Understand open-shell versus closed-shell options and the effect on subspace construction

In this guide, we will show how to choose subspace dimensions in the qiskit_addon_sqd package to post-process quantum samples using the self-consistent configuration recovery technique.

More importantly, this guide also highlights some differences in the behavior in the subspace construction when run in open_shell = False or open_shell = True modes:

open_shell = False only works when the number of spin-up and spin-down electrons is the same.
open_shell = True must be used when the number of spin-up and spin-down electrons is different. It can also be used when the number of spin-up and spin-down electrons is the same. However, in this last case, there is a difference in the sizes of the subspaces generated between open_shell = False and open_shell = True, as discussed in this guide.

NOTE: Some of the electronic-configuration (bitstring) manipulations in this package have as a goal to preserve the total spin symmetry $S^2$ . Standard Selected Configuration Interaction (SCI) solvers cannot impose $S^2$ conservation exactly. Consequently, they do so approximately via a Lagrange multiplier.

The choice of electronic configurations entering the eigenstate solver can also have a strong effect in the conservation of spin. For example, in a (2-electron, 2-orbital) system, one can sample the configuration $|1001\rangle$ (having a single spin-up excitation over the RHF state $|0101\rangle$ ), which is a linear combination of the open-shell singlet and triplet states, respectively $(|1001\rangle ± |0110\rangle) /\sqrt{2}$ . If the configuration |0110⟩ is not sampled, one can construct neither eigenfunction of total spin, leading to spin contamination or redundancy (the configuration |1001⟩ is involved in a CI calculation, but has coefficient 0 in the CI vector). Given that a single sample $|1001\rangle$ is generated in the quantum computer, this is how the sqd package handles this situation:

open_shell = False:
1. The $1001$ bitstring is split in half, representing spin-up and spin-down configurations: $10$ (up) and $01$ (down).
2. The list of unique spin-polarized configurations is constructed: $\mathcal{U} = [01, 10]$ .
3. We then consider all possible combinations of $\mathcal{U}$ elements to form the basis: $\left \{ |0101\rangle, |0110\rangle , |1001\rangle , |1010\rangle \right \}$ , which contains the singlet and triplet states.
open_shell = True:
1. Contrary to the open_shell = False case, we do not combine the halves of the bitstring to form the basis.

Closed-shell

This example shows how the bitstrings are manipulated in a (2-electron, 4-orbital) system.

# Specify molecule properties
num_orbitals = 4
num_elec_a = num_elec_b = 1
open_shell = False

Specify by hand a dictionary of measurement outcomes:

counts_dict = {
    "00010010": 1 / 2.0 - 0.01,
    "01001000": 1 / 2.0 - 0.01,
    "00010001": 0.02,
}

Transform the counts dict into a bitstring matrix and probability array for post-processing:

from qiskit_addon_sqd.counts import counts_to_arrays

# Convert counts into bitstring and probability arrays
bitstring_matrix_full, probs_arr_full = counts_to_arrays(counts_dict)
print(bitstring_matrix_full)
print(probs_arr_full)

Output:

[[False False False  True False False  True False]
 [False  True False False  True False False False]
 [False False False  True False False False  True]]
[0.49 0.49 0.02]

Subsample a single batch of size two:

n_batches = 1: Number of batches of configurations used by the different calls to the eigenstate solver
samples_per_batch = 2: Number of unique configurations to include in each batch

from qiskit_addon_sqd.subsampling import postselect_and_subsample

n_batches = 1
samples_per_batch = 2

# seed for random number generator
rand_seed = 48

# Generate the batches
batches = postselect_and_subsample(
    bitstring_matrix_full,
    probs_arr_full,
    hamming_right=num_elec_a,
    hamming_left=num_elec_b,
    samples_per_batch=samples_per_batch,
    num_batches=n_batches,
    rand_seed=rand_seed,
)

print(batches[0])

Output:

[[False False False  True False False  True False]
 [False  True False False  True False False False]]

Obtain decimal representation of the spin-up and spin-down bitstrings used by the eigenstate solver.

