Code circuit builder -> integrate stim

src/qiskit_qec/circuits/css_code.py

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+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2019.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+# pylint: disable=invalid-name
+
+"""Generates circuits for CSS codes."""
+from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
+from qiskit_aer.noise import depolarizing_error, pauli_error
+
+from qiskit_qec.circuits.code_circuit import CodeCircuit
+
+
+class CssCodeCircuit(CodeCircuit):
+    """
+    CodeCircuit class for generic CSS codes.
+    """
+
+    def __init__(self, code, T, basis="z", round_schedule="zx", p_depol=0, p_meas=0):
+        """
+        Args:
+            code: A CSS code
+            T: Number of syndrome measurement rounds
+            basis: basis for encoding ('x' or 'z')
+            round_schedule: Order in which to measureme gauge operators ('zx' or 'xz')
+            p_depol: Probabity of depolarizing noise on code qubits between rounds
+            p_meas: Probability of measurement errors
+        """
+
+        super().__init__()
+
+        self.code = code
+        self.T = T
+        self.basis = basis
+        self.base = "0"
+        self.round_schedule = round_schedule
+        self.p_depol = p_depol
+        self.p_meas = p_meas
+        self._noise = p_depol > 0 or p_meas > 0
+
+        self._depol_error = depolarizing_error(p_depol, 1)
+        self._meas_error = pauli_error([("X", p_meas), ("I", 1 - p_meas)])
+
+        self.circuit = {}
+        for state in ["0", "1"]:
+            qc = QuantumCircuit()
+            qregs = []
+            qregs.append(QuantumRegister(code.n, name="code qubits"))
+            qregs.append(QuantumRegister(len(code.z_gauges), name="z auxs"))
+            qregs.append(QuantumRegister(len(code.x_gauges), name="x auxs"))
+            for qreg in qregs:
+                qc.add_register(qreg)
+            # prepare initial state
+            if state == "1":
+                if basis == "z":
+                    qc.x(code.logical_x[0])
+                else:
+                    qc.x(code.logical_z[0])
+            if basis == "x":
+                qc.h(qregs[0])
+            # peform syndrome measurements
+            for t in range(T):
+                if self._noise:
+                    for q in qregs[0]:
+                        qc.append(self._depol_error, [q])
+                # gauge measurements
+                if round_schedule == "zx":
+                    self._z_gauge_measurements(qc, t)
+                    self._x_gauge_measurements(qc, t)
+                elif round_schedule == "xz":
+                    self._x_gauge_measurements(qc, t)
+                    self._z_gauge_measurements(qc, t)
+                else:
+                    print("Round schedule " + round_schedule + " not supported.")
+            # final readout
+            creg = ClassicalRegister(code.n, name="final_readout")
+            qc.add_register(creg)
+            if basis == "x":
+                qc.h(qregs[0])
+            if self._noise:
+                for q in qregs[0]:
+                    qc.append(self._meas_error, [q])
+            qc.measure(qregs[0], creg)
+            self.circuit[state] = qc
+
+        self._gauges4stabilizers = []
+        self._stabilizers = [code.x_stabilizers, code.z_stabilizers]
+        self._gauges = [code.x_gauges, code.z_gauges]
+        for j in range(2):
+            self._gauges4stabilizers.append([])
+            for stabilizer in self._stabilizers[j]:
+                gauges = []
+                for g, gauge in enumerate(self._gauges[j]):
+                    if set(stabilizer).intersection(set(gauge)) == set(gauge):
+                        gauges.append(g)
+                self._gauges4stabilizers[j].append(gauges)
+
+    def _z_gauge_measurements(self, qc, t):
+        creg = ClassicalRegister(len(self.code.z_gauges), name="round_" + str(t) + "_z_bits")
+        qc.add_register(creg)
+        for g, z_gauge in enumerate(self.code.z_gauges):
+            for q in z_gauge:
+                qc.cx(qc.qregs[0][q], qc.qregs[1][g])
+            if self._noise:
+                qc.append(self._meas_error, [qc.qregs[1][g]])
+            qc.measure(qc.qregs[1][g], creg[g])
+            qc.reset(qc.qregs[1][g])
+
+    def _x_gauge_measurements(self, qc, t):
+        creg = ClassicalRegister(len(self.code.x_gauges), name="round_" + str(t) + "_x_bits")
+        qc.add_register(creg)
+        for g, x_gauge in enumerate(self.code.x_gauges):
+            for q in x_gauge:
+                qc.h(qc.qregs[0][q])
+                qc.cx(qc.qregs[0][q], qc.qregs[2][g])
+                qc.h(qc.qregs[0][q])
+            if self._noise:
+                qc.append(self._meas_error, [qc.qregs[2][g]])
+            qc.measure(qc.qregs[2][g], creg[g])
+            qc.reset(qc.qregs[2][g])
+
+    def string2nodes(self, string, **kwargs):
+        """
+        Convert output string from circuits into a set of nodes for
+        `DecodingGraph`.
