Add expectation value measurement error mitigation

README.md

@@ -27,8 +27,12 @@ can do this with `pip install matplotlib` or when you install ignis with
 `pip install qiskit-ignis[visualization]`. If you're going to use a cvx fitter
 for running tomogography you'll need to install cvxpy. You can do this with
 `pip install cvxpy` or when you install install ignis with
-`pip install qiskit-ignis[cvx]`. If you want to install both when you install
-ignis you can run `pip install qiskit-ignis[visualization,cvx]`.
+`pip install qiskit-ignis[cvx]`. When performing expectation value measurement
+error mitigation using the CTMP method performance can be improved using
+just-in-time compiling if Numbda is installed. You can do this with
+`pip install numba` or when you install install ignis with
+`pip install qiskit-ignis[jit]`. If you want to install all extra requirements
+when you install ignis you can run `pip install qiskit-ignis[visualization,cvx,jit]`.
 
 ## Creating your first quantum experiment with Qiskit Ignis
 Now that you have Qiskit Ignis installed, you can start creating experiments, to reveal information about the device quality. Here is a basic example:

qiskit/ignis/mitigation/__init__.py

@@ -36,7 +36,31 @@
    CompleteMeasFitter
    TensoredMeasFitter
 
+Expectation Value Measurement
+=============================
+
+The following classes allow mitigation of measurement errors
+in expectation value computations.
+
+.. autosummary::
+   :toctree: ../stubs/
+
+   expectation_value
+   counts_probability_vector
+   expval_meas_mitigator_circuits
+   ExpvalMeasMitigatorFitter
+   CTMPExpvalMeasMitigator
+   CompleteExpvalMeasMitigator
 """
+
 from .measurement import (complete_meas_cal, tensored_meas_cal,
                           MeasurementFilter, TensoredFilter,
                           CompleteMeasFitter, TensoredMeasFitter)
+
+from .expval import (expectation_value,
+                     counts_probability_vector,
+                     expval_meas_mitigator_circuits,
+                     ExpvalMeasMitigatorFitter,
+                     CompleteExpvalMeasMitigator,
+                     TensoredExpvalMeasMitigator,
+                     CTMPExpvalMeasMitigator)

qiskit/ignis/mitigation/expval/__init__.py

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2020.
+#
+# 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.
+"""
+Expectation value measurement error mitigation module
+"""
+
+from .circuits import expval_meas_mitigator_circuits
+from .fitter import ExpvalMeasMitigatorFitter
+from .complete_mitigator import CompleteExpvalMeasMitigator
+from .tensored_mitigator import TensoredExpvalMeasMitigator
+from .ctmp_mitigator import CTMPExpvalMeasMitigator
+from .utils import expectation_value, counts_probability_vector

