Experimental

Note

Functions here are untested and under-documented.

Continuous Variables

This module contains a collection functions for calculating continuous variable quantities from fock-basis representation of the state of multi-mode fields.

correlation_matrix(basis, rho=None)

Given a basis set of operators \(\{a\}_n\), calculate the correlation matrix:

\[C_{mn} = \langle a_m a_n \rangle\]

Parameters:

basislist

List of operators that defines the basis for the correlation matrix.

rhoQobj, optional

Density matrix for which to calculate the correlation matrix. If rho is None, then a matrix of correlation matrix operators is returned instead of expectation values of those operators.

Returns:

corr_matndarray

A 2-dimensional array of correlation values or operators.

correlation_matrix_field(a1, a2, rho=None)

Calculates the correlation matrix for given field operators \(a_1\) and \(a_2\). If a density matrix is given the expectation values are calculated, otherwise a matrix with operators is returned.

Parameters:

a1Qobj

Field operator for mode 1.

a2Qobj

Field operator for mode 2.

rhoQobj, optional

Density matrix for which to calculate the covariance matrix.

Returns:

cov_matndarray

Array of complex numbers or Qobj’s A 2-dimensional array of covariance values, or, if rho=0, a matrix of operators.

correlation_matrix_quadrature(a1, a2, rho=None, g=1.4142135623730951)

Calculate the quadrature correlation matrix with given field operators \(a_1\) and \(a_2\). If a density matrix is given the expectation values are calculated, otherwise a matrix with operators is returned.

Parameters:

a1Qobj

Field operator for mode 1.

a2Qobj

Field operator for mode 2.

rhoQobj, optional

Density matrix for which to calculate the covariance matrix.

gfloat, default: sqrt(2)

Scaling factor for a = 0.5 * g * (x + iy), default g = sqrt(2). The value of g is related to the value of hbar in the commutation relation [x, y] = i * hbar via hbar=2/g ** 2 giving the default value hbar=1.

Returns:

corr_matndarray

Array of complex numbers or Qobj’s A 2-dimensional array of covariance values for the field quadratures, or, if rho=0, a matrix of operators.

covariance_matrix(basis, rho, symmetrized=True)

Given a basis set of operators \(\{a\}_n\), calculate the covariance matrix:

\[V_{mn} = \frac{1}{2}\langle a_m a_n + a_n a_m \rangle - \langle a_m \rangle \langle a_n\rangle\]

or, if of the optional argument symmetrized=False,

\[V_{mn} = \langle a_m a_n\rangle - \langle a_m \rangle \langle a_n\rangle\]

Parameters:

basislist

List of operators that defines the basis for the covariance matrix.

rhoQobj

Density matrix for which to calculate the covariance matrix.

symmetrizedbool, default: True

Flag indicating whether the symmetrized (default) or non-symmetrized correlation matrix is to be calculated.

Returns:

corr_matndarray

A 2-dimensional array of covariance values.

logarithmic_negativity(V, g=1.4142135623730951)

Calculates the logarithmic negativity given a symmetrized covariance matrix, see qutip.continuous_variables.covariance_matrix. Note that the two-mode field state that is described by V must be Gaussian for this function to applicable.

Parameters:

Vndarray

The covariance matrix.

gfloat, default: sqrt(2)

Scaling factor for a = 0.5 * g * (x + iy), default g = sqrt(2). The value of g is related to the value of hbar in the commutation relation [x, y] = i * hbar via hbar=2/g ** 2 giving the default value hbar=1.

Returns:

Nfloat

The logarithmic negativity for the two-mode Gaussian state that is described by the the Wigner covariance matrix V.

wigner_covariance_matrix(

a1=None,

a2=None,

R=None,

rho=None,

g=1.4142135623730951,

)

Calculates the Wigner covariance matrix \(V_{ij} = \frac{1}{2}(R_{ij} + R_{ji})\), given the quadrature correlation matrix \(R_{ij} = \langle R_{i} R_{j}\rangle - \langle R_{i}\rangle \langle R_{j}\rangle\), where \(R = (q_1, p_1, q_2, p_2)^T\) is the vector with quadrature operators for the two modes.

Alternatively, if R = None, and if annihilation operators a1 and a2 for the two modes are supplied instead, the quadrature correlation matrix is constructed from the annihilation operators before then the covariance matrix is calculated.

Parameters:

a1Qobj, optional

Field operator for mode 1.

a2Qobj, optional

Field operator for mode 2.

Rndarray, optional

The quadrature correlation matrix.

rhoQobj, optional

Density matrix for which to calculate the covariance matrix.

gfloat, default: sqrt(2)

Scaling factor for a = 0.5 * g * (x + iy), default g = sqrt(2). The value of g is related to the value of hbar in the commutation relation [x, y] = i * hbar via hbar=2/g ** 2 giving the default value hbar=1.

Returns:

cov_matndarray

A 2-dimensional array of covariance values.