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I have a Hamiltonian as following $$ H= \sum_{i}^{n-1}(c_nX_iX_{i+1}+b_nZ_iZ_{i+1})+\sum_{i=1}^{n}b_nY$$ where $n:$ number of qubits.

I want to define this Hamiltonian in qiskit and use it as an observable to calculate the expectation value in an eigensolver. I have implemented it in OpenFermion:

def create_xy_hamiltonian(nqubit: int, cn: float, bn: float, r: float) -> Observable:
    """
    Args:
        nqubit: `int`, number of qubits
        cn: `float`, 0 - 1, coupling constant
        bn: `float`, 0 - 1, magnetic field
        r: `float`, 0 - 1, anisotropy param
    
    Returns:
        qulacs observable
    """
    hami = QubitOperator()
    for i in range(nqubit-1):
        hami += (0.5*cn*(1+r)) * QubitOperator(f"X{i} X{i+1}")
        #hami += (0.5*cn*(1-r)) * QubitOperator(f"Y{i} Y{i+1}")
        hami += (0.5*cn*(1-r)) * QubitOperator(f"Z{i} Z{i+1}")
    for j in range(nqubit):
        hami += bn*QubitOperator(f"Y{j}")

    return (hami)```
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1 Answer 1

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In Qiskit, observables as usually defined as instances of SparsePauliOp class. Its newly added static method from_sparse_list() makes it easy to construct Hamiltonians where each term acts non-trivially on a very few number of qubits.

sparse_list = []
for m in range(num_qubits - 1):
    sparse_list.append(("XX", [m, m + 1], cn))
    sparse_list.append(("ZZ", [m, m + 1], bn))

for m in range(num_qubits):
    sparse_list.append(("Y", [m], bn))

hamiltonian = SparsePauliOp.from_sparse_list(sparse_list, num_qubits=num_qubits)

print(hamiltonian)
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  • $\begingroup$ Thank you so much for your solution. Really appreciate it. $\endgroup$
    – Bakaneko
    Commented May 27 at 8:01

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