The first element in the tuple corresponds to the decimal representation of the spin-up configurations, while the second element in the tuple corresponds to the decimal representation of the spin-down configurations:

from qiskit_addon_sqd.fermion import bitstring_matrix_to_ci_strs

ci_strs = bitstring_matrix_to_ci_strs(batches[0], open_shell=open_shell)
print(ci_strs)

Output:

(array([1, 2, 4, 8], dtype=int64), array([1, 2, 4, 8], dtype=int64))

Note that while the number of samples per batch is two, and the sampled bitstrings are: $00010010$ and $01001000$ , four electronic configurations are generated per spin-species. In this case, the set of unique spin-polarized configurations is given by:

\mathcal{U} = \{ 0001, 0010, 0100, 1000 \}

whose base-10 decimal representation is

\mathcal{U}_{10} = \{ 1, 2, 4, 8 \}

Basis of the subspace:

The eigenstate solver takes all possible pairs of spin-up and spin-down bitstrings to construct the basis $\mathcal{B}$ of the subspace:

Element 1: $|00010001\rangle$
Element 2: $|00010010\rangle$
Element 3: $|00010100\rangle$
Element 4: $|00011000\rangle$
Element 5: $|00100001\rangle$
Element 6: $|00100010\rangle$
Element 7: $|00100100\rangle$
Element 8: $|00101000\rangle$
Element 9: $|01000001\rangle$
Element 10: $|01000010\rangle$
Element 11: $|01000100\rangle$
Element 12: $|01001000\rangle$
Element 13: $|10000001\rangle$
Element 14: $|10000010\rangle$
Element 15: $|10000100\rangle$
Element 16: $|10001000\rangle$

ci_strs_up, ci_strs_dn = ci_strs

print("Basis elements of the subspace:")

for ci_str_up in ci_strs_up:
    for ci_str_dn in ci_strs_dn:
        format_name = "{0:0" + str(num_orbitals) + "b}"
        print(
            "|"
            + format_name.format(ci_str_up)
            + format_name.format(ci_str_dn)
            + ">"
        )

Output:

Basis elements of the subspace:
|00010001>
|00010010>
|00010100>
|00011000>
|00100001>
|00100010>
|00100100>
|00101000>
|01000001>
|01000010>
|01000100>
|01001000>
|10000001>
|10000010>
|10000100>
|10001000>

The subspace dimension is upper-bounded by: $2 \cdot$ (samples_per_batch) $^2$ .

Open-shell

This example shows how the bitstrings are manipulated in a (2-electron, 4-orbital) system.

# Specify molecule properties
num_orbitals = 4
num_elec_a = num_elec_b = 1
open_shell = True

Specify by hand a dictionary of measurement outcomes:

counts_dict = {
    "00010010": 1 / 2.0 - 0.01,
    "01001000": 1 / 2.0 - 0.01,
    "00010001": 0.02,
}

Transform the counts dict into a bitstring matrix and probability array for post-processing:

# Convert counts into bitstring and probability arrays
bitstring_matrix_full, probs_arr_full = counts_to_arrays(counts_dict)
print(bitstring_matrix_full)
print(probs_arr_full)

Output:

[[False False False  True False False  True False]
 [False  True False False  True False False False]
 [False False False  True False False False  True]]
[0.49 0.49 0.02]

Subsample a single batch of size two:

n_batches = 1: Number of batches of configurations used by the different calls to the eigenstate solver
samples_per_batch = 2: Number of unique configurations to include in each batch

n_batches = 1
samples_per_batch = 2

# seed for random number generator
rand_seed = 48

# Generate the batches
batches = postselect_and_subsample(
    bitstring_matrix_full,
    probs_arr_full,
    hamming_right=num_elec_a,
    hamming_left=num_elec_b,
    samples_per_batch=samples_per_batch,
    num_batches=n_batches,
    rand_seed=rand_seed,
)

print(batches[0])

Output:

[[False False False  True False False  True False]
 [False  True False False  True False False False]]

Obtain decimal representation of the spin-up and spin-down bitstrings used by the eigenstate solver.

ci_strs = bitstring_matrix_to_ci_strs(batches[0], open_shell=open_shell)
print(ci_strs)

Output:

(array([2, 8], dtype=int64), array([1, 4], dtype=int64))

If we specify that open_shell = True, we do not include all unique half-bitstrings as spin-up and spin-down configurations, thus yielding a smaller basis than when specifying open_shell = False.

Basis of the subspace:

The eigenstate solver takes all possible pairs of spin-up and spin-down bitstrings to construct the basis $\mathcal{B}$ of the subspace:

Element 1: $|00010010\rangle$
Element 2: $|00011000\rangle$
Element 3: $|01000010\rangle$
Element 4: $|01001000\rangle$

ci_strs_up, ci_strs_dn = ci_strs

print("Basis elements of the subspace:")

for ci_str_up in ci_strs_up:
    for ci_str_dn in ci_strs_dn:
        format_name = "{0:0" + str(num_orbitals) + "b}"
        print(
            "|"
            + format_name.format(ci_str_up)
            + format_name.format(ci_str_dn)
            + ">"
        )

Output:

Basis elements of the subspace:
|00100001>
|00100100>
|10000001>
|10000100>

The subspace dimension is upper-bounded by: (samples_per_batch) $^2$ .

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