{highlight}$v(r)\geqslant 0$} and {\color{highlight}$v\in L_1(\mathbb R^3)$}, Hamiltonian:
+  $$
+    H_N:=
+    -\frac12\sum_{i=1}^N\Delta_i
+    +\sum_{1\leqslant i<j\leqslant N}v(|x_i-x_j|)
+  $$
+  \vskip-5pt
+  \item Ground state: $\psi_0$, energy $E_0$.
+  \item Observables in the {\color{highlight}thermodynamic limit}: ground state energy per particle and condensate fraction: $P_i$: projection onto condensate state
+  $$
+    e_0:=\lim_{\displaystyle\mathop{\scriptstyle V,N\to\infty}_{{\color{highlight}\frac NV=\rho}}}\frac{E_0}N
+    ,\quad
+    \eta_0:=\lim_{\displaystyle\mathop{\scriptstyle V,N\to\infty}_{{\color{highlight}\frac NV=\rho}}}\frac1N\sum_{i=1}^N\left<\psi_0\right|P_i\left|\psi_0\right>
+    .
+  $$
+\end{itemize}
+\vfill
+\eject
+
+\title{Low density}
+\begin{itemize}
+  \item Bogolyubov theory: {\color{highlight}approximation scheme} that reduces the problem to an effective {\color{highlight}1-particle problem}.
+  \item Predictions \href{https://doi.org/10.1103/PhysRev.106.1135}{[Lee, Huang, Yang, 1957]}:
+  \begin{itemize}
+    \item Energy:
+    $$
+      {\color{highlight}e_0=2\pi\rho a\left(1+\frac{128}{15\sqrt\pi}\sqrt{\rho a^3}+o(\sqrt\rho)\right)}
+    $$
+    \vskip-10pt
+    \item Condensate fraction:
+    $$
+      {\color{highlight}1-\eta_0\sim\frac{8\sqrt{\rho a^3}}{3\sqrt\pi}}
+    $$
+  \end{itemize}
+\end{itemize}
+\vfill
+\eject
+
+\title{Low density}
+\begin{itemize}
+  \item Energy asymptotics: {\color{highlight} proved}:
+  \href{https://doi.org/10.1103/PhysRevLett.80.2504}{[Lieb, Yngvason, 1998]},
+  \href{https://doi.org/10.1007/s10955-009-9792-3}{[Yau, Yin, 2009]},
+  \href{https://doi.org/10.4007/annals.2020.192.3.5}{[Fournais, Solovej, 2020]}.
+
+  \item Condensate fraction: {\color{highlight}still open} in the theormodynamic limit, but there are proofs of condensation in the Gross-Pitaevskii regime (ultra-dilute):
+  \href{https://doi.org/10.1103/PhysRevLett.88.170409}{[Lieb, Seiringer, 2002]},
+  \href{https://doi.org/10.1007/s00220-017-3016-5}{[Boccato, Brennecke, Cenatiempo, Schlein, 2018]}.
+
+\end{itemize}
+\vfill
+\eject
+
+\title{High density}
+\begin{itemize}
+  \item [Bogolyubov, 1947]: if $\hat v\geqslant 0$.
+  $$
+    {\color{highlight}e_0\sim\frac\rho2\int v}
+  $$
+  {\color{highlight}Hartree} (mean field) energy.
+  \item {\color{highlight}Proved} in \href{https://doi.org/10.1103/PhysRev.130.2518}{[Lieb, 1963]}.
+  \item Condensate fraction
+  $$
+    \eta\to1
+  $$
+  {\color{highlight}open}.
+\end{itemize}
+\vfill
+\eject
+
+\title{Energy as a function of density}
+For $v(x)=e^{-|x|}$:
+
+\hfil\includegraphics[height=5.5cm]{erho_lowhigh.pdf}
+\vfill
+\eject
+
+\addtocounter{page}{-1}
+\title{Energy as a function of density}
+For $v(x)=e^{-|x|}$:
+
+\hfil\includegraphics[height=5.5cm]{erho_effective.pdf}
+\vfill
+\eject
+
+\title{Derivation of the equation}
+\begin{itemize}
+  \item \href{https://doi.org/10.1103/PhysRev.130.2518}{[Lieb, 1963]}.
