There are several Tutorials from Qiskit demonstrating the use of PySCF driver. Usually, it takes as input the geometric structure of the molecule -> second quantised-operators -> JW or Parity mapping -> VQE.

However, I do not want to input a molecule but other Fermionic based operators, e.g. Fermi-Hubbard model. How do I do that?


You can do this through FermionicOperator, it is Deprecated in the newer update of qiskit though so you will get warning about this issue.

Here is an example, I just made up some random second-quantized fermionic one-body operator (h1) to demonstrate how this works under the parity mapping:

b = np.random.rand(2,2)
one_body_operator = (b + b.T)/2
ferOp = FermionicOperator(h1=one_body_operator )
qubitOp = ferOp.mapping(map_type='parity' , threshold=0.00000001)
print( qubitOp.print_details() )

which gives you the qubits representation:

II  (0.4098900912867185+0j)
IZ  (-0.23235537601756617+0j)
ZX  (-0.12228344289218121+0j)
IX  (0.12228344289218121+0j)
ZZ  (-0.17753471526915232+0j)

If you want to use Jordan-Wigner, then specify map_type = 'jordan_wigner' . Similarly, map_type = 'bravyi_kitaev' for Bravyi Kitaev mapping.

Here is the link to Qiskit Ferminonic operator for more details.

  • $\begingroup$ Really helpful, thanks a lot! Just a further clarification: Your example uses a given matrix, however, how would the solution look like for string manipulations? Because I cannot scale up using your method, or can I? $\endgroup$ – quantumdip May 30 at 17:20

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