openfermion.ops.QuadOperator

QuadOperator stores a sum of products of canonical quadrature operators.

Inherits From: SymbolicOperator

Used in the notebooks

Used in the tutorials

They are defined in terms of the bosonic ladder operators: q = sqrt{hbar/2}(b+b^) p = -isqrt{hbar/2}(b-b^) where hbar is a constant appearing in the commutator of q and p: [q, p] = i hbar

In OpenFermion, we describe the canonical quadrature operators acting on quantum modes 'i' and 'j' using the shorthand: 'qi' = q_i 'pj' = p_j where ['qi', 'pj'] = i hbar delta_ij is the commutator.

The QuadOperator class is designed (in general) to store sums of these terms. For instance, an instance of QuadOperator might represent

.. code-block:: python

H = 0.5 * QuadOperator('q0 p5') + 0.3 * QuadOperator('q0')

Note for a QuadOperator to be a Hamiltonian which is a hermitian operator, the coefficients of all terms must be real.

QuadOperator is a subclass of SymbolicOperator. Importantly, it has attributes set as follows::

actions = ('q', 'p')
action_strings = ('q', 'p')
action_before_index = True
different_indices_commute = True

See the documentation of SymbolicOperator for more details.

Example:

.. code-block:: python

H = (QuadOperator('p0 q3', 0.5)

       + 0.6 * QuadOperator('p3 q0'))
# Equivalently
H2 = QuadOperator('p0 q3', 0.5)
H2 += QuadOperator('p3 q0', 0.6)

Note:

Adding QuadOperator is faster using += (as this is done by in-place addition). Specifying the coefficient during initialization is faster than multiplying a QuadOperator with a scalar.

action_before_index Whether action comes before index in string representations.
action_strings The string representations of the allowed actions.
actions The allowed actions.
constant The value of the constant term.
different_indices_commute Whether factors acting on different indices commute.

Methods

accumulate

View source

Sums over SymbolicOperators.

compress

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Eliminates all terms with coefficients close to zero and removes small imaginary and real parts.

Args
abs_tol(float): Absolute tolerance, must be at least 0.0

get_operator_groups

View source

Gets a list of operators with a few terms.

Args
num_groups(int): How many operators to get in the end.

Returns
operators([self.class]): A list of operators summing up to self.

get_operators

View source

Gets a list of operators with a single term.

Returns
operators([self.class]): A generator of the operators in self.

identity

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Returns: multiplicative_identity (SymbolicOperator): A symbolic operator u with the property that ux = xu = x for all operators x of the same class.

induced_norm

View source

Compute the induced p-norm of the operator.

If we represent an operator as :math: \sum_{j} w_j H_j where :math: w_j are scalar coefficients then this norm is :math: \left(\sum_{j} \| w_j \|^p \right)^{\frac{1}{p} } where :math:p` is the order of the induced norm

Args
order(int): the order of the induced norm.

is_gaussian

View source

Query whether the term is quadratic or lower in the quadrature operators.

is_normal_ordered

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Return whether or not term is in normal order.

In our convention, q operators come first. Note that unlike the Fermion operator, due to the commutation of quadrature operators with different indices, the QuadOperator sorts quadrature operators by index.

isclose

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Check if other (SymbolicOperator) is close to self.

Comparison is done for each term individually. Return True if the difference between each term in self and other is less than EQ_TOLERANCE

Args
other(SymbolicOperator): SymbolicOperator to compare against.

many_body_order

View source

Compute the many-body order of a SymbolicOperator.

The many-body order of a SymbolicOperator is the maximum length of a term with nonzero coefficient.

Returns
int

zero

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Returns: additive_identity (SymbolicOperator): A symbolic operator o with the property that o+x = x+o = x for all operators x of the same class.

__add__

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Args: addend (SymbolicOperator): The operator to add.

Returns
sum (SymbolicOperator)

__div__

View source

For compatibility with Python 2.

__eq__

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Approximate numerical equality (not true equality).

__iter__

View source

__mul__

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Return self * multiplier for a scalar, or a SymbolicOperator.

Args
multiplier A scalar, or a SymbolicOperator.

Returns
product (SymbolicOperator)

Raises
TypeError Invalid type cannot be multiply with SymbolicOperator.

__ne__

View source

Return self!=value.

__neg__

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Returns: negation (SymbolicOperator)

__pow__

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Exponentiate the SymbolicOperator.

Args
exponent (int): The exponent with which to raise the operator.

Returns
exponentiated (SymbolicOperator)

Raises
ValueError Can only raise SymbolicOperator to non-negative integer powers.

__radd__

View source

Args: addend (SymbolicOperator): The operator to add.

Returns
sum (SymbolicOperator)

__rmul__

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Return multiplier * self for a scalar.

We only define rmul for scalars because the left multiply exist for SymbolicOperator and left multiply is also queried as the default behavior.

Args
multiplier A scalar to multiply by.

Returns
product A new instance of SymbolicOperator.

Raises
TypeError Object of invalid type cannot multiply SymbolicOperator.

__rsub__

View source

Args: subtrahend (SymbolicOperator): The operator to subtract.

Returns
difference (SymbolicOperator)

__sub__

View source

Args: subtrahend (SymbolicOperator): The operator to subtract.

Returns
difference (SymbolicOperator)

__truediv__

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Return self / divisor for a scalar.

Note:

This is always floating point division.

Args
divisor A scalar to divide by.

Returns
A new instance of SymbolicOperator.

Raises
TypeError Cannot divide local operator by non-scalar type.