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quantum.py
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quantum.py
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# Copyright 2019 Xanadu Quantum Technologies Inc.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
Quantum algorithms
==================
.. currentmodule:: thewalrus.quantum
This submodule provides access to various utility functions that act on Gaussian
quantum states.
For more details on how the hafnian relates to various properties of Gaussian quantum
states, see:
* Kruse, R., Hamilton, C. S., Sansoni, L., Barkhofen, S., Silberhorn, C., & Jex, I.
"Detailed study of Gaussian boson sampling." `Physical Review A 100, 032326 (2019)
<https://journals.aps.org/pra/abstract/10.1103/PhysRevA.100.032326>`_
* Hamilton, C. S., Kruse, R., Sansoni, L., Barkhofen, S., Silberhorn, C., & Jex, I.
"Gaussian boson sampling." `Physical Review Letters, 119(17), 170501 (2017)
<https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.170501>`_
* Quesada, N.
"Franck-Condon factors by counting perfect matchings of graphs with loops."
`Journal of Chemical Physics 150, 164113 (2019) <https://aip.scitation.org/doi/10.1063/1.5086387>`_
* Quesada, N., Helt, L. G., Izaac, J., Arrazola, J. M., Shahrokhshahi, R., Myers, C. R., & Sabapathy, K. K.
"Simulating realistic non-Gaussian state preparation." `Physical Review A 100, 022341 (2019)
<https://journals.aps.org/pra/abstract/10.1103/PhysRevA.100.022341>`_
Fock states and tensors
-----------------------
.. autosummary::
pure_state_amplitude
state_vector
density_matrix_element
density_matrix
fock_tensor
probabilities
update_probabilities_with_loss
update_probabilities_with_noise
Details
^^^^^^^
.. autofunction::
pure_state_amplitude
.. autofunction::
state_vector
.. autofunction::
density_matrix_element
.. autofunction::
density_matrix
.. autofunction::
fock_tensor
.. autofunction::
probabilities
.. autofunction::
update_probabilities_with_loss
.. autofunction::
update_probabilities_with_noise
Utility functions
-----------------
.. autosummary::
reduced_gaussian
Xmat
Qmat
Covmat
Amat
Beta
Means
prefactor
find_scaling_adjacency_matrix
mean_number_of_clicks
find_scaling_adjacency_matrix_torontonian
gen_Qmat_from_graph
photon_number_mean
photon_number_mean_vector
photon_number_covar
photon_number_covmat
is_valid_cov
is_pure_cov
is_classical_cov
total_photon_num_dist_pure_state
gen_single_mode_dist
gen_multi_mode_dist
Details
^^^^^^^
"""
# pylint: disable=too-many-arguments
from itertools import count, product, chain
import numpy as np
from scipy.optimize import root_scalar
from scipy.special import factorial as fac
from scipy.stats import nbinom
from numba import jit
from thewalrus.symplectic import expand, sympmat, is_symplectic
from ._hafnian import hafnian, hafnian_repeated, reduction
from ._hermite_multidimensional import hermite_multidimensional, hafnian_batched
def reduced_gaussian(mu, cov, modes):
r""" Returns the vector of means and the covariance matrix of the specified modes.
Args:
mu (array): a length-:math:`2N` ``np.float64`` vector of means.
cov (array): a :math:`2N\times 2N` ``np.float64`` covariance matrix
representing an :math:`N` mode quantum state.
modes (int of Sequence[int]): indices of the requested modes
Returns:
tuple (means, cov): where means is an array containing the vector of means,
and cov is a square array containing the covariance matrix.
"""
N = len(mu) // 2
# reduce rho down to specified subsystems
if isinstance(modes, int):
modes = [modes]
if np.any(np.array(modes) > N):
raise ValueError("Provided mode is larger than the number of subsystems.")
if len(modes) == N:
# reduced state is full state
return mu, cov
ind = np.concatenate([np.array(modes), np.array(modes) + N])
rows = ind.reshape(-1, 1)
cols = ind.reshape(1, -1)
return mu[ind], cov[rows, cols]
def Xmat(N):
r"""Returns the matrix :math:`X_n = \begin{bmatrix}0 & I_n\\ I_n & 0\end{bmatrix}`
Args:
N (int): positive integer
Returns:
array: :math:`2N\times 2N` array
"""
I = np.identity(N)
O = np.zeros_like(I)
X = np.block([[O, I], [I, O]])
return X
def Qmat(cov, hbar=2):
r"""Returns the :math:`Q` Husimi matrix of the Gaussian state.
