Correct XY Hamiltonian interaction term (#744)

* Correct XY interaction term calculation

* Adapt tutorial

Author: HGSilveri

pulser-simulation/pulser_simulation/hamiltonian.py

@@ -457,22 +457,20 @@ def make_xy_term(q1: QubitId, q2: QubitId) -> qutip.Qobj:
             The units are given so that the coefficient
             includes a 1/hbar factor.
             """
-            dist = np.linalg.norm(self._qdict[q1] - self._qdict[q2])
-            coords_dim = len(self._qdict[q1])
-            mag_field = cast(np.ndarray, self.samples_obj._magnetic_field)[
-                :coords_dim
-            ]
+            diff_vector = np.zeros(3, dtype=float)
+            diff_vector[: len(self._qdict[q1])] = (
+                self._qdict[q1] - self._qdict[q2]
+            )
+            dist = np.linalg.norm(diff_vector)
+            mag_field = cast(np.ndarray, self.samples_obj._magnetic_field)
             mag_norm = np.linalg.norm(mag_field)
-            if mag_norm < 1e-8:
-                cosine = 0.0
-            else:
-                cosine = np.dot(
-                    (self._qdict[q1] - self._qdict[q2]),
-                    mag_field,
-                ) / (dist * mag_norm)
+            assert mag_norm > 0, "There must be a magnetic field in XY mode."
+            cosine = np.dot(
+                diff_vector,
+                mag_field,
+            ) / (dist * mag_norm)
             U = (
-                0.5
-                * cast(float, self._device.interaction_coeff_xy)
+                cast(float, self._device.interaction_coeff_xy)
                 * (1 - 3 * cosine**2)
                 / dist**3
             )

tests/test_simulation.py

@@ -1189,11 +1189,10 @@ def test_get_xy_hamiltonian():
         simple_sim.get_hamiltonian(-10)
     # Constant detuning, so |ud><du| term is C_3/r^3 - 2*detuning for any time
     simple_ham = simple_sim.get_hamiltonian(143)
-    assert simple_ham[1, 2] == 0.5 * MockDevice.interaction_coeff_xy / 10**3
+    assert simple_ham[1, 2] == MockDevice.interaction_coeff_xy / 10**3
     assert (
         np.abs(
-            simple_ham[1, 4]
-            - (-2 * 0.5 * MockDevice.interaction_coeff_xy / 10**3)
+            simple_ham[1, 4] - (-2 * MockDevice.interaction_coeff_xy / 10**3)
         )
         < 1e-10
     )
@@ -1236,10 +1235,10 @@ def test_run_xy():
     assert sim.samples_obj._measurement == "XY"
 
 
-res1 = {"0000": 892, "1000": 47, "0100": 25, "0001": 19, "0010": 17}
-res2 = {"0000": 962, "0010": 13, "1000": 13, "0100": 12}
-res3 = {"0000": 904, "0100": 43, "0010": 24, "1000": 19, "0001": 10}
-res4 = {"0000": 969, "0001": 18, "1000": 13}
+res1 = {"0000": 950, "0100": 19, "0001": 21, "0010": 10}
+res2 = {"0000": 944, "0010": 15, "1000": 33, "0100": 8}
+res3 = {"0000": 950, "0100": 19, "0010": 10, "0001": 21}
+res4 = {"0000": 951, "0100": 19, "1000": 30}
 
 
 @pytest.mark.parametrize(