From 6a9c864d166daf2965454b213ff0bdb5db3aeede Mon Sep 17 00:00:00 2001
From: Haggai Landa <haggai.landa@ibm.com>
Date: Sun, 26 May 2024 18:44:07 +0300
Subject: [PATCH] Added placeholders for a bit flip Lindbladian term

---
 src/Pauli.cc      | 16 ++++++++++++++--
 src/Pauli.h       |  3 ++-
 src/lindbladian.h |  8 ++++++--
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/src/Pauli.cc b/src/Pauli.cc
index a18e390..b146150 100644
--- a/src/Pauli.cc
+++ b/src/Pauli.cc
@@ -141,6 +141,10 @@ ITensor PauliSite::op(const string &opname, const Args &args) const
     Op.set(ud_, ud, -1.0);
     Op.set(uu_, uu, +1.0);
   }
+  else if (opname == "Sx_Sx")
+  {
+    Error("Operator " + opname + " missing implementation.");
+  }
   else if (opname == "_S-S+")
   {
     Op.set(dd_, uu, 1.0);
@@ -231,7 +235,7 @@ ITensor PauliSite::op(const string &opname, const Args &args) const
   }
   else
   {
-    Error("Operator " + opname + " name not recognized");
+    Error("Operator " + opname + " name not recognized.");
   }
   return Op;
 }
@@ -603,7 +607,8 @@ Cplx SpinHalfSystem::Expect(const vector <string>& opnames, const vector <int>&
   return (re);
 }
 
-void SpinHalfSystem::AddSingleSpinBath(double GammaPlus, double GammaMinus, double GammaDephasing, int site)
+void SpinHalfSystem::AddSingleSpinBath(double GammaPlus, double GammaMinus, double GammaDephasing,
+    double GammaBitFlip, int site)
 {
   if (GammaMinus)
   {
@@ -626,6 +631,13 @@ void SpinHalfSystem::AddSingleSpinBath(double GammaPlus, double GammaMinus, doub
       Lindbladian += -z * 0.5, "Id", site;
       Lindbladian += -z * 0.5, "Id", site;
   }
+  if (GammaBitFlip)
+  {
+      Cplx z = GammaBitFlip * Cplx_i;
+      Lindbladian += z, "Sx_Sx", site;
+      Lindbladian += -z * 0.5, "Id", site;
+      Lindbladian += -z * 0.5, "Id", site;
+  }
 }
 //_____________________________________________________
 
diff --git a/src/Pauli.h b/src/Pauli.h
index baafd82..84bcd7d 100644
--- a/src/Pauli.h
+++ b/src/Pauli.h
@@ -62,7 +62,8 @@ class SpinHalfSystem {
   AutoMPO Lindbladian;
 
   //Adds a single-spin term to the Lindbladian above
-  void AddSingleSpinBath(double GammaPlus, double GammaMinus, double GammaDephasing, int site);
+  void AddSingleSpinBath(double GammaPlus, double GammaMinus, double GammaDephasing,
+    double GammaBitFlip, int site);
   
   //Trace of rho
   complex<double> trace_rho() const;
diff --git a/src/lindbladian.h b/src/lindbladian.h
index 50ecdb7..165245e 100644
--- a/src/lindbladian.h
+++ b/src/lindbladian.h
@@ -164,13 +164,17 @@ bool SetLindbladian(SpinHalfSystem &C, ModelParameters param, Lattice2d L)
     if (g_2_len == 1)
         g_2 = vector<double>(N, g_2[0]);
 
+    g_3 = vector<double>(N, 0.);
+    // TODO: support parameter input
+
     for (int i = 1; i <= int(N); i++)
-        if (g_0[i - 1] || g_1[i - 1] || g_2[i - 1])
+        if (g_0[i - 1] || g_1[i - 1] || g_2[i - 1] || g_3[i - 1])
         {
-            C.AddSingleSpinBath(g_0[i - 1], g_1[i - 1], g_2[i - 1], i);
+            C.AddSingleSpinBath(g_0[i - 1], g_1[i - 1], g_2[i - 1], g_3[i - 1], i);
             // The first argument is the rate of dissipative processes where a spin goes from down to up
             // The second argument is the rate of dissipative processes where a spin goes from up to down
             // The third argument is the rate of energy-conserving (pure) dephasing processes
+            // The fourth argument is the rate of bit flip processes
             b_time_evolution = true;
         }
     return b_time_evolution;