Args:
cov (array): :math:`2N\times 2N xp-` Wigner covariance matrix
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
Returns:
array: the :math:`Q` matrix.
"""
# number of modes
N = len(cov) // 2
I = np.identity(N)
x = cov[:N, :N] * 2 / hbar
xp = cov[:N, N:] * 2 / hbar
p = cov[N:, N:] * 2 / hbar
# the (Hermitian) matrix elements <a_i^\dagger a_j>
aidaj = (x + p + 1j * (xp - xp.T) - 2 * I) / 4
# the (symmetric) matrix elements <a_i a_j>
aiaj = (x - p + 1j * (xp + xp.T)) / 4
# calculate the covariance matrix sigma_Q appearing in the Q function:
# Q(alpha) = exp[-(alpha-beta).sigma_Q^{-1}.(alpha-beta)/2]/|sigma_Q|
Q = np.block([[aidaj, aiaj.conj()], [aiaj, aidaj.conj()]]) + np.identity(2 * N)
return Q
def Covmat(Q, hbar=2):
r"""Returns the Wigner covariance matrix in the :math:`xp`-ordering of the Gaussian state.
This is the inverse function of Qmat.
Args:
Q (array): :math:`2N\times 2N` Husimi Q matrix
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
Returns:
array: the :math:`xp`-ordered covariance matrix in the xp-ordering.
"""
# number of modes
n = len(Q) // 2
I = np.identity(n)
N = Q[0:n, 0:n] - I
M = Q[n : 2 * n, 0:n]
mm11a = 2 * (N.real + M.real) + np.identity(n)
mm22a = 2 * (N.real - M.real) + np.identity(n)
mm12a = 2 * (M.imag + N.imag)
cov = np.block([[mm11a, mm12a], [mm12a.T, mm22a]])
return (hbar / 2) * cov
def Amat(cov, hbar=2, cov_is_qmat=False):
r"""Returns the :math:`A` matrix of the Gaussian state whose hafnian gives the photon number probabilities.
Args:
cov (array): :math:`2N\times 2N` covariance matrix
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
cov_is_qmat (bool): if ``True``, it is assumed that ``cov`` is in fact the Q matrix.
Returns:
array: the :math:`A` matrix.
"""
# number of modes
N = len(cov) // 2
X = Xmat(N)
# inverse Q matrix
if cov_is_qmat:
Q = cov
else:
Q = Qmat(cov, hbar=hbar)
Qinv = np.linalg.inv(Q)
# calculate Hamilton's A matrix: A = X.(I-Q^{-1})*
A = X @ (np.identity(2 * N) - Qinv).conj()
return A
def Beta(mu, hbar=2):
r"""Returns the vector of complex displacements and conjugate displacements.
Args:
mu (array): length-:math:`2N` means vector
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
Returns:
array: the expectation values
:math:`[\langle a_1\rangle, \langle a_2\rangle,\dots,\langle a_N\rangle, \langle a^\dagger_1\rangle, \dots, \langle a^\dagger_N\rangle]`
"""
N = len(mu) // 2
# mean displacement of each mode
alpha = (mu[:N] + 1j * mu[N:]) / np.sqrt(2 * hbar)
# the expectation values (<a_1>, <a_2>,...,<a_N>, <a^\dagger_1>, ..., <a^\dagger_N>)
return np.concatenate([alpha, alpha.conj()])
def Means(beta, hbar=2):
r"""Returns the vector of real quadrature displacements.
Args:
beta (array): length-:math:`2N` means bivector
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
Returns:
array: the quadrature expectation values
:math:`[\langle q_1\rangle, \langle q_2\rangle,\dots,\langle q_N\rangle, \langle p_1\rangle, \dots, \langle p_N\rangle]`
"""
N = len(beta) // 2
alpha = beta[0:N]
return np.sqrt(2 * hbar) * np.concatenate([alpha.real, alpha.imag])
def prefactor(mu, cov, hbar=2):
r"""Returns the prefactor.
