# Source code for qutip.metrics

```
# -*- coding: utf-8 -*-
# This file is part of QuTiP: Quantum Toolbox in Python.
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"""
This module contains a collection of functions for calculating metrics
(distance measures) between states and operators.
"""
__all__ = ['fidelity', 'tracedist', 'bures_dist', 'bures_angle',
'hellinger_dist', 'hilbert_dist', 'average_gate_fidelity',
'process_fidelity', 'unitarity', 'dnorm']
import numpy as np
from scipy import linalg as la
import scipy.sparse as sp
from qutip.sparse import sp_eigs
from qutip.states import ket2dm
from qutip.superop_reps import to_kraus, to_stinespring, to_choi, _super_to_superpauli, to_super
from qutip.superoperator import operator_to_vector, vector_to_operator
from qutip.operators import qeye
from qutip.semidefinite import dnorm_problem
import qutip.settings as settings
import qutip.logging_utils as logging
logger = logging.get_logger()
try:
import cvxpy
except:
cvxpy = None
[docs]def fidelity(A, B):
"""
Calculates the fidelity (pseudo-metric) between two density matrices.
See: Nielsen & Chuang, "Quantum Computation and Quantum Information"
Parameters
----------
A : qobj
Density matrix or state vector.
B : qobj
Density matrix or state vector with same dimensions as A.
Returns
-------
fid : float
Fidelity pseudo-metric between A and B.
Examples
--------
>>> x = fock_dm(5,3)
>>> y = coherent_dm(5,1)
>>> fidelity(x,y)
0.24104350624628332
"""
if A.isket or A.isbra:
# Take advantage of the fact that the density operator for A
# is a projector to avoid a sqrtm call.
sqrtmA = ket2dm(A)
# Check whether we have to turn B into a density operator, too.
if B.isket or B.isbra:
B = ket2dm(B)
else:
if B.isket or B.isbra:
# Swap the order so that we can take a more numerically
# stable square root of B.
return fidelity(B, A)
# If we made it here, both A and B are operators, so
# we have to take the sqrtm of one of them.
sqrtmA = A.sqrtm()
if sqrtmA.dims != B.dims:
raise TypeError('Density matrices do not have same dimensions.')
# We don't actually need the whole matrix here, just the trace
# of its square root, so let's just get its eigenenergies instead.
# We also truncate negative eigenvalues to avoid nan propagation;
# even for positive semidefinite matrices, small negative eigenvalues
# can be reported.
eig_vals = (sqrtmA * B * sqrtmA).eigenenergies()
return float(np.real(np.sqrt(eig_vals[eig_vals > 0]).sum()))
[docs]def process_fidelity(U1, U2, normalize=True):
"""
Calculate the process fidelity given two process operators.
"""
if normalize:
return (U1 * U2).tr() / (U1.tr() * U2.tr())
else:
return (U1 * U2).tr()
[docs]def average_gate_fidelity(oper, target=None):
"""
Given a Qobj representing the supermatrix form of a map, returns the
average gate fidelity (pseudo-metric) of that map.
Parameters
----------
A : Qobj
Quantum object representing a superoperator.
target : Qobj
Quantum object representing the target unitary; the inverse
is applied before evaluating the fidelity.
Returns
-------
fid : float
Fidelity pseudo-metric between A and the identity superoperator,
or between A and the target superunitary.
"""
kraus_form = to_kraus(oper)
d = kraus_form[0].shape[0]
if kraus_form[0].shape[1] != d:
return TypeError("Average gate fidelity only implemented for square "
"superoperators.")
if target is None:
return (d + np.sum([np.abs(A_k.tr())**2
for A_k in kraus_form])) / (d**2 + d)
else:
return (d + np.sum([np.abs((A_k * target.dag()).tr())**2
for A_k in kraus_form])) / (d**2 + d)
[docs]def tracedist(A, B, sparse=False, tol=0):
"""
Calculates the trace distance between two density matrices..
See: Nielsen & Chuang, "Quantum Computation and Quantum Information"
Parameters
----------!=
A : qobj
Density matrix or state vector.
B : qobj
Density matrix or state vector with same dimensions as A.
tol : float
Tolerance used by sparse eigensolver, if used. (0=Machine precision)
sparse : {False, True}
Use sparse eigensolver.
Returns
-------
tracedist : float
Trace distance between A and B.
