Merge branch 'main' into dec2023rap

README.md

@@ -37,7 +37,7 @@ h2.run_pyscf()
 hamiltonian = h2.hamiltonian()
 
 # Build a UCC circuit ansatz for running VQE
-circuit = hf_circuit(h2.nso, sum(h2.nelec))
+circuit = hf_circuit(h2.nso, h2.nelec)
 circuit += ucc_circuit(h2.nso, [0, 1, 2, 3])
 
 # Create and run the VQE, starting with random initial parameters

doc/source/api-reference/ansatz.rst

@@ -23,6 +23,9 @@ Unitary Coupled Cluster
 
 .. autofunction:: qibochem.ansatz.ucc.ucc_ansatz
 
+.. autofunction:: qibochem.ansatz.qeb.qeb_circuit
+
+
 Basis rotation
 --------------
 

doc/source/tutorials/ansatz.rst

@@ -138,6 +138,41 @@ An example of how to build a UCC doubles circuit ansatz for the :math:`H_2` mole
     q3: ... ─────o─RX─RX─o────────────o─RX─
 
 
+
+UCC with Qubit-Excitation-Based n-tuple Excitation
+--------------------------------------------------
+
+A CNOT depth-efficient quantum circuit for employing the UCC ansatz, dubbed the Qubit-Excitation-Based (QEB) n-tuple excitations for UCC, was constructed by Yordanov et al. [#f7]_ and Magoulas et al. [#f8]_, avoiding the exponential number of CNOT cascades in those developed before. [#f5]_ The quantum circuits generated for :math:`N` qubits have a reduction of CNOTs from :math:`(2N-1)2^{2N}` to :math:`2^{2N-1}+4N-2`.
+
+An example for the :math:`H_2` molecule is given here:
+
+
+.. code-block:: python
+
+    from qibochem.driver.molecule import Molecule
+    from qibochem.ansatz import hf_circuit, qeb_circuit
+
+    mol = Molecule([("H", (0.0, 0.0, 0.0)), ("H", (0.0, 0.0, 0.74804))])
+    mol.run_pyscf()
+    hamiltonian = mol.hamiltonian()
+
+    # Set parameters for the rest of the experiment
+    n_qubits = mol.nso
+    n_electrons = mol.nelec
+
+    # Build UCCD circuit
+    circuit = hf_circuit(n_qubits, n_electrons) # Start with HF circuit
+    circuit += qeb_circuit(n_qubits, [0, 1, 2, 3]) # Then add the double excitation circuit ansatz
+
+    print(circuit.draw())
+
+.. code-block:: output
+
+    q0: ─X─X─────X─o──X─────X─
+    q1: ─X─o───X───o────X───o─
+    q2: ─────X─|─X─o──X─|─X───
+    q3: ─────o─o───RY───o─o───
+
 ..
    _Basis rotation ansatz
 
@@ -233,3 +268,7 @@ The orthonormal molecular orbitals :math:`\phi` are optimized by a direct minimi
 .. [#f6] Piela, L. (2007). 'Ideas of Quantum Chemistry'. Elsevier B. V., the Netherlands.
 
 .. [#f7] Clements, W. R. et al., 'Optimal Design for Universal Multiport Interferometers', Optica 3 (2016) 1460.
+
+.. [#f8] Yordanov Y. S. et al., 'Efficient Quantum Circuits for Quantum Computational Chemistry', Phys Rev A 102 (2020) 062612.
+
+.. [#f9] Magoulas, I. and Evangelista, F. A., 'CNOT-Efficient Circuits for Arbitrary Rank Many-Body Fermionic and Qubit Excitations', J. Chem. Theory Comput. 19 (2023) 822.

src/qibochem/ansatz/__init__.py

@@ -5,6 +5,7 @@
 )
 from qibochem.ansatz.hardware_efficient import he_circuit
 from qibochem.ansatz.hf_reference import hf_circuit
+from qibochem.ansatz.qeb import qeb_circuit
 from qibochem.ansatz.symmetry import symm_preserving_circuit
 from qibochem.ansatz.ucc import ucc_ansatz, ucc_circuit
 from qibochem.ansatz.util import generate_excitations, mp2_amplitude, sort_excitations