qiskit/ignis/mitigation/expval/base_meas_mitigator.py

@@ -0,0 +1,267 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2020.
+#
+# 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.
+"""
+Full A-matrix measurement migitation generator.
+"""
+
+from abc import ABC, abstractmethod
+from typing import Optional, List, Dict, Tuple
+import numpy as np
+
+try:
+    import matplotlib.pyplot as plt
+    _HAS_MATPLOTLIB = True
+except ImportError:
+    _HAS_MATPLOTLIB = False
+
+
+class BaseExpvalMeasMitigator(ABC):
+    """Base measurement error mitigator class."""
+
+    @abstractmethod
+    def expectation_value(self,
+                          counts: Dict,
+                          diagonal: Optional[np.ndarray] = None,
+                          clbits: Optional[List[int]] = None,
+                          qubits: Optional[List[int]] = None
+                          ) -> Tuple[float, float]:
+        r"""Compute the mitigated expectation value of a diagonal observable.
+
+        This computes the mitigated estimator of
+        :math:`\langle O \rangle = \mbox{Tr}[\rho. O]` of a diagonal observable
+        :math:`O = \sum_{x\in\{0, 1\}^n} O(x)|x\rangle\!\langle x|` where
+        :math:`|O(x)|\le 1`.
+
+        Args:
+            counts: counts object
+            diagonal: Optional, values of the diagonal observable. If None the
+                      observable is set to :math:`Z^\otimes n`.
+            clbits: Optional, marginalize counts to just these bits.
+            qubits: qubits the count bitstrings correspond to.
+
+        Returns:
+            (float, float): the expectation value and standard deviation.
+
+        Additional Information:
+            The observable :math:`O` is input using the ``diagonal`` kwarg as a
+            Numpy array :math:`[O(0), ..., O(2^n -1)]`. If no diagonal is specified
+            the Pauli-Z operator `O = Z^{\otimes n}` is used.
+
+            The ``clbits`` kwarg is used to marginalize the input counts dictionary
+            over the specified bit-values, and the ``qubits`` kwarg is used to specify
+            which physical qubits these bit-values correspond to as
+            ``circuit.measure(qubits, clbits)``.
+        """
+
+    @abstractmethod
+    def mitigation_matrix(self, qubits: List[int] = None) -> np.ndarray:
+        r"""Return the measurement mitigation matrix for the specified qubits.
+
+        The mitigation matrix :math:`A^{-1}` is defined as the inverse of the
+        :meth:`assignment_matrix` :math:`A`.
+
+        Args:
+            qubits: Optional, qubits being measured for operator expval.
+
+        Returns:
+            np.ndarray: the measurement error mitigation matrix :math:`A^{-1}`.
+        """
+
+    @abstractmethod
+    def assignment_matrix(self, qubits: List[int] = None) -> np.ndarray:
+        r"""Return the measurement assignment matrix for specified qubits.
+
+        The assignment matrix is the stochastic matrix :math:`A` which assigns
+        a noisy measurement probability distribution to an ideal input
+        measurement distribution: :math:`P(i|j) = \langle i | A | j \rangle`.
+
+        Args:
+            qubits: Optional, qubits being measured for operator expval.
+
+        Returns:
+            np.ndarray: the assignment matrix A.
+        """
+
+    @abstractmethod
+    def _compute_gamma(self, qubits: Optional[List[int]] = None) -> float:
+        """Compute gamma for N-qubit mitigation."""
+
+    def assignment_fidelity(self, qubits: Optional[List[int]] = None) -> float:
+        r"""Return the measurement assignment fidelity on the specified qubits.
+
+        The assignment fidelity on N-qubits is defined as
+        :math:`\sum_{x\in\{0, 1\}^n} P(x|x) / 2^n`, where
+        :math:`P(x|x) = \rangle x|A|x\langle`, and :math:`A` is the
+        :meth:`assignment_matrix`.
+
+        Args:
+            qubits: Optional, qubits being measured for operator expval.
+
+        Returns:
+            float: the assignment fidelity.
+        """
+        return self.assignment_matrix(qubits=qubits).diagonal().mean()
+
+    def required_shots(self, delta: float, qubits: Optional[List[int]] = None) -> int:
+        r"""Return the number of shots required for expectation value estimation.
+
+        This is the number of shots required so that
+        :math:`|\langle O \rangle_{est} - \langle O \rangle_{true}| < \delta`
+        with high probability (at least 2/3) and is given by
+        :math:`4\delta^2 \Gamma^2` where :math:`\Gamma^2` is the
+        :meth:`mitigation_overhead`.
+
+        Args:
+            delta: Error tolerance for expectation value estimator.
+            qubits: Optional, qubits being measured for operator expval.
+
+        Returns:
+            int: the required shots.
+        """
+        gamma = self._compute_gamma(qubits=qubits)
+        return int(np.ceil((4 * gamma ** 2) / (delta ** 2)))
