qml.fermi.from_string

from_string(fermi_string)

Return a fermionic operator object from its string representation.

The string representation is a compact format that uses the orbital index and '+' or '-' symbols to indicate creation and annihilation operators, respectively. For instance, the string representation for the operator \(a^{\dagger}_0 a_1 a^{\dagger}_0 a_1\) is '0+ 1- 0+ 1-'. The '-' symbols can be optionally dropped such that '0+ 1 0+ 1' represents the same operator. The format commonly used in OpenFermion to represent the same operator, '0^ 1 0^ 1' , is also supported.

Parameters:

fermi_string (str) – string representation of the fermionic object

Returns:

the fermionic operator object

Return type:

FermiWord

Example

>>> from pennylane.fermi import from_string
>>> print(from_string('0+ 1- 0+ 1-'))
a⁺(0) a(1) a⁺(0) a(1)

>>> print(from_string('0+ 1 0+ 1'))
a⁺(0) a(1) a⁺(0) a(1)

>>> print(from_string('0^ 1 0^ 1'))
a⁺(0) a(1) a⁺(0) a(1)

>>> op1 = qml.FermiC(0) * qml.FermiA(1) * qml.FermiC(2) * qml.FermiA(3)
>>> op2 = from_string('0+ 1- 2+ 3-')
>>> op1 == op2
True

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