.. math:: prefactor = \frac{e^{-\beta Q^{-1}\beta^*/2}}{n_1!\cdots n_m! \sqrt{|Q|}}
Args:
mu (array): length-:math:`2N` vector of mean values :math:`[\alpha,\alpha^*]`
cov (array): length-:math:`2N` `xp`-covariance matrix
Returns:
float: the prefactor
"""
Q = Qmat(cov, hbar=hbar)
beta = Beta(mu, hbar=hbar)
Qinv = np.linalg.inv(Q)
return np.exp(-0.5 * beta @ Qinv @ beta.conj()) / np.sqrt(np.linalg.det(Q))
def density_matrix_element(mu, cov, i, j, include_prefactor=True, tol=1e-10, hbar=2):
r"""Returns the :math:`\langle i | \rho | j \rangle` element of the density matrix
of a Gaussian state defined by covariance matrix cov.
Args:
mu (array): length-:math:`2N` quadrature displacement vector
cov (array): length-:math:`2N` covariance matrix
i (list): list of density matrix rows
j (list): list of density matrix columns
include_prefactor (bool): if ``True``, the prefactor is automatically calculated
used to scale the result.
tol (float): tolerance for determining if displacement is negligible
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
Returns:
complex: the density matrix element
"""
rpt = i + j
beta = Beta(mu, hbar=hbar)
A = Amat(cov, hbar=hbar)
if np.linalg.norm(beta) < tol:
# no displacement
if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
A_rpt = reduction(A, rpt)
haf = hafnian(A_rpt)
else:
haf = hafnian_repeated(A, rpt)
else:
# replace the diagonal of A with gamma
gamma = beta.conj() - A @ beta
if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
A_rpt = reduction(A, rpt)
np.fill_diagonal(A_rpt, reduction(gamma, rpt))
haf = hafnian(A_rpt, loop=True)
else:
haf = hafnian_repeated(A, rpt, mu=gamma, loop=True)
if include_prefactor:
haf *= prefactor(mu, cov, hbar=hbar)
return haf / np.sqrt(np.prod(fac(rpt)))
def density_matrix(mu, cov, post_select=None, normalize=False, cutoff=5, hbar=2):
r"""Returns the density matrix of a (PNR post-selected) Gaussian state.
The resulting density matrix will have shape
.. math:: \underbrace{D\times D \times \cdots \times D}_{2M}
where :math:`D` is the Fock space cutoff, and :math:`M` is the
number of *non* post-selected modes, i.e. ``M = len(mu)//2 - len(post_select)``.
Note that we use the Strawberry Fields convention for indexing the density
matrix; the first two dimensions correspond to subsystem 1, the second two
dimensions correspond to subsystem 2, etc.
If post_select is None then the density matrix elements are calculated using
the multidimensional Hermite polynomials which provide a significantly faster
evaluation.
Args:
mu (array): length-:math:`2N` means vector in xp-ordering
cov (array): :math:`2N\times 2N` covariance matrix in xp-ordering
post_select (dict): dictionary containing the post-selected modes, of
the form ``{mode: value}``. If post_select is None the whole non post-selected density matrix
is calculated directly using (multidimensional) Hermite polynomials, which is significantly faster
than calculating one hafnian at a time.
normalize (bool): If ``True``, a post-selected density matrix is re-normalized.
cutoff (dim): the final length (i.e., Hilbert space dimension) of each
mode in the density matrix.
