override FastDataBlock.n_logical_qubits (#896)

* override FastDataBlock.n_logical_qubits

* lint

---------

Co-authored-by: Matthew Harrigan <mpharrigan@google.com>

qualtran/surface_code/data_block.py

@@ -81,8 +81,9 @@ def footprint(self, n_algo_qubits: int) -> int:
 
     def data_error(self, n_algo_qubits: int, n_cycles: int, phys_err: float) -> float:
         """The error associated with storing data on `n_algo_qubits` for `n_cycles`."""
-        data_cells = self.n_logical_qubits(n_algo_qubits) * n_cycles
-        return data_cells * self.qec_scheme.logical_error_rate(
+        # spacetime_volue = number of data cells x number of cycles they will live for.
+        spacetime_volume = self.n_logical_qubits(n_algo_qubits) * n_cycles
+        return spacetime_volume * self.qec_scheme.logical_error_rate(
             physical_error_rate=phys_err, code_distance=self.data_d
         )
 
@@ -177,11 +178,16 @@ def footprint(self, n_algo_qubits: int) -> int:
         return FastDataBlock.grid_size(n_algo_qubits)
 
     def data_error(self, n_algo_qubits: int, n_cycles: int, phys_err: float) -> float:
-        data_cells = self.n_logical_qubits(n_algo_qubits) * n_cycles
-        return data_cells * self.qec_scheme.logical_error_rate(
+        """The error associated with storing data on `n_algo_qubits` for `n_cycles`."""
+        # spacetime_volue = number of data cells x number of cycles they will live for.
+        spacetime_volume = self.n_logical_qubits(n_algo_qubits) * n_cycles
+        return spacetime_volume * self.qec_scheme.logical_error_rate(
             physical_error_rate=phys_err, code_distance=self.data_d
         )
 
+    def n_logical_qubits(self, n_algo_qubits: int) -> int:
+        return FastDataBlock.grid_size(n_algo_qubits)
+
     def n_cycles_to_consume_a_magic_state(self) -> int:
         return self.data_d
 

qualtran/surface_code/data_block_test.py

@@ -19,7 +19,11 @@
 
 
 @pytest.mark.parametrize(
-    ["logical_qubits", "logical_qubits_with_routing"], [[100, 230], [1318, 2740], [12581, 25481]]
+    ["logical_qubits", "logical_qubits_with_routing", "data_error"],
+    [[100, 230, 0.69], [1318, 2740, 8.22], [12581, 25481, 76.443]],
 )
-def test_fast_block(logical_qubits, logical_qubits_with_routing):
+def test_fast_block(logical_qubits, logical_qubits_with_routing, data_error):
     assert FastDataBlock.grid_size(n_algo_qubits=logical_qubits) == logical_qubits_with_routing
+    assert FastDataBlock(3).data_error(
+        n_algo_qubits=logical_qubits, n_cycles=3, phys_err=1e-3
+    ) == pytest.approx(data_error)