# This file is part of QuTiP: Quantum Toolbox in Python.
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# Copyright (c) 2011 and later, Paul D. Nation and Robert J. Johansson.
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__all__ = ['expect', 'variance']
import numpy as np
import scipy.sparse as sp
from qutip.qobj import Qobj, isoper
from qutip.eseries import eseries
from qutip.cy.spmatfuncs import (cy_expect_rho_vec, cy_expect_psi, cy_spmm_tr)
expect_rho_vec = cy_expect_rho_vec
expect_psi = cy_expect_psi
[docs]def expect(oper, state):
'''Calculates the expectation value for operator(s) and state(s).
Parameters
----------
oper : qobj/array-like
A single or a `list` or operators for expectation value.
state : qobj/array-like
A single or a `list` of quantum states or density matrices.
Returns
-------
expt : float/complex/array-like
Expectation value. ``real`` if `oper` is Hermitian, ``complex``
otherwise. A (nested) array of expectaction values of state or operator
are arrays.
Examples
--------
>>> expect(num(4), basis(4, 3))
3
'''
if isinstance(state, Qobj) and isinstance(oper, Qobj):
return _single_qobj_expect(oper, state)
elif isinstance(oper, Qobj) and isinstance(state, eseries):
return _single_eseries_expect(oper, state)
elif isinstance(oper, (list, np.ndarray)):
if isinstance(state, Qobj):
if (all([op.isherm for op in oper]) and
(state.isket or state.isherm)):
return np.array([_single_qobj_expect(o, state) for o in oper])
else:
return np.array([_single_qobj_expect(o, state) for o in oper],
dtype=complex)
else:
return [expect(o, state) for o in oper]
elif isinstance(state, (list, np.ndarray)):
if oper.isherm and all([(op.isherm or op.type == 'ket')
for op in state]):
return np.array([_single_qobj_expect(oper, x) for x in state])
else:
return np.array([_single_qobj_expect(oper, x) for x in state],
dtype=complex)
else:
raise TypeError('Arguments must be quantum objects or eseries')
def _single_qobj_expect(oper, state):
"""
Private function used by expect to calculate expectation values of Qobjs.
"""
if isoper(oper):
if oper.dims[1] != state.dims[0]:
raise Exception('Operator and state do not have same tensor ' +
'structure: %s and %s' %
(oper.dims[1], state.dims[0]))
if state.type == 'oper':
# calculates expectation value via TR(op*rho)
return cy_spmm_tr(oper.data, state.data,
oper.isherm and state.isherm)
elif state.type == 'ket':
# calculates expectation value via <psi|op|psi>
return cy_expect_psi(oper.data, state.full(squeeze=True),
oper.isherm)
else:
raise TypeError('Invalid operand types')
def _single_eseries_expect(oper, state):
"""
Private function used by expect to calculate expectation values for
eseries.
"""
out = eseries()
if isoper(state.ampl[0]):
out.rates = state.rates
out.ampl = np.array([expect(oper, a) for a in state.ampl])
else:
out.rates = np.array([])
out.ampl = np.array([])
for m in range(len(state.rates)):
op_m = state.ampl[m].data.conj().T * oper.data
for n in range(len(state.rates)):
a = op_m * state.ampl[n].data
if isinstance(a, sp.spmatrix):
a = a.todense()
out.rates = np.append(out.rates, state.rates[n] -
state.rates[m])
out.ampl = np.append(out.ampl, a)
return out
[docs]def variance(oper, state):
"""
Variance of an operator for the given state vector or density matrix.
Parameters
----------
oper : qobj
Operator for expectation value.
state : qobj/list
A single or `list` of quantum states or density matrices..
Returns
-------
var : float
Variance of operator 'oper' for given state.
"""
return expect(oper ** 2, state) - expect(oper, state) ** 2