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
Returns:
np.array[complex]: the density matrix of the Gaussian state
"""
N = len(mu) // 2
pref = prefactor(mu, cov, hbar=hbar)
if post_select is None:
A = Amat(cov, hbar=hbar).conj()
sf_order = tuple(chain.from_iterable([[i, i + N] for i in range(N)]))
if np.allclose(mu, np.zeros_like(mu)):
tensor = np.real_if_close(pref) * hermite_multidimensional(
-A, cutoff, renorm=True, modified=True
)
return tensor.transpose(sf_order)
beta = Beta(mu, hbar=hbar)
y = beta - A @ beta.conj()
tensor = np.real_if_close(pref) * hermite_multidimensional(
-A, cutoff, y=y, renorm=True, modified=True
)
return tensor.transpose(sf_order)
M = N - len(post_select)
rho = np.zeros([cutoff] * (2 * M), dtype=np.complex128)
for idx in product(range(cutoff), repeat=2 * M):
el = []
counter = count(0)
modes = (np.arange(2 * N) % N).tolist()
el = [post_select[i] if i in post_select else idx[next(counter)] for i in modes]
el = np.array(el).reshape(2, -1)
el0 = el[0].tolist()
el1 = el[1].tolist()
sf_idx = np.array(idx).reshape(2, -1)
sf_el = tuple(sf_idx[::-1].T.flatten())
rho[sf_el] = density_matrix_element(mu, cov, el0, el1, include_prefactor=False, hbar=hbar)
rho *= pref
if normalize:
# construct the standard 2D density matrix, and take the trace
new_ax = np.arange(2 * M).reshape([M, 2]).T.flatten()
tr = np.trace(rho.transpose(new_ax).reshape([cutoff ** M, cutoff ** M])).real
# renormalize
rho /= tr
return rho
def pure_state_amplitude(mu, cov, i, include_prefactor=True, tol=1e-10, hbar=2, check_purity=True):
r"""Returns the :math:`\langle i | \psi\rangle` element of the state ket
of a Gaussian state defined by covariance matrix cov.
Args:
mu (array): length-:math:`2N` quadrature displacement vector
cov (array): length-:math:`2N` covariance matrix
i (list): list of amplitude elements
include_prefactor (bool): if ``True``, the prefactor is automatically calculated
used to scale the result.
tol (float): tolerance for determining if displacement is negligible
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
check_purity (bool): if ``True``, the purity of the Gaussian state is checked
before calculating the state vector.
Returns:
complex: the pure state amplitude
"""
if check_purity:
if not is_pure_cov(cov, hbar=hbar, rtol=1e-05, atol=1e-08):
raise ValueError("The covariance matrix does not correspond to a pure state")
rpt = i
beta = Beta(mu, hbar=hbar)
Q = Qmat(cov, hbar=hbar)
A = Amat(cov, hbar=hbar)
(n, _) = cov.shape
N = n // 2
B = A[0:N, 0:N].conj()
alpha = beta[0:N]
if np.linalg.norm(alpha) < tol:
# no displacement
if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
B_rpt = reduction(B, rpt)
haf = hafnian(B_rpt)
else:
haf = hafnian_repeated(B, rpt)
else:
gamma = alpha - B @ np.conj(alpha)
if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
B_rpt = reduction(B, rpt)
np.fill_diagonal(B_rpt, reduction(gamma, rpt))
haf = hafnian(B_rpt, loop=True)
else:
haf = hafnian_repeated(B, rpt, mu=gamma, loop=True)
if include_prefactor:
pref = np.exp(-0.5 * (np.linalg.norm(alpha) ** 2 - alpha @ B @ alpha))
haf *= pref
return haf / np.sqrt(np.prod(fac(rpt)) * np.sqrt(np.linalg.det(Q)))
def state_vector(
mu, cov, post_select=None, normalize=False, cutoff=5, hbar=2, check_purity=True, **kwargs
):
r"""Returns the state vector of a (PNR post-selected) Gaussian state.
The resulting density matrix will have shape
.. math:: \underbrace{D\times D \times \cdots \times D}_M
where :math:`D` is the Fock space cutoff, and :math:`M` is the
number of *non* post-selected modes, i.e. ``M = len(mu)//2 - len(post_select)``.
If post_select is None then the density matrix elements are calculated using
the multidimensional Hermite polynomials which provide a significantly faster
evaluation.
Args:
mu (array): length-:math:`2N` means vector in xp-ordering
cov (array): :math:`2N\times 2N` covariance matrix in xp-ordering
post_select (dict): dictionary containing the post-selected modes, of
the form ``{mode: value}``.
normalize (bool): If ``True``, a post-selected density matrix is re-normalized.
cutoff (dim): the final length (i.e., Hilbert space dimension) of each
mode in the density matrix.
hbar (float): the value of :math:`\hbar` in the commutation
relation :math:`[\x,\p]=i\hbar`.
check_purity (bool): if ``True``, the purity of the Gaussian state is checked
before calculating the state vector.
