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[feature] Always canonicalise MPS before 2q gate #82

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Apr 3, 2024
Merged

[feature] Always canonicalise MPS before 2q gate #82

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19 changes: 4 additions & 15 deletions pytket/extensions/cutensornet/structured_state/mps_gate.py
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Original file line number Diff line number Diff line change
Expand Up @@ -96,27 +96,16 @@ def _apply_2q_gate(self, positions: tuple[int, int], gate: Op) -> MPSxGate:
l_pos = min(positions)
r_pos = max(positions)

# Always canonicalise. Even in the case of exact simulation (no truncation)
# canonicalisation may reduce the bond dimension (thanks to reduced QR).
self.canonicalise(l_pos, r_pos)

# Figure out the new dimension of the shared virtual bond
new_dim = 2 * min(
self.get_virtual_dimensions(l_pos)[0],
self.get_virtual_dimensions(r_pos)[1],
)

# Canonicalisation may be required if `new_dim` is larger than `chi`
# or if set by `truncation_fidelity`
if new_dim > self._cfg.chi or self._cfg.truncation_fidelity < 1:
# If truncation required, convert to canonical form before
# contracting. Avoids the need to apply gauge transformations
# to the larger tensor resulting from the contraction.
self.canonicalise(l_pos, r_pos)

# Since canonicalisation may change the dimension of the bonds,
# we need to recalculate the value of `new_dim`
new_dim = 2 * min(
self.get_virtual_dimensions(l_pos)[0],
self.get_virtual_dimensions(r_pos)[1],
)

# Load the gate's unitary to the GPU memory
gate_unitary = gate.get_unitary().astype(dtype=self._cfg._complex_t, copy=False)
gate_tensor = cp.asarray(gate_unitary, dtype=self._cfg._complex_t)
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