pychemiq.Utils#

Module Contents#

Functions#

pychemiq.Utils.get_cc_n_term(n_qubits, n_elec, excited_level)#

Obtain the number of coupled cluster operator terms at the specified excitation level. For example, for four qubits, the single and double excited coupled cluster operator of a 2-electron system, with spin orbitals 0 and 1 as occupied states, has five coupled cluster terms: 0->2,0->3,1->2,1->3,01->23.

Parameters:

  • n_qubits (int) – Enter the number of qubits required for calculation.

  • n_elec (int) – Enter the number of electrons of the molecular system.

  • excited_level (str) – The excitation level of the input coupled cluster operator. Currently available: CCS, CCD, CCSD.

Returns:

Output the number of coupled cluster operator terms at the specified excitation level. Integer type.

pychemiq.Utils.get_cc(n_qubits, n_elec, para, excited_level='SD')#

Obtain the coupled cluster operator for the specified excitation level with parameters. For example, for four qubits, the single and double excited coupled cluster operator of a 2-electron system has spin orbitals 0 and 1 as occupied states, so the excited coupled cluster term is: 0->2,0->3,1->2,1->3,01->23. The output Fermion operators are: {{“2+0”: para [0]}, {“3+0”: para [1]}, {“2+1”: para [2]}, {“3+1”: para [3]}, {“3+2+10”: para [4]}

Parameters:

  • n_qubits (int) – Enter the number of qubits required for calculation.

  • n_elec (int) – Enter the number of electrons of the molecular system.

  • para (list[float]) – Enter the initial parameter list.

  • excited_level (str) – The excitation level of the input coupled cluster operator. Currently available: CCS, CCD, CCSD. The default is the single double excited coupled cluster operator (CCSD).

Returns:

Output the coupled cluster operator for the specified excitation level. Fermion operator class.

pychemiq.Utils.transCC2UCC(Pauli)#

Only unitary operators can be placed on quantum circuits for simulation. On the basis of the coupled cluster operator, this function removes the operator that is not a Hermitian matrix and constructs a “unitary operator version” of the coupled cluster operator.

Parameters:

Pauli (PauliOperator) – Input coupled cluster operator. Pauli operator class.

Returns:

Output the unitary coupled cluster operator. Pauli operator class.

Interface example:

In the following example, we calculate the single and double excited coupled cluster operators for 4 quantum bits and 2 electron systems, and convert them into unitary coupled cluster operators that can directly construct quantum circuits.

from pychemiq.Utils import get_cc_n_term,get_cc,transCC2UCC
from pychemiq.Transform.Mapping import jordan_wigner
import numpy as np

# Calculate the number of terms for the single and double excited coupled cluster operators of 4 quantum bits and 2 electron systems to initialize the parameter list. Here, we first set the initial parameters to a list of all 1
n_para = get_cc_n_term(4,2,"CCSD")
para = np.ones(n_para)

# After printing and setting the initial parameters, the single and double excited coupled cluster operator for a 4-qubit, 2-electron system
cc_fermion = get_cc(4,2,para,"CCSD")
print(cc_fermion)

The printed result is:

{
2+ 0 : 1.000000
3+ 0 : 1.000000
2+ 1 : 1.000000
3+ 1 : 1.000000
3+ 2+ 1 0 : 1.000000
}

# Next, use JW mapping to map the Fermion operator to the Pauli operator
cc_pauli = jordan_wigner(cc_fermion)
# Delete the non unitary coupled cluster operator and leave the unitary coupled cluster operator using transCC2UCC function
ucc_pauli = transCC2UCC(cc_pauli)
print(ucc_pauli)

The printed result is:

{
"X0 X1 X2 Y3" : -0.125000,
"X0 X1 Y2 X3" : -0.125000,
"X0 Y1 X2 X3" : 0.125000,
"X0 Y1 Y2 Y3" : -0.125000,
"X0 Z1 Y2" : 0.500000,
"X0 Z1 Z2 Y3" : 0.500000,
"X1 Y2" : 0.500000,
"X1 Z2 Y3" : 0.500000,
"Y0 X1 X2 X3" : 0.125000,
"Y0 X1 Y2 Y3" : -0.125000,
"Y0 Y1 X2 Y3" : 0.125000,
"Y0 Y1 Y2 X3" : 0.125000,
"Y0 Z1 X2" : -0.500000,
"Y0 Z1 Z2 X3" : -0.500000,
"Y1 X2" : -0.500000,
"Y1 Z2 X3" : -0.500000
}