src/qibochem/ansatz/qeb.py

@@ -0,0 +1,43 @@
+from qibo import Circuit, gates
+
+
+def qeb_circuit(n_qubits, excitation, theta=0.0) -> Circuit:
+    r"""
+    Qubit-excitation-based (QEB) circuit corresponding to the unitary coupled-cluster ansatz for a single excitation
+
+    Supports only Jordan-Wigner encoded circuits
+
+    Ref: arXiv:2210.05771
+
+    Args:
+        n_qubits: Number of qubits in the quantum circuit
+        excitation: Iterable of orbitals involved in the excitation; must have an even number of elements
+            E.g. ``[0, 1, 2, 3]`` represents the excitation of electrons in orbitals ``(0, 1)`` to ``(2, 3)``
+        theta: UCC parameter. Defaults to 0.0
+        trotter_steps: Number of Trotter steps; i.e. number of times this ansatz is applied with ``theta`` = ``theta / trotter_steps``. Default: 1
+
+    Returns:
+        Qibo ``Circuit``: Circuit corresponding to a single UCC excitation
+    """
+
+    n_orbitals = len(excitation)
+    assert n_orbitals % 2 == 0, f"{excitation} must have an even number of items"
+
+    n_tuples = len(excitation) // 2
+    i_array = excitation[:n_tuples]
+    a_array = excitation[n_tuples:]
+    fwd_gates = [gates.CNOT(i_array[-1], _i) for _i in i_array[-2::-1]]
+    fwd_gates += [gates.CNOT(a_array[-1], _a) for _a in a_array[-2::-1]]
+    fwd_gates.append(gates.CNOT(a_array[-1], i_array[-1]))
+    fwd_gates += [gates.X(_ia) for _ia in excitation if _ia not in (i_array[-1], a_array[-1])]
+    circuit = Circuit(n_qubits)
+    circuit.add(gate for gate in fwd_gates)
+    # MCRY
+    # multi-controlled RY gate,
+    # negative controls i, a
+    # positive control on i_n
+    mcry_controls = excitation[:-1]
+    ry_angle = 2.0 * theta
+    circuit.add(gates.RY(a_array[-1], ry_angle).controlled_by(*mcry_controls))
+    circuit.add(gate for gate in fwd_gates[::-1])
+    return circuit

tests/test_ucc.py

@@ -10,6 +10,7 @@
 from qibo.hamiltonians import SymbolicHamiltonian
 
 from qibochem.ansatz import hf_circuit
+from qibochem.ansatz.qeb import qeb_circuit
 from qibochem.ansatz.ucc import expi_pauli, ucc_ansatz, ucc_circuit
 from qibochem.ansatz.util import generate_excitations, mp2_amplitude, sort_excitations
 from qibochem.driver import Molecule
@@ -95,6 +96,51 @@ def test_ucc_circuit(excitation, mapping, basis_rotations, coeffs):
     assert len(test_circuit.get_parameters()) == len(basis_rotations)
 
 
+@pytest.mark.parametrize(
+    "excitation,mapping,basis_rotations,coeffs",
+    [
+        ([0, 2], None, ("Y0 X2", "X0 Y2"), (0.5, -0.5)),  # JW singles
+        (
+            [0, 1, 2, 3],
+            None,
+            (
+                "X0 X1 Y2 X3",
+                "Y0 Y1 Y2 X3",
+                "Y0 X1 X2 X3",
+                "X0 Y1 X2 X3",
+                "Y0 X1 Y2 Y3",
+                "X0 Y1 Y2 Y3",
+                "X0 X1 X2 Y3",
+                "Y0 Y1 X2 Y3",
+            ),
+            (-0.25, 0.25, 0.25, 0.25, -0.25, -0.25, -0.25, 0.25),
+        ),  # JW doubles
+    ],
+)
+def test_qeb_circuit(excitation, mapping, basis_rotations, coeffs):
+    """Build QEB circuit"""
+    theta = 0.1
+    n_qubits = 4
+
+    # Build the control array using SymbolicHamiltonian.circuit
+    # But need to multiply theta by some coefficient introduced by the fermion->qubit mapping
+    control_circuit = Circuit(n_qubits)
+    for coeff, basis_rotation in zip(coeffs, basis_rotations):
+        n_terms = len(basis_rotation)
+        pauli_term = SymbolicHamiltonian(
+            symbols.I(n_qubits - 1)
+            * reduce(lambda x, y: x * y, (getattr(symbols, _op)(int(qubit)) for _op, qubit in basis_rotation.split()))
+        )
+        control_circuit += pauli_term.circuit(-coeff * theta)
+    control_result = control_circuit(nshots=1)
+    control_state = control_result.state(True)
+
+    test_circuit = qeb_circuit(n_qubits, excitation, theta=theta)
+    test_result = test_circuit(nshots=1)
+    test_state = test_result.state(True)
+    assert np.allclose(control_state, test_state)
+
+
 def test_ucc_ferm_qubit_map_error():
     """If unknown fermion to qubit map used"""
     with pytest.raises(KeyError):