Examples
--------
>>> x=fock_dm(5,3)
>>> y=coherent_dm(5,1)
>>> tracedist(x,y)
0.9705143161472971
"""
if A.isket or A.isbra:
A = ket2dm(A)
if B.isket or B.isbra:
B = ket2dm(B)
if A.dims != B.dims:
raise TypeError("A and B do not have same dimensions.")
diff = A - B
diff = diff.dag() * diff
vals = sp_eigs(diff.data, diff.isherm, vecs=False, sparse=sparse, tol=tol)
return float(np.real(0.5 * np.sum(np.sqrt(np.abs(vals)))))
[docs]def hilbert_dist(A, B):
"""
Returns the Hilbert-Schmidt distance between two density matrices A & B.
Parameters
----------
A : qobj
Density matrix or state vector.
B : qobj
Density matrix or state vector with same dimensions as A.
Returns
-------
dist : float
Hilbert-Schmidt distance between density matrices.
Notes
-----
See V. Vedral and M. B. Plenio, Phys. Rev. A 57, 1619 (1998).
"""
if A.isket or A.isbra:
A = ket2dm(A)
if B.isket or B.isbra:
B = ket2dm(B)
if A.dims != B.dims:
raise TypeError('A and B do not have same dimensions.')
return ((A - B)**2).tr()
[docs]def bures_dist(A, B):
"""
Returns the Bures distance between two density matrices A & B.
The Bures distance ranges from 0, for states with unit fidelity,
to sqrt(2).
Parameters
----------
A : qobj
Density matrix or state vector.
B : qobj
Density matrix or state vector with same dimensions as A.
Returns
-------
dist : float
Bures distance between density matrices.
"""
if A.isket or A.isbra:
A = ket2dm(A)
if B.isket or B.isbra:
B = ket2dm(B)
if A.dims != B.dims:
raise TypeError('A and B do not have same dimensions.')
dist = np.sqrt(2.0 * (1.0 - fidelity(A, B)))
return dist
[docs]def bures_angle(A, B):
"""
Returns the Bures Angle between two density matrices A & B.
The Bures angle ranges from 0, for states with unit fidelity, to pi/2.
Parameters
----------
A : qobj
Density matrix or state vector.
B : qobj
Density matrix or state vector with same dimensions as A.
Returns
-------
angle : float
Bures angle between density matrices.
"""
if A.isket or A.isbra:
A = ket2dm(A)
if B.isket or B.isbra:
B = ket2dm(B)
if A.dims != B.dims:
raise TypeError('A and B do not have same dimensions.')
return np.arccos(fidelity(A, B))
def hellinger_dist(A, B, sparse=False, tol=0):
"""
Calculates the quantum Hellinger distance between two density matrices.
Formula:
hellinger_dist(A, B) = sqrt(2-2*Tr(sqrt(A)*sqrt(B)))
See: D. Spehner, F. Illuminati, M. Orszag, and W. Roga, "Geometric
measures of quantum correlations with Bures and Hellinger distances"
arXiv:1611.03449
Parameters
----------
A : :class:`qutip.Qobj`
Density matrix or state vector.
B : :class:`qutip.Qobj`
Density matrix or state vector with same dimensions as A.
tol : float
Tolerance used by sparse eigensolver, if used. (0=Machine precision)
sparse : {False, True}
Use sparse eigensolver.
Returns
-------
hellinger_dist : float
Quantum Hellinger distance between A and B. Ranges from 0 to sqrt(2).
Examples
--------
>>> x=fock_dm(5,3)
>>> y=coherent_dm(5,1)
>>> hellinger_dist(x,y)
1.3725145002591095
"""
if A.dims != B.dims:
raise TypeError("A and B do not have same dimensions.")
if A.isket or A.isbra:
sqrtmA = ket2dm(A)
else:
sqrtmA = A.sqrtm(sparse=sparse, tol=tol)
if B.isket or B.isbra:
sqrtmB = ket2dm(B)
else:
sqrtmB = B.sqrtm(sparse=sparse, tol=tol)
product = sqrtmA*sqrtmB
eigs = sp_eigs(product.data,
isherm=product.isherm, vecs=False, sparse=sparse, tol=tol)
#np.maximum() is to avoid nan appearing sometimes due to numerical
#instabilities causing np.sum(eigs) slightly (~1e-8) larger than 1
#when hellinger_dist(A, B) is called for A=B
return np.sqrt(2.0 * np.maximum(0., (1.0 - np.real(np.sum(eigs)))))
def dnorm(A, B=None, solver="CVXOPT", verbose=False, force_solve=False):
"""
Calculates the diamond norm of the quantum map q_oper, using
the simplified semidefinite program of [Wat12]_.
The diamond norm SDP is solved by using CVXPY_.
Parameters
----------
A : Qobj
Quantum map to take the diamond norm of.
