4 Ts per CSwap (#1642)

Fixes #1463 

 - change default ts per cswap to 4
 - leaves it as inconsistently 7 in `to_legacy_t_complexity` with a note

Author: mpharrigan

qualtran/bloqs/chemistry/df/double_factorization_test.py

@@ -80,8 +80,7 @@ def test_compare_cost_to_openfermion():
     inner_prep_qrom_diff = 8
     prog_rot_qrom_diff = 60
     missing_toffoli = 4  # need one more toffoli for second application of CZ
-    swap_cost = 4 * (7 - 4) * num_spin_orb // 2
-    qual_cost = t_counts - inner_prep_qrom_diff - prog_rot_qrom_diff + missing_toffoli - swap_cost
+    qual_cost = t_counts - inner_prep_qrom_diff - prog_rot_qrom_diff + missing_toffoli
     # correct the expected cost by using a different uniform superposition algorithm
     # see: https://github.com/quantumlib/Qualtran/issues/611
     eta = power_two(num_aux + 1)

qualtran/bloqs/chemistry/hubbard_model/qubitization/select_hubbard_test.py

@@ -35,7 +35,7 @@ def test_select_t_complexity(dim):
     assert cost == GateCounts(
         cswap=2 * logN, and_bloq=5 * (N // 2) - 2, measurement=5 * (N // 2) - 2, clifford=ANY
     )
-    assert cost.total_t_count() == 10 * N + 14 * logN - 8
+    assert cost.total_t_count() == 10 * N + 8 * logN - 8
 
 
 def test_adjoint_controlled():

qualtran/bloqs/chemistry/pbc/first_quantization/projectile/select_and_prepare_test.py

@@ -53,7 +53,7 @@ def test_select_t_costs():
     cost = get_cost_value(sel_first_quant, QECGatesCost()).total_t_count()
 
     # Swaps
-    expected_cost = 7 * (12 * eta * num_bits_p + 6 * num_bits_n) + 4 * (4 * eta - 6)  #
+    expected_cost = 4 * (12 * eta * num_bits_p + 6 * num_bits_n) + 4 * (4 * eta - 6)  #
     # SELT
     expected_cost += 4 * (5 * (num_bits_n - 1) + 2 + 1)
     # SELUV

qualtran/bloqs/chemistry/pbc/first_quantization/select_and_prepare_test.py

@@ -51,7 +51,7 @@ def test_select_t_costs():
     )
     cost += get_cost_value(sel_first_quant, QECGatesCost()).total_t_count()
 
-    expected_cost = 7 * (12 * eta * num_bits_p) + 4 * (4 * eta - 8)
+    expected_cost = 4 * (12 * eta * num_bits_p) + 4 * (4 * eta - 8)
     expected_cost += 4 * (5 * (num_bits_p - 1) + 2)
     expected_cost += 4 * (24 * num_bits_p)
     expected_cost += 4 * (

qualtran/resource_counting/_bloq_counts.py

@@ -186,7 +186,7 @@ def _is_nonzero(v):
     def total_t_count(
         self,
         ts_per_toffoli: int = 4,
-        ts_per_cswap: int = 7,
+        ts_per_cswap: int = 4,
         ts_per_and_bloq: int = 4,
         ts_per_rotation: int = 11,
     ) -> int:
@@ -236,6 +236,12 @@ def to_legacy_t_complexity(
         add per `self.and_bloq`. To fully match the exact legacy `t_complexity` numbers, you
         must enable `QECGatesCost(legacy_shims=True)`, which will enable a shim that directly
         adds on clifford counts for the X-gates used to invert the And control lines.
+
+        **Warning:** The number of Ts per Toffoli defaults to 4, whereas the
+        number of Ts per CSwap defaults to 7. This preserves a legacy inconsistency. Really,
+        these numbers should either both be 7 or both be 4 for a proper accounting.
+        In `total_t_count`, we default to 4 T gates per CSwap or Toffoli. See
+        https://github.com/quantumlib/Qualtran/issues/1463 for more discussion.
         """
         from qualtran.cirq_interop.t_complexity_protocol import TComplexity
 

qualtran/resource_counting/_bloq_counts_test.py

@@ -61,7 +61,7 @@ def test_gate_counts():
     assert GateCounts(t=10) * 2 == GateCounts(t=20)
     assert 2 * GateCounts(t=10) == GateCounts(t=20)
 
-    assert GateCounts(toffoli=1, cswap=1, and_bloq=1).total_t_count() == 4 + 7 + 4
+    assert GateCounts(toffoli=1, cswap=1, and_bloq=1).total_t_count() == 4 + 4 + 4
 
     gc2 = GateCounts(t=sympy.Symbol('n'), toffoli=sympy.sympify('0'), cswap=2)
     assert str(gc2) == 't: n, cswap: 2'

qualtran/resource_counting/classify_bloqs_test.py

@@ -113,7 +113,7 @@ def test_classify_bloq_counts_with_custom_bloq_classification():
     classified_bloqs = classify_t_count_by_bloq_type(
         test_bloq, bloq_classification=bloq_classification
     )
-    assert classified_bloqs == {'swaps': 42 * 10 * 7, 'other': 3 * 4 * (4 - 1)}
+    assert classified_bloqs == {'swaps': 42 * 10 * 4, 'other': 3 * 4 * (4 - 1)}
     assert get_cost_value(test_bloq, QECGatesCost()).total_t_count() == sum(
         classified_bloqs.values()
     )