Fix CSwap bloq in mod division (#1528)

* fix mod division cswap

* cswapapprox doc - relative phase

Author: anurudhp

qualtran/bloqs/mod_arithmetic/mod_division.py

@@ -38,16 +38,13 @@
 from qualtran.bloqs.arithmetic.bitwise import BitwiseNot, XorK
 from qualtran.bloqs.arithmetic.comparison import LinearDepthHalfGreaterThan
 from qualtran.bloqs.arithmetic.controlled_addition import CAdd
-from qualtran.bloqs.basic_gates import CNOT, TwoBitCSwap, XGate
+from qualtran.bloqs.basic_gates import CNOT, CSwap, TwoBitCSwap, XGate
 from qualtran.bloqs.mcmt import And, MultiAnd
 from qualtran.bloqs.mod_arithmetic.mod_multiplication import ModDbl
-from qualtran.bloqs.swap_network import CSwapApprox
-from qualtran.resource_counting import BloqCountDictT
-from qualtran.resource_counting._call_graph import SympySymbolAllocator
 from qualtran.symbolics import HasLength, is_symbolic
 
 if TYPE_CHECKING:
-    from qualtran.resource_counting import BloqCountDictT
+    from qualtran.resource_counting import BloqCountDictT, SympySymbolAllocator
     from qualtran.simulation.classical_sim import ClassicalValT
     from qualtran.symbolics import SymbolicInt
 
@@ -260,14 +257,12 @@ def on_classical_vals(
     def build_composite_bloq(
         self, bb: 'BloqBuilder', u: Soquet, v: Soquet, r: Soquet, s: Soquet, a: Soquet
     ) -> Dict[str, 'SoquetT']:
-        # CSwapApprox is a CSWAP with a phase flip.
-        # Since we are doing two SWAPs the overal phase is correct.
-        a, u, v = bb.add(CSwapApprox(self.bitsize), ctrl=a, x=u, y=v)
-        a, r, s = bb.add(CSwapApprox(self.bitsize), ctrl=a, x=r, y=s)
+        a, u, v = bb.add(CSwap(self.bitsize), ctrl=a, x=u, y=v)
+        a, r, s = bb.add(CSwap(self.bitsize), ctrl=a, x=r, y=s)
         return {'u': u, 'v': v, 'r': r, 's': s, 'a': a}
 
-    def build_call_graph(self, ssa: SympySymbolAllocator) -> 'BloqCountDictT':
-        return {CSwapApprox(self.bitsize): 2}
+    def build_call_graph(self, ssa: 'SympySymbolAllocator') -> 'BloqCountDictT':
+        return {CSwap(self.bitsize): 2}
 
 
 @frozen
@@ -379,8 +374,8 @@ def build_composite_bloq(
 
         r = bb.add(ModDbl(QMontgomeryUInt(self.bitsize), self.mod), x=r)
 
-        a, u, v = bb.add(CSwapApprox(self.bitsize), ctrl=a, x=u, y=v)
-        a, r, s = bb.add(CSwapApprox(self.bitsize), ctrl=a, x=r, y=s)
+        a, u, v = bb.add(CSwap(self.bitsize), ctrl=a, x=u, y=v)
+        a, r, s = bb.add(CSwap(self.bitsize), ctrl=a, x=r, y=s)
 
         s_arr = bb.split(s)
         s_arr[-1] = bb.add(XGate(), q=s_arr[-1])
@@ -395,7 +390,7 @@ def build_call_graph(self, ssa: 'SympySymbolAllocator') -> 'BloqCountDictT':
             CNOT(): 3,
             XGate(): 2,
             ModDbl(QMontgomeryUInt(self.bitsize), self.mod): 1,
-            CSwapApprox(self.bitsize): 2,
+            CSwap(self.bitsize): 2,
             TwoBitCSwap(): self.bitsize - 1,
         }
 

qualtran/bloqs/mod_arithmetic/mod_division_test.py

@@ -74,10 +74,8 @@ def test_kaliski_mod_bloq_counts(bitsize, mod):
 def test_kaliski_symbolic_cost():
     n, p = sympy.symbols('n p')
     b = KaliskiModInverse(n, p)
-    cost = get_cost_value(b, QECGatesCost()).total_t_and_ccz_count()
-    # We have some T gates since we use CSwapApprox instead of n CSWAPs.
-    total_toff = (cost['n_t'] / 4 + cost['n_ccz']) * sympy.Integer(1)
-    total_toff = total_toff.expand()
+    total_toff = get_cost_value(b, QECGatesCost()).total_toffoli_only()
+    total_toff = sympy.expand(total_toff)
 
     # The toffoli cost from Litinski https://arxiv.org/abs/2306.08585 is 26n^2 + 2n.
     # The cost of Kaliski is 2*n*(cost of an iteration) + (cost of computing $p - x$)

qualtran/bloqs/swap_network/cswap_approx.py

@@ -42,7 +42,7 @@ class CSwapApprox(GateWithRegisters):
     r"""Approximately implements a multi-target controlled swap unitary using only $4n$ T-gates.
 
     Implements $\mathrm{CSWAP}_n = |0 \rangle\langle 0| I + |1 \rangle\langle 1| \mathrm{SWAP}_n$
-    such that the output state is correct up to a global phase factor of +1 / -1.
+    such that the output state is correct up to a relative phase factor of +1/-1 in the standard basis.
 
     This is useful when the incorrect phase can be absorbed in a garbage state of an algorithm
     and thus ignored. See the reference for more details.

qualtran/bloqs/swap_network/swap_network.ipynb

@@ -41,7 +41,7 @@
     "Approximately implements a multi-target controlled swap unitary using only $4n$ T-gates.\n",
     "\n",
     "Implements $\\mathrm{CSWAP}_n = |0 \\rangle\\langle 0| I + |1 \\rangle\\langle 1| \\mathrm{SWAP}_n$\n",
-    "such that the output state is correct up to a global phase factor of +1 / -1.\n",
+    "such that the output state is correct up to a relative phase factor of +1/-1 in the standard basis.\n",
     "\n",
     "This is useful when the incorrect phase can be absorbed in a garbage state of an algorithm\n",
     "and thus ignored. See the reference for more details.\n",