Keyword Args:
choi_r (float or None): Value of the two-mode squeezing parameter used in Choi-Jamiolkoski
trick in :func:`~.fock_tensor`. This keyword argument should only be used when ``state_vector``
is called by :func:`~.fock_tensor`.
Returns:
np.array[complex]: the state vector of the Gaussian state
"""
if check_purity:
if not is_pure_cov(cov, hbar=hbar, rtol=1e-05, atol=1e-08):
raise ValueError("The covariance matrix does not correspond to a pure state")
beta = Beta(mu, hbar=hbar)
A = Amat(cov, hbar=hbar)
Q = Qmat(cov, hbar=hbar)
(n, _) = cov.shape
N = n // 2
B = A[0:N, 0:N]
alpha = beta[0:N]
gamma = np.conj(alpha) - B @ alpha
prefexp = -0.5 * (np.linalg.norm(alpha) ** 2 - alpha @ B @ alpha)
pref = np.exp(prefexp.conj())
if post_select is None:
choi_r = kwargs.get("choi_r", None)
if choi_r is None:
denom = np.sqrt(np.sqrt(np.linalg.det(Q).real))
else:
rescaling = np.concatenate(
[np.ones([N // 2]), (1.0 / np.tanh(choi_r)) * np.ones([N // 2])]
)
B = np.diag(rescaling) @ B @ np.diag(rescaling)
gamma = rescaling * gamma
denom = np.sqrt(np.sqrt(np.linalg.det(Q / np.cosh(choi_r)).real))
psi = (
np.real_if_close(pref)
* hafnian_batched(B.conj(), cutoff, mu=gamma.conj(), renorm=True)
/ denom
)
else:
M = N - len(post_select)
psi = np.zeros([cutoff] * (M), dtype=np.complex128)
for idx in product(range(cutoff), repeat=M):
el = []
counter = count(0)
modes = (np.arange(N)).tolist()
el = [post_select[i] if i in post_select else idx[next(counter)] for i in modes]
psi[idx] = pure_state_amplitude(
mu, cov, el, check_purity=False, include_prefactor=False, hbar=hbar
)
psi = psi * pref
if normalize:
norm = np.sqrt(np.sum(np.abs(psi) ** 2))
psi = psi / norm
return psi
def mean_number_of_clicks(A):
r""" Given an adjacency matrix this function calculates the mean number of clicks.
For this to make sense the user must provide a matrix with singular values
less than or equal to one. See Appendix A.3 of <https://arxiv.org/abs/1902.00462>`_
by Banchi et al.
Args:
A (array): rescaled adjacency matrix
Returns:
float: mean number of clicks
"""
n, _ = A.shape
idn = np.identity(n)
X = np.block([[0 * idn, idn], [idn, 0 * idn]])
B = np.block([[A, 0 * A], [0 * A, np.conj(A)]])
Q = np.linalg.inv(np.identity(2 * n) - X @ B)
meanc = 1.0 * n
for i in range(n):
det_val = np.real(Q[i, i]*Q[i+n, i+n] - Q[i+n, i]*Q[i, i+n])
meanc -= 1.0 / np.sqrt(det_val)
return meanc
def find_scaling_adjacency_matrix_torontonian(A, c_mean):
r""" Returns the scaling parameter by which the adjacency matrix A
should be rescaled so that the Gaussian state that encodes it has
give a mean number of clicks equal to ``c_mean`` when measured with
threshold detectors.
Args:
A (array): adjacency matrix
c_mean (float): mean photon number of the Gaussian state
Returns:
float: scaling parameter
"""
n, _ = A.shape
if c_mean < 0 or c_mean > n:
raise ValueError("The mean number of clicks should be smaller than the number of modes")
vals = np.linalg.svd(A, compute_uv=False)
localA = A / vals[0] # rescale the matrix so that the singular values are between 0 and 1.
def cost(x):
r""" Cost function giving the difference between the wanted number of clicks and the number
of clicks at a given scaling value. It assumes that the adjacency matrix has been rescaled
so that it has singular values between 0 and 1.