+  \item Integrate $H_N\psi_0=E_0\psi_0$:
+  $$
+    \int dx_1\cdots dx_N\ 
+    \left(
+      -\frac12\sum_{i=1}^N\Delta_i\psi_0
+      +\sum_{1\leqslant i<j\leqslant N} v(x_i-x_j)\psi_0
+    \right)
+    =E_0\int dx_1\cdots dx_N\ \psi_0
+  $$
+  \item Therefore,
+  $$
+    \frac{N(N-1)}2\int dx_1dx_2\ v(x_1-x_2)\frac{\int dx_3\cdots dx_N\ \psi_0}{\int dx_1\cdots dx_N\ \psi_0}
+    =E_0
+  $$
+\end{itemize}
+\vfill
+\eject
+
+\title{Derivation of the equation}
+\begin{itemize}
+  \item Thus,
+  $$
+    \frac{E_0}N=\frac{N-1}{2V}\int dx\ v(x)g_2(0,x)
+  $$
+  \item {\color{highlight}$\psi_0\geqslant 0$}, so it can be thought of as a probability distribution.
+  \item $g_n$: {\color{highlight}correlation functions} of $V^{-N}\psi_0$
+  $$
+    g_n(x_1,\cdots,x_n):=\frac{V^n\int dx_{n+1}\cdots dx_N\ \psi_0(x_1,\cdots,x_N)}{\int dx_1\cdots dx_N\ \psi_0(x_1,\cdots,x_N)}
+  $$
+\end{itemize}
+\vfill
+\eject
+
+\title{Hierarchy}
+\vskip-10pt
+\begin{itemize}
+  \item Equation for $g_2$: integrate $H_N\psi_0=E_0\psi_0$ with respect to $x_3,\cdots,x_N$:
+  $$
+    \begin{array}{>\displaystyle l}
+      -\frac12(\Delta_x+\Delta_y) g_2(x,y)
+      +\frac{N-2}V\int dz\ (v(x-z)+v(y-z))g_3(x,y,z)
+      \\[0.5cm]\hfill
+      +v(x-y)g_2(x,y)
+      +\frac{(N-2)(N-3)}{2V^2}\int dzdt\ v(z-t)g_4(x,y,z,t)
+      =E_0g_2(x,y)
+    \end{array}
+  $$
+  \item Factorization {\color{highlight}assumption}:
+  $$
+    g_3(x_1,x_2,x_3)=g_2(x_1,x_2)g_2(x_1,x_3)g_2(x_2,x_3)
+  $$
+  $$
+    g_4(x_1,x_2,x_3,x_4)=\prod_{i<j}(g_2(x_i,x_j)+O(V^{-1}))
+  $$
+\end{itemize}
+\vfill
+\eject
+
+\title{{\color{ipurple}Big equation}}
+\begin{itemize}
+\item In the thermodynamic limit, if ${\color{highlight}u(x):=1-g_2(0,x)}$,
+  $$
+    -\Delta u(x)
+    =
+    (1-u(x))\left(v(x)-2\rho K(x)+\rho^2 L(x)\right)
+  $$
+  $$
+    K:=
+    u\ast S
+    ,\quad
+    S(y):=(1-u(y))v(y)
+  $$
+  $$
+    L:=
+    u\ast u\ast S
+    -2u\ast(u(u\ast S))
+    +\frac12
+    \int dydz\ u(y)u(z-x)u(z)u(y-x)S(z-y)
+    .
+  $$
+
+  \item {\color{ipurple}``Big'' equation}:
+  $$
+    L\approx
+    u\ast u\ast S
+    -2u\ast (u(u\ast S))
+  $$
+\end{itemize}
+\vfill
+\eject
+  
+\title{{\color{iblue}Simple equation}}
+\vskip-10pt
+\begin{itemize}
+  \item Further approximate $S(x)\approx\frac{2e}\rho\delta(x)$ and $u\ll 1$.
+  \item Simple equation
+  $$
+    {\color{iblue}-\Delta u(x)=(1-u(x))v(x)- 4eu(x)+2e\rho\ u\ast u(x)}
+  $$
+  $$
+    {\color{iblue}e=\frac\rho2\int dx\ (1-u(x))v(x)}
+  $$
+  \item {\bf Theorem 1}:
+  If $v(x)\geqslant 0$ and $v\in L_1\cap L_2(\mathbb R^3)$, then the {\color{iblue}simple equation} has an {\color{highlight}integrable solution} (proved constructively), with $0\leqslant u\leqslant 1$.