+        Args:
+            string (string): Results string to convert.
+            kwargs (dict): Any additional keyword arguments.
+        """
+        output = string.split(" ")[::-1]
+        gauge_outs = [[], []]
+        for t in range(self.T):
+            gauge_outs[0].append(
+                [int(b) for b in output[2 * t + self.round_schedule.find("x")]][::-1]
+            )
+            gauge_outs[1].append(
+                [int(b) for b in output[2 * t + self.round_schedule.find("z")]][::-1]
+            )
+        final_outs = [int(b) for b in output[-1]]
+
+        stabilizer_outs = []
+        for j in range(2):
+            stabilizer_outs.append([])
+            for t in range(self.T):
+                round_outs = []
+                for gs in self._gauges4stabilizers[j]:
+                    out = 0
+                    for g in gs:
+                        out += gauge_outs[j][t][g]
+                    out = out % 2
+                    round_outs.append(out)
+                stabilizer_outs[j].append(round_outs)
+
+        bases = ["x", "z"]
+        j = bases.index(self.basis)
+        round_outs = []
+        for stabilizer in self._stabilizers[j]:
+            out = 0
+            for q in stabilizer:
+                out += final_outs[q]
+            out = out % 2
+            round_outs.append(out)
+        stabilizer_outs[j].append(round_outs)
+
+        stabilizer_changes = []
+        for j in range(2):
+            stabilizer_changes.append([])
+            for t in range(self.T + (bases[j] == self.basis)):
+                stabilizer_changes[j].append([])
+                for e in range(len(stabilizer_outs[j][t])):
+                    if t == 0 and j == bases.index(self.basis):
+                        stabilizer_changes[j][t].append(stabilizer_outs[j][t][e])
+                    else:
+                        stabilizer_changes[j][t].append(
+                            (stabilizer_outs[j][t][e] + stabilizer_outs[j][t - 1][e]) % 2
+                        )
+
+        nodes = []
+        for j in range(2):
+            for t, round_changes in enumerate(stabilizer_changes[j]):
+                for e, change in enumerate(round_changes):
+                    if change == 1:
+                        node = {
+                            "time": t,
+                            "basis": bases[j],
+                            "qubits": self._stabilizers[j][e],
+                            "element": e,
+                            "is_boundary": False,
+                        }
+                        nodes.append(node)
+
+        j = bases.index(self.basis)
+        boundary = [self.code.x_boundary, self.code.z_boundary]
+        boundary_qubits = [q[0] for q in boundary[j]]
+
+        boundary_out = 0
+        for q in boundary_qubits:
+            boundary_out += final_outs[q]
+        boundary_out = boundary_out % 2
+
+        if boundary_out == 1:
+            node = {
+                "time": 0,
+                "basis": self.basis,
+                "qubits": boundary_qubits,
+                "element": 0,
+                "is_boundary": True,
+            }
+            nodes.append(node)
+
+        return nodes