Alvin Gonzales (Jan 06 2025).
Abstract: This work introduces a method for correcting the output distribution of a quantum computