[Refact] Factor 2 for Hamevo spectral gap in GPSR (#526)

Co-authored-by: Roland-djee <[email protected]>

qadence/blocks/utils.py

@@ -263,11 +263,23 @@ def expression_to_uuids(block: AbstractBlock) -> dict[Expr, list[str]]:
     return expr_to_uuid
 
 
-def uuid_to_eigen(block: AbstractBlock) -> dict[str, Tensor]:
+def uuid_to_eigen(
+    block: AbstractBlock, rescale_eigenvals_timeevo: bool = False
+) -> dict[str, Tensor]:
     """Creates a mapping between a parametric block's param_id and its' eigenvalues.
 
     This method is needed for constructing the PSR rules for a given block.
 
+    Args:
+        block (AbstractBlock): Block input
+        rescale_eigenvals_timeevo (bool, optional): If True, rescale
+        eigenvalues by 2 for the TimeEvolutionBlock case to allow
+        differientiating with Hamevo.
+        Defaults to False.
+
+    Returns:
+        dict[str, Tensor]: Mapping between block's param_id and eigenvalues.
+
     !!! warn
         Will ignore eigenvalues of AnalogBlocks that are not yet computed.
     """
@@ -278,6 +290,11 @@ def uuid_to_eigen(block: AbstractBlock) -> dict[str, Tensor]:
             if b.eigenvalues_generator.numel() > 0:
                 result[uuid] = b.eigenvalues_generator
 
+                # GPSR assumes a factor 0.5 for differentiation
+                # so need rescaling
+                if isinstance(b, TimeEvolutionBlock) and rescale_eigenvals_timeevo:
+                    result[uuid] *= 2.0
+
                 # leave only angle parameter uuid with eigenvals for ConstantAnalogRotation block
                 if isinstance(block, ConstantAnalogRotation):
                     break

qadence/engines/torch/differentiable_expectation.py

@@ -215,7 +215,7 @@ def construct_rules(
     ) -> dict[str, Callable]:
         """Create a mapping between parameters and PSR functions."""
 
-        uuid_to_eigs = uuid_to_eigen(circuit.block)
+        uuid_to_eigs = uuid_to_eigen(circuit.block, rescale_eigenvals_timeevo=True)
         # We currently rely on implicit ordering to match the PSR to the parameter,
         # because we want to cache PSRs.
 

tests/metrics.py

@@ -16,7 +16,7 @@
 
 JS_ACCEPTANCE = 7.5e-2
 PSR_ACCEPTANCE = 1e-5
-GPSR_ACCEPTANCE = 1e-1
+GPSR_ACCEPTANCE = 1e-2
 ADJOINT_ACCEPTANCE = ATOL_E6
 PULSER_GPSR_ACCEPTANCE = 6.0e-2
 ATOL_DICT = {