+
+    def mitigation_overhead(self, qubits: Optional[List[int]] = None) -> int:
+        """Return the mitigation overhead for expectation value estimation.
+
+        This is the multiplicative factor of extra shots required for
+        estimating a mitigated expectation value with the same accuracy as
+        an unmitigated expectation value.
+
+        Args:
+            qubits: Optional, qubits being measured for operator expval.
+
+        Returns:
+            int: the mitigation overhead factor.
+        """
+        gamma = self._compute_gamma(qubits=qubits)
+        return int(np.ceil(gamma ** 2))
+
+    def stddev_upper_bound(self, shots: int = 1, qubits: Optional[List[int]] = None) -> float:
+        """Return an upper bound on standard deviation of expval estimator.
+
+        Args:
+            shots: Number of shots used for expectation value measurement.
+            qubits: qubits being measured for operator expval.
+
+        Returns:
+            float: the standard deviation upper bound.
+        """
+        gamma = self._compute_gamma(qubits=qubits)
+        return gamma / np.sqrt(shots)
+
+    def plot_assignment_matrix(self,
+                               qubits: Optional[List[int]] = None,
+                               ax: Optional[plt.axes] = None) -> plt.axes:
+        """Matrix plot of the readout error assignment matrix.
+
+        Args:
+            qubits: Optional, qubits being measured for operator expval.
+            ax: Optional. Axes object to add plot to.
+
+        Returns:
+            plt.axes: the figure axes object.
+
+        Raises:
+            ImportError: if matplotlib is not installed.
+        """
+        if not _HAS_MATPLOTLIB:
+            raise ImportError(
+                'Plotting functions require the optional matplotlib package.'
+                ' Install with `pip install matplotlib`.')
+
+        mat = self.assignment_matrix(qubits)
+        if ax is None:
+            figure = plt.figure()
+            ax = figure.gca()
+        axim = ax.matshow(mat, cmap=plt.cm.binary, clim=[0, 1])
+        ax.figure.colorbar(axim)
+        ax = self._plot_axis(mat, ax=ax)
+        return ax
+
+    def plot_mitigation_matrix(self,
+                               qubits: Optional[List[int]] = None,
+                               ax: Optional[plt.axes] = None) -> plt.axes:
+        """Matrix plot of the readout error mitigation matrix.
+
+        Args:
+            qubits: Optional, qubits being measured for operator expval.
+            ax: Optional. Axes object to add plot to.
+
+        Returns:
+            plt.axes: the figure axes object.
+
+        Raises:
+            ImportError: if matplotlib is not installed.
+        """
+        if not _HAS_MATPLOTLIB:
+            raise ImportError(
+                'Plotting functions require the optional matplotlib package.'
+                ' Install with `pip install matplotlib`.')
+
+        # Matrix parameters
+        mat = self.mitigation_matrix(qubits)
+        mat_max = np.max(mat)
+        mat_min = np.min(mat)
+        lim = max(abs(mat_max), abs(mat_min), 1)
+
+        if ax is None:
+            figure = plt.figure()
+            ax = figure.gca()
+        axim = ax.matshow(mat, cmap=plt.cm.RdGy, clim=[-lim, lim])
+        ax.figure.colorbar(axim, values=np.linspace(mat_min, mat_max, 100))
+        ax = self._plot_axis(mat, ax=ax)
+        return ax
+
+    @staticmethod
+    def _z_diagonal(dim, dtype=float):
+        r"""Return the diagonal for the operator :math:`Z^\otimes n`"""
+        parity = np.zeros(dim, dtype=dtype)
+        for i in range(dim):
+            parity[i] = bin(i)[2:].count('1')
+        return (-1)**np.mod(parity, 2)
+
+    @staticmethod
+    def _int_to_bitstring(i, num_qubits=None):
+        """Convert an integer to a bitstring."""
+        label = bin(i)[2:]
+        if num_qubits:
+            pad = num_qubits - len(label)
+            if pad > 0:
+                label = pad * '0' + label
+        return label
+
+    @staticmethod
+    def _plot_axis(mat: np.ndarray, ax: plt.axes) -> plt.axes:
+        """Helper function for setting up axes for plots.
+
+        Args:
+            mat: the N-qubit matrix to plot.
+            ax: Optional. Axes object to add plot to.
+
+        Returns:
+            plt.axes: the figure object and axes object.
+        """
+        dim = len(mat)
+        num_qubits = int(np.log2(dim))
+        bit_labels = [BaseExpvalMeasMitigator._int_to_bitstring(i, num_qubits) for i in range(dim)]
+
+        ax.set_xticks(np.arange(dim))
+        ax.set_yticks(np.arange(dim))
+        ax.set_xticklabels(bit_labels, rotation=90)
+        ax.set_yticklabels(bit_labels)
+        ax.set_xlabel('Prepared State')
+        ax.xaxis.set_label_position('top')
+        ax.set_ylabel('Measured State')
+        return ax