Grover controlled-diffuser (CU_s)

The Grover controlled-diffuser (CU_s) is a quantum diffusion operator for Grover’s algorithm that searches for all solutions for Boolean oracles only, since the standard Grover diffuser (U_s) fails to find correct solutions for Boolean oracles in some logical structures! The CU_s operator relies on the states of the output qubit (as the reflection of Boolean decisions from a Boolean oracle), without relying on the phase kickback as illustrated below.

Thereby, the CU_s operator successfully searches for all solutions for Boolean oracles regardless of their logical structures, e.g., POS, SOP, DSOP, ESOP, ANF (Reed-Muller), XOR SAT (CNF-XOR SAT and DNF-XOR SAT), just to name a few.

Therefore, the CU_s operator can replace the standard U_s operator for Grover’s algorithm of Boolean oracles representing practical applications in the topics of digital synthesizers, robotics, computer vision, machine learning, etc., in the quantum domain.

For more information, please read our paper entitled "A concept of controlling Grover diffusion operator: a new approach to solve arbitrary Boolean-based problems", available at https://doi.org/10.1038/s41598-024-74587-y

Installation

Install the latest version of CU_s operator using the pip command:

pip install git+https://github.com/albayaty/grover_controlled_diffuser@main

Instead, the CU_s operator can be manually installed as stated in the following steps:

1. Download this repository to your computer, as a ZIP file.

2. Extract this file to a folder, e.g., grover_controlled_diffuser.

3. Use the terminal (or the Command Prompt) to cd to the grover_controlled_diffuser folder.

4. Install the setup.py file using the following command:

    python setup.py install

   Or, using this command:

   python3 -m pip install .

Usage

First of all, please be sure that the following prerequisite packages have been installed:

• qiskit (version >= 1.0).
• qiskit_aer (simulating quantum circuits locally).
• qiskit_ibm_runtime (transpiling and executing quantum circuits on IBM quantum computers).
• qiskit.visualization (plotting histograms, distributions, etc.).
• matplotlib (drawing quantum circuits).

Next, the callable function of the CU_s operator is expressed as follows.

CUs(quantum_circuit, inputs, output, barriers=False)

Where,

quantum_circuit: the quantum circuit of Grover's algorithm,

inputs: the list of input qubits' indices of a Boolean oracle,

output: the index of output qubit of a Boolean oracle, and

barriers: draw barriers (separators) around the CU_s operator, its default value is False.

Finally, the CUs function returns the quantum circuit of Grover's algorithm with the constructed CU_s operator. Note that this function does not add the measurement gates.

Examples

Initially, import the required Python and Qiskit libraries (including our grover_controlled_diffuser):

from qiskit import *
from qiskit_aer import AerSimulator
from qiskit.visualization import *
from grover_controlled_diffuser import *
import matplotlib.pyplot as plt
%matplotlib inline

Then, let's construct and use the CU_s operator in different scenarios as follows.

1. The CU_s operator (2 inputs and 1 output) surrounded by barriers:

   inputs = [0,1]
   output = 2
   IN  = QuantumRegister( len(inputs), name = 'input'  )
   OUT = QuantumRegister( 1, name = 'output' )
   qc = QuantumCircuit(IN, OUT)
   qc = CUs(qc, inputs, output, barriers=True)     # Grover controlled-diffuser (CUs)
   qc.draw(output='mpl', style='bw', scale=1.0, fold=-1);

   

2. Construct Grover's algorithm to solve a 4-bit Toffoli gate (as a Boolean oracle) using the CU_s operator, in one Grover iteration (loop), and then measure the outcomes as the highest probabilities as solutions:

   inputs = [0,1,2]
   output = 3
   IN  = QuantumRegister( len(inputs), name = 'input'  )
   OUT = QuantumRegister( 1, name = 'output' )
   MEAS = ClassicalRegister( len(inputs), name = 'clbits' )
   qc = QuantumCircuit(IN, OUT, MEAS)
   qc.h(inputs)
   qc.mcx(inputs, output)   # The Boolean oracle
   qc = CUs(qc, inputs, output, barriers=True)     # Grover controlled-diffuser (CUs)
   qc.measure(inputs, list(range(len(inputs))))
   qc.draw(output='mpl', style='bw', scale=1.0, fold=-1);
   simulator = AerSimulator()
   results = simulator.run(qc).result()
   counts  = results.get_counts(0)
   plot_distribution(counts, bar_labels=True, title="One solution is found when all inputs are in the |1> states");

   

3. Construct Grover's algorithm to solve an arbitrary Boolean oracle in POS structure, as (a + b + ¬c)(¬a + c)(¬b + c), using the CU_s operator, in one Grover iteration (loop), and then measure the outcomes as the highest probabilities as solutions. Note that such a Boolean oracle in POS structure is not solvable using the standard Grover diffuser (U_s)!

   inputs = [0,1,2]
   ancillae = [3,4,5]
   output = 6
   IN  = QuantumRegister( len(inputs), name = 'input'  )
   ANC = QuantumRegister( len(ancillae), name = 'anc' )
   OUT = QuantumRegister( 1, name = 'output' )
   MEAS = ClassicalRegister( len(inputs), name = 'clbits' )
   qc = QuantumCircuit(IN, ANC, OUT, MEAS)
   qc.h(inputs)
   qc.barrier()
   # The Boolean oracle in POS structure:
   qc.x([0,1,3])
   qc.mcx([0,1,2],3)
   qc.x([0,1])
   qc.x([2,4])
   qc.ccx(0,2,4)
   qc.x(5)
   qc.ccx(1,2,5)
   qc.mcx([3,4,5],6)
   # The mirror (uncomputing):
   qc.ccx(1,2,5)
   qc.x(5)
   qc.ccx(0,2,4)
   qc.x([2,4])
   qc.x([0,1])
   qc.mcx([0,1,2],3)
   qc.x([0,1,3])
   qc = CUs(qc, inputs, output, barriers=True)     # Grover controlled-diffuser (CUs)
   qc.measure(inputs, list(range(len(inputs))))
   qc.draw(output='mpl', style='bw', scale=1.0, fold=-1);
   simulator = AerSimulator()
   results = simulator.run(qc).result()
   counts  = results.get_counts(0)
   plot_distribution(counts, bar_labels=True, title="Solutions");

   

Reference

In case you are utilizing our Grover controlled-diffuser (CU_s) in your research work, we would be grateful if you cited our publication:

A. Al-Bayaty and M. Perkowski, "A concept of controlling Grover diffusion operator: a new approach to solve arbitrary Boolean-based problems," Scientific Reports, vol. 14, p. 23570, 2024. [Online]. Available: https://doi.org/10.1038/s41598-024-74587-y

Or, using BibTeX style:

@article{grovercontrolleddiffuser,
    title={A concept of controlling Grover diffusion operator: a new approach to solve arbitrary Boolean-based problems},
    author={Al-Bayaty, Ali and Perkowski, Marek},
    journal={Scientific Reports},
    note={Grover controlled-diffuser is available at \url{https://doi.org/10.1038/s41598-024-74587-y}},
    volume={14},
    pages={23570},
    year={2024},
    publisher={Nature Publishing Group}
}