Args:
x (float): scaling value
Return:
float: difference between desired and obtained mean number of clicks
"""
if x >= 1.0:
return c_mean - n
if x <= 0:
return c_mean
return c_mean - mean_number_of_clicks(x * localA)
res = root_scalar(cost, x0=0.5, bracket=(0.0, 1.0)) # Do the optimization
if not res.converged:
raise ValueError("The search for a scaling value failed")
return res.root / vals[0]
def find_scaling_adjacency_matrix(A, n_mean):
r""" Returns the scaling parameter by which the adjacency matrix A
should be rescaled so that the Gaussian state that endodes it has
a total mean photon number n_mean.
Args:
A (array): Adjacency matrix
n_mean (float): Mean photon number of the Gaussian state
Returns:
float: Scaling parameter
"""
eps = 1e-10
ls = np.linalg.svd(A, compute_uv=False)
max_sv = ls[0]
a_lim = 0.0
b_lim = 1.0 / (eps + max_sv)
x_init = 0.5 * b_lim
if 1000 * eps >= max_sv:
raise ValueError("The singular values of the matrix A are too small.")
def mean_photon_number(x, vals):
r""" Returns the mean number of photons in the Gaussian state that
encodes the adjacency matrix x*A where vals are the singular values of A
Args:
x (float): Scaling parameter
vals (array): Singular values of the matrix A
Returns:
n_mean: Mean photon number in the Gaussian state
"""
vals2 = (x * vals) ** 2
n = np.sum(vals2 / (1.0 - vals2))
return n
# The following function is implicitly tested in test_find_scaling_adjacency_matrix
def grad_mean_photon_number(x, vals): # pragma: no cover
r""" Returns the gradient od the mean number of photons in the Gaussian state that
encodes the adjacency matrix x*A with respect to x.
vals are the singular values of A
Args:
x (float): Scaling parameter
vals (array): Singular values of the matrix A
Returns:
d_n_mean: Derivative of the mean photon number in the Gaussian state
with respect to x
"""
vals1 = vals * x
dn = (2.0 / x) * np.sum((vals1 / (1 - vals1 ** 2)) ** 2)
return dn
f = lambda x: mean_photon_number(x, ls) - n_mean
df = lambda x: grad_mean_photon_number(x, ls)
res = root_scalar(f, fprime=df, x0=x_init, bracket=(a_lim, b_lim))
if not res.converged:
raise ValueError("The search for a scaling value failed")
return res.root
def gen_Qmat_from_graph(A, n_mean):
r""" Returns the Qmat xp-covariance matrix associated to a graph with
adjacency matrix :math:`A` and with mean photon number :math:`n_{mean}`.
Args:
A (array): a :math:`N\times N` ``np.float64`` (symmetric) adjacency matrix
n_mean (float): mean photon number of the Gaussian state
Returns:
array: the :math:`2N\times 2N` Q matrix.
"""
n, m = A.shape
if n != m:
raise ValueError("Matrix must be square.")
sc = find_scaling_adjacency_matrix(A, n_mean)
Asc = sc * A
A = np.block([[Asc, 0 * Asc], [0 * Asc, Asc.conj()]])
I = np.identity(2 * n)
X = Xmat(n)
Q = np.linalg.inv(I - X @ A)
return Q
def photon_number_mean(mu, cov, j, hbar=2):
r""" Calculate the mean photon number of mode j of a Gaussian state.
Args:
mu (array): vector of means of the Gaussian state using the ordering
:math:`[q_1, q_2, \dots, q_n, p_1, p_2, \dots, p_n]`
cov (array): the covariance matrix of the Gaussian state
j (int): the j :sup:`th` mode
hbar (float): the ``hbar`` convention used in the commutation
relation :math:`[q, p]=i\hbar`
Returns:
float: the mean photon number in mode :math:`j`.
"""
num_modes = len(mu) // 2
return (
mu[j] ** 2
+ mu[j + num_modes] ** 2
+ cov[j, j]
+ cov[j + num_modes, j + num_modes]
- hbar
) / (2 * hbar)
def photon_number_covar(mu, cov, j, k, hbar=2):
r""" Calculate the variance/covariance of the photon number distribution
of a Gaussian state.