+\end{itemize}
+\vfill
+\eject
+
+\title{Energy for the {\color{iblue}simple equation}}
+\vskip-10pt
+\begin{itemize}
+  \item {\bf Theorem 2}:
+  $$
+    \frac{e}{\rho}\mathop{\longrightarrow}_{\rho\to\infty}\frac12\int dx\ v(x)
+  $$
+  (note that there is no condition that $\hat v\geqslant 0$).
+  This coincides with the {\color{highlight}Hartree energy}.
+  \item {\bf Theorem 3}:
+  $$
+    e=2\pi\rho a\left(1+\frac{128}{15\sqrt\pi}\sqrt{\rho a^3}+o(\sqrt\rho)\right)
+  $$
+  This coincides with the {\color{highlight}Lee-Huang-Yang prediction}.
+\end{itemize}
+\vfill
+\eject
+
+\title{Energy}
+$v(x)=e^{-|x|}$, Blue: {\color{iblue}simple equation}; purple: {\color{ipurple}big equation}; red: {\color{ired}Monte Carlo}
+
+\hfil\includegraphics[height=5.5cm]{erho_fulleq.pdf}
+\vfill
+\eject
+
+\title{Energy}
+$v(x)=e^{-|x|}$, Blue: {\color{iblue}simple equation}; red: {\color{ired}Jastrow}; purple: {\color{ipurple}big equation}
+
+\hfil\includegraphics[height=5.5cm]{cmp_energy1.pdf}
+\vfill
+\eject
+
+\title{Condensate fraction}
+\begin{itemize}
+  \item Add a parameter $\mu$ to the Hamiltonian:
+  $$
+    H_N(\mu):=
+    -\frac12\sum_{i=1}^N\Delta_i
+    +\sum_{1\leqslant i<j\leqslant N}v(x_i-x_j)
+    {\color{highlight}-\mu\sum_{i=1}^NP_i}
+  $$
+  \item Projection onto condensate wavefunction: $P_i$.
+  \item Condensate fraction:
+  $$
+    {\color{highlight}\eta_0}:=\frac1N\left<\psi_0\right|\sum_{i=1}^NP_i\left|\psi_0\right>
+    =-\frac1N\partial_\mu \left<\psi_0\right|H_N(\mu)\left|\psi_0\right>|_{\mu_0}
+    \equiv
+    {\color{highlight}-\partial_\mu e_0(\mu)|_{\mu=0}}
+  $$
+\end{itemize}
+\vfill
+\eject
+
+\title{Condensate fraction}
+\begin{itemize}
+  \item {\bf Theorem 4}:
+  For the {\color{iblue}simple equation}, as $\rho\to0$
+  $$
+    1-\eta\sim\frac{8\sqrt{\rho a^3}}{3\sqrt\pi}
+  $$
+  which coincides with {\color{highlight}Bogolyubov's prediction}.
+\end{itemize}
+\vfill
+\eject
+
+\title{Condensate fraction}
+$v(x)=e^{-|x|}$, Blue: {\color{iblue}simple equation}; purple: {\color{ipurple}big equation}; red: {\color{ired}Monte Carlo}
+
+\hfil\includegraphics[height=5.5cm]{condensate.pdf}
+\vfill
+\eject
+
+\title{Two point correlation function}
+$v(x)=16e^{-|x|}$, Blue: {\color{iblue}simple equation}; purple: {\color{ipurple}big equation}; red: {\color{ired}Monte Carlo}
+
+\hfil\includegraphics[height=5.5cm]{correlation.pdf}
+\vfill
+\eject
+
+\title{Summary and outlook}
+\begin{itemize}
+  \item Two {\color{highlight}effective equations}: the {\color{ipurple}big equation} and the {\color{iblue}simple equation}, which are {\color{highlight}non-linear 1-particle equations}.
+  \item Reproduce the known results for both {\color{highlight}small and large densities}.
+  \item Their derivation is {\color{highlight}different from Bogolyubov theory}, so they may give new insights onto studying the Bose gas in these asymptotic regimes.
+  \item The {\color{ipurple}big equation} is {\color{highlight}quantitatively accurate} at intermediate densities.
+  \item This opens up the possibility of studying the physics of the {\color{highlight}Bose gas at intermediate densities}.