B : Qobj or None
If provided, the diamond norm of :math:`A - B` is
taken instead.
solver : str
Solver to use with CVXPY. One of "CVXOPT" (default)
or "SCS". The latter tends to be significantly faster,
but somewhat less accurate.
verbose : bool
If True, prints additional information about the
solution.
force_solve : bool
If True, forces dnorm to solve the associated SDP, even if a special
case is known for the argument.
Returns
-------
dn : float
Diamond norm of q_oper.
Raises
------
ImportError
If CVXPY cannot be imported.
.. _cvxpy: http://www.cvxpy.org/en/latest/
"""
if cvxpy is None: # pragma: no cover
raise ImportError("dnorm() requires CVXPY to be installed.")
# We follow the strategy of using Watrous' simpler semidefinite
# program in its primal form. This is the same strategy used,
# for instance, by both pyGSTi and SchattenNorms.jl. (By contrast,
# QETLAB uses the dual problem.)
# Check if A and B are both unitaries. If so, then we can without
# loss of generality choose B to be the identity by using the
# unitary invariance of the diamond norm,
# || A - B ||_♢ = || A B⁺ - I ||_♢.
# Then, using the technique mentioned by each of Johnston and
# da Silva,
# || A B⁺ - I ||_♢ = max_{i, j} | \lambda_i(A B⁺) - \lambda_j(A B⁺) |,
# where \lambda_i(U) is the ith eigenvalue of U.
if (
# There's a lot of conditions to check for this path.
not force_solve and B is not None and
# Only check if they aren't superoperators.
A.type == "oper" and B.type == "oper" and
# The difference of unitaries optimization is currently
# only implemented for d == 2. Much of the code below is more general,
# though, in anticipation of generalizing the optimization.
A.shape[0] == 2
):
# Make an identity the same size as A and B to
# compare against.
I = qeye(A.dims[0])
# Compare to B first, so that an error is raised
# as soon as possible.
Bd = B.dag()
if (
(B * Bd - I).norm() < 1e-6 and
(A * A.dag() - I).norm() < 1e-6
):
# Now we are on the fast path, so let's compute the
# eigenvalues, then find the diameter of the smallest circle
# containing all of them.
#
# For now, this is only implemented for dim = 2, such that
# generalizing here will allow for generalizing the optimization.
# A reasonable approach would probably be to use Welzl's algorithm
# (https://en.wikipedia.org/wiki/Smallest-circle_problem).
U = A * B.dag()
eigs = U.eigenenergies()
eig_distances = np.abs(eigs[:, None] - eigs[None, :])
return np.max(eig_distances)
# Force the input superoperator to be a Choi matrix.
J = to_choi(A)
if B is not None:
J -= to_choi(B)
# Watrous 2012 also points out that the diamond norm of Lambda
# is the same as the completely-bounded operator-norm (∞-norm)
# of the dual map of Lambda. We can evaluate that norm much more
# easily if Lambda is completely positive, since then the largest
# eigenvalue is the same as the largest singular value.
if not force_solve and J.iscp:
S_dual = to_super(J.dual_chan())
vec_eye = operator_to_vector(qeye(S_dual.dims[1][1]))
op = vector_to_operator(S_dual * vec_eye)
# The 2-norm was not implemented for sparse matrices as of the time
# of this writing. Thus, we must yet again go dense.
return la.norm(op.data.todense(), 2)
# If we're still here, we need to actually solve the problem.
# Assume square...
dim = np.prod(J.dims[0][0])
# The constraints only depend on the dimension, so
# we can cache them efficiently.
problem, Jr, Ji, X, rho0, rho1 = dnorm_problem(dim)
# Load the parameters with the Choi matrix passed in.
J_dat = J.data
Jr.value = sp.csr_matrix((J_dat.data.real, J_dat.indices, J_dat.indptr),
shape=J_dat.shape)
Ji.value = sp.csr_matrix((J_dat.data.imag, J_dat.indices, J_dat.indptr),
shape=J_dat.shape)
# Finally, set up and run the problem.
problem.solve(solver=solver, verbose=verbose)
return problem.value
def unitarity(oper):
"""
Returns the unitarity of a quantum map, defined as the Frobenius norm
of the unital block of that map's superoperator representation.
Parameters
----------
oper : Qobj
Quantum map under consideration.
Returns
-------
u : float
Unitarity of ``oper``.
"""
Eu = _super_to_superpauli(oper).full()[1:, 1:]
#return np.real(np.trace(np.dot(Eu, Eu.conj().T))) / len(Eu)
return np.linalg.norm(Eu, 'fro')**2 / len(Eu)
```