Implements the covariance matrix of the photon number distribution of a
Gaussian state according to the Last two eq. of Part II. in
`'Multidimensional Hermite polynomials and photon distribution for polymode
mixed light', Dodonov et al. <https://journals.aps.org/pra/abstract/10.1103/PhysRevA.50.813>`_
.. math::
\sigma_{n_j n_j} &= \frac{1}{2}\left(T_j^2 - 2d_j - \frac{1}{2}\right)
+ \left<\mathbf{Q}_j\right>\mathcal{M}_j\left<\mathbf{Q}_j\right>, \\
\sigma_{n_j n_k} &= \frac{1}{2}\mathrm{Tr}\left(\Lambda_j \mathbf{M} \Lambda_k \mathbf{M}\right)
+ \frac{1}{2}\left<\mathbf{Q}\right>\Lambda_j \mathbf{M} \Lambda_k\left<\mathbf{Q}\right>,
where :math:`T_j` and :math:`d_j` are the trace and the determinant of
:math:`2 \times 2` matrix :math:`\mathcal{M}_j` whose elements coincide
with the nonzero elements of matrix :math:`\mathbf{M}_j = \Lambda_j \mathbf{M} \Lambda_k`
while the two-vector :math:`\mathbf{Q}_j` has the components :math:`(q_j, p_j)`.
:math:`2N \times 2N` projector matrix :math:`\Lambda_j` has only two nonzero
elements: :math:`\left(\Lambda_j\right)_{jj} = \left(\Lambda_j\right)_{j+N,j+N} = 1`.
Note that the convention for ``mu`` used here differs from the one used in Dodonov et al.,
They both provide the same results in this particular case.
Args:
mu (array): vector of means of the Gaussian state using the ordering
:math:`[q_1, q_2, \dots, q_n, p_1, p_2, \dots, p_n]`
cov (array): the covariance matrix of the Gaussian state
j (int): the j :sup:`th` mode
k (int): the k :sup:`th` mode
hbar (float): the ``hbar`` convention used in the commutation
relation :math:`[q, p]=i\hbar`
Returns:
float: the covariance for the photon numbers at modes :math:`j` and :math:`k`.
"""
# renormalise the covariance matrix
cov = cov / hbar
N = len(mu) // 2
mu = np.array(mu) / np.sqrt(hbar)
lambda_1 = np.zeros((2 * N, 2 * N))
lambda_1[j, j] = lambda_1[j + N, j + N] = 1
lambda_2 = np.zeros((2 * N, 2 * N))
lambda_2[k, k] = lambda_2[k + N, k + N] = 1
if j == k:
idxs = ((j, j, j + N, j + N), (j, j + N, j, j + N))
M = (lambda_1 @ cov @ lambda_2)[idxs].reshape(2, 2)
term_1 = (np.trace(M) ** 2 - 2 * np.linalg.det(M) - 0.5) / 2
term_2 = mu[[j, j + N]] @ M @ mu[[j, j + N]]
else:
term_1 = np.trace(lambda_1 @ cov @ lambda_2 @ cov) / 2
term_2 = (mu @ lambda_1 @ cov @ lambda_2 @ mu) / 2
return term_1 + term_2
def photon_number_covmat(mu, cov, hbar=2):
r""" Calculate the covariance matrix of the photon number distribution of a
Gaussian state.
Args:
mu (array): vector of means of the Gaussian state using the ordering
:math:`[q_1, q_2, \dots, q_n, p_1, p_2, \dots, p_n]`
cov (array): the covariance matrix of the Gaussian state
hbar (float): the ``hbar`` convention used in the commutation
relation :math:`[q, p]=i\hbar`
Returns:
array: the covariance matrix of the photon number distribution
"""
N = len(mu) // 2
pnd_cov = np.zeros((N, N))
for i in range(N):
for j in range(i+1):
pnd_cov[i][j] = photon_number_covar(mu, cov, i, j, hbar=hbar)
pnd_cov[j][i] = pnd_cov[i][j]
return pnd_cov
def photon_number_mean_vector(mu, cov, hbar=2):
r""" Calculate the mean photon number of each of the modes in a Gaussian state
Args:
mu (array): vector of means of the Gaussian state using the ordering
:math:`[q_1, q_2, \dots, q_n, p_1, p_2, \dots, p_n]`
cov (array): the covariance matrix of the Gaussian state
hbar (float): the ``hbar`` convention used in the commutation
relation :math:`[q, p]=i\hbar`
Returns:
array: the vector of means of the photon number distribution
"""
N = len(mu) // 2
return np.array([photon_number_mean(mu, cov, j, hbar=hbar) for j in range(N)])
def is_valid_cov(cov, hbar=2, rtol=1e-05, atol=1e-08):
r""" Checks if the covariance matrix is a valid quantum covariance matrix.
