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Let's start from a basic example: suppose I have the following list containing $N=8$ non-negative ints:

L = [2, 1, 2, 0, 0, 1, 3, 2]

I want to encode the whole list $L$ into a quantum state $|\psi\rangle$ by using basis encoding as follows:

$$|\psi \rangle = \frac{1}{\sqrt{N}} \sum_{i=0}^{N-1} |L_i\rangle |i\rangle$$

where $|L_i\rangle$ is the $i$-th element in the list, and $|i\rangle$ is its corresponding index (both represented as bit-strings). In this example, I would get an equal superposition of the states $|10\rangle|000\rangle$ ($L_0=2$), $|01\rangle|001\rangle$ ($L_1=1$), and so on until $|10\rangle|111\rangle$ ($L_7=2$).

Is there a way in Qiskit to prepare this state given an arbitrary Python list? And, if not, how could it be implemented?

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2 Answers 2

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This is the implementation of my own solution (it only works well when $L_i\in \mathbb{N}$ and len(L) is a power of 2 but could be easily extended to a more general case):

import numpy as np
from qiskit import QuantumCircuit
from qiskit.circuit.library import StatePreparation

L = [2, 1, 2, 0, 0, 1, 3, 2]

len_qr1 = int(np.ceil(np.log2(len(L))))
len_qr2 = int(np.log2(max(L))) + 1
num_qubits = len_qr1 + len_qr2
statevector = np.zeros(2**num_qubits)

for i, el in enumerate(L):
    index_reg = '{0:b}'.format(i).zfill(len_qr1)
    element_reg = '{0:b}'.format(el).zfill(len_qr2)
    statevector[int(element_reg + index_reg, 2)] = 1

statevector /= np.linalg.norm(statevector)

qc = QuantumCircuit(num_qubits)
sp = StatePreparation(statevector)
qc.append(sp, range(num_qubits))

Running the qc circuit 10000 times by using the qasm_simulator to measure the final state $|\psi\rangle$, I get the expected distribution (equal superposition of the 8 bit-strings):

enter image description here

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import numpy as np
import numpy.linalg as la

L = [2, 1, 2, 0, 0, 1, 3, 2]
L = np.array(L, dtype=float)

qc = QuantumCircuit(3)
qc.prepare_state(L / la.norm(L))
qc.draw()

You can also use initialize instead of prepare_state. The former resets the qubits to zero first (which isn't necessary here) while the latter generates an invertible gate.

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  • $\begingroup$ Ok but this is "amplitude encoding". I want to encode my list by using "basis encoding" as described in the question $\endgroup$ Commented Dec 14, 2022 at 23:21
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    $\begingroup$ So just create a state vector with 32 elements and the items that you want to have equal probability set to 1 and the rest set to 0. Normalize and call prepare_state as above. $\endgroup$ Commented Dec 15, 2022 at 0:58

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