+\end{itemize}
+\vfill
+\eject
+
+\title{Open problems}
+\vfill
+\begin{itemize}
+  \item {\color{highlight}Uniqueness} of the solution of the {\color{iblue}simple equation} (done for small and large $\rho$).
+  \vfill
+
+  \item LHY as an {\color{highlight}upper bound} at low density using the {\color{iblue}simple equation} to construct an Ansatz.
+  \vfill
+
+  \item {\color{highlight}Existence} (and uniqueness) of the solution of the {\color{ipurple}big equation}.
+
+\end{itemize}
+\vfill
+\eject
+
+\title{The uniqueness problem}
+  $$
+    -\Delta u(x)=(1-u(x))v(x)- 4eu(x)+2e\rho\ u\ast u(x)
+    ,\quad
+    e=\frac\rho2\int dx\ (1-u(x))v(x)
+  $$
+\begin{itemize}
+  \item Change the point of view: {\color{highlight}fix $e>0$}, and compute $\rho$ and $u$.
+
+  \item {\color{highlight}Iteration}: $u_0=0$,
+  $$
+    (-\Delta+4e+v)u_n=v+2e\rho_{n-1}u_{n-1}\ast u_{n-1}
+    ,\quad
+    \rho_n:=\frac{2e}{\int dx\ (1-u_n(x))v(x)}
+    .
+  $$
+\end{itemize}
+\vfill
+\eject
+
+\title{The uniqueness problem}
+\begin{itemize}
+  \item {\bf Lemma}: $u_n(x)$ is an {\color{highlight}increasing} sequence, and is {\color{highlight}bounded} $u_n(x)\leqslant 1$.
+  It converges to a function $u$, which is the {\color{highlight}unique} integrable solution of the equation {\color{highlight}with $e$ fixed}.
+
+  \item {\bf Lemma}: $e\mapsto\rho(e)$ is {\color{highlight}continuous}, and $\rho(0)=0$ and $\rho(\infty)=\infty$, which allows us to  compute solutions for the problem at fixed $\rho$.
+
+  \item We thus have a {\color{highlight}restricted} notion of uniqueness.
+  The full uniqueness would follow from a proof that $e\mapsto\rho(r)$ is {\color{highlight}monotone increasing} (which must be true for the physics to make sense).
+\end{itemize}
+\vfill
+\eject
+
+\title{Upper bound at low density}
+\begin{itemize}
+  \item \href{https://doi.org/10.1007/s10955-009-9792-3}{[Yau, Yin, 2009]}: proof for {\color{highlight}weak, smooth}, rapidly decaying potentials.
+  \item \href{https://arxiv.org/abs/2101.06222}{[Basti, Cenatiempo, Schlein, 2021]}: extended for {\color{highlight}$L_3$} (and compactly supported) potentials (excludes hard-core interactions).
+  \item {\color{iblue}Simple Equation}: our analysis holds for the hard-core, so if one could find a good {\color{highlight}Ansatz} from it, one might get an upper bound for the energy in this case.
+  \item Idea for Ansatz? (Jastrow, Dyson-Jastrow?).
+\end{itemize}
+\vfill
+\eject
+
+\title{Upper bound at low density: Jastrow function}
+\begin{itemize}
+  \item Idea:
+  $$
+    \psi=\prod_{i<j}e^{-u(x_i-x_j)}
+  $$
+
+  \item Why this: $\rho\ll1$, and {\color{highlight}if $\rho\|u\|_1\ll1$},
+  $$
+    g_2\sim1-u
+    .
+  $$
+
+  \item Again, {\color{highlight}if $\rho\|u\|_1\ll1$}, we would be able to compute the energy of $\psi$ using a cluster expansion!
+
+  \item However, {\color{highlight}$\|u\|_1=\frac1\rho$}!
+\end{itemize}
+\vfill
+\eject
+
+\title{Existence for the {\color{ipurple}Big Equation}}
+\begin{itemize}
+  \item Numerical method: {\color{highlight}Newton algorithm}.
+
+  \item For the existence of a solution, it would suffice to prove that the Newton algorithm has a {\color{highlight}Basin of attraction}. (Kantorovich-like theorem?)
+
+  \item Such a result, applied to the {\color{iblue}Simple Equation}, would imply the {\color{highlight}uniqueness} of a solution (provided we have convergence in an appropriate norm).
+\end{itemize}
+
+\end{document}
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