Args:
cov (array): a covariance matrix
hbar (float): value of hbar in the uncertainty relation
rtol (float): the relative tolerance parameter used in `np.allclose`
atol (float): the absolute tolerance parameter used in `np.allclose`
Returns:
(bool): whether the given covariance matrix is a valid covariance matrix
"""
(n, m) = cov.shape
if n != m:
# raise ValueError("The input matrix must be square")
return False
if not np.allclose(cov, np.transpose(cov), rtol=rtol, atol=atol):
# raise ValueError("The input matrix is not symmetric")
return False
if n % 2 != 0:
# raise ValueError("The input matrix is of even dimension")
return False
nmodes = n // 2
vals = np.linalg.eigvalsh(cov + 0.5j * hbar * sympmat(nmodes))
vals[np.abs(vals) < atol] = 0.0
if np.all(vals >= 0):
# raise ValueError("The input matrix violates the uncertainty relation")
return True
return False
def is_pure_cov(cov, hbar=2, rtol=1e-05, atol=1e-08):
r""" Checks if the covariance matrix is a valid quantum covariance matrix
that corresponds to a quantum pure state
Args:
cov (array): a covariance matrix
hbar (float): value of hbar in the uncertainty relation
rtol (float): the relative tolerance parameter used in `np.allclose`
atol (float): the absolute tolerance parameter used in `np.allclose`
Returns:
(bool): whether the given covariance matrix corresponds to a pure state
"""
if is_valid_cov(cov, hbar=hbar, rtol=rtol, atol=atol):
purity = 1 / np.sqrt(np.linalg.det(2 * cov / hbar))
if np.allclose(purity, 1.0, rtol=rtol, atol=atol):
return True
return False
def is_classical_cov(cov, hbar=2, atol=1e-08):
r""" Checks if the covariance matrix can be efficiently sampled.
Args:
cov (array): a covariance matrix
hbar (float): value of hbar in the uncertainty relation
Returns:
(bool): whether the given covariance matrix corresponds to a classical state
"""
if is_valid_cov(cov, hbar=hbar, atol=atol):
(n, _) = cov.shape
vals = np.linalg.eigvalsh(cov - 0.5 * hbar * np.identity(n))
vals[np.abs(vals) < atol] = 0.0
if np.all(vals >= 0):
return True
return False
def gen_single_mode_dist(s, cutoff=50, N=1):
"""Generate the photon number distribution of :math:`N` identical single mode squeezed states.
Args:
s (float): squeezing parameter
cutoff (int): Fock cutoff
N (float): number of squeezed states
Returns:
(array): Photon number distribution
"""
r = 0.5 * N
q = 1.0 - np.tanh(s) ** 2
N = cutoff // 2
ps_tot = np.zeros(cutoff)
if cutoff % 2 == 0:
ps = nbinom.pmf(np.arange(N), p=q, n=r)
ps_tot[0::2] = ps
else:
ps = nbinom.pmf(np.arange(N + 1), p=q, n=r)
ps_tot[0:-1][0::2] = ps[0:-1]
ps_tot[-1] = ps[-1]
return ps_tot
def gen_multi_mode_dist(s, cutoff=50, padding_factor=2):
"""Generates the total photon number distribution of single mode squeezed states with different squeezing values.
Args:
s (array): array of squeezing parameters
cutoff (int): Fock cutoff
Returns:
(array[int]): total photon number distribution
"""
scale = padding_factor
cutoff_sc = scale * cutoff
ps = np.zeros(cutoff_sc)
ps[0] = 1.0
for s_val in s:
ps = np.convolve(ps, gen_single_mode_dist(s_val, cutoff_sc))[0:cutoff_sc]
return ps
def total_photon_num_dist_pure_state(cov, cutoff=50, hbar=2, padding_factor=2):
r""" Calculates the total photon number distribution of a pure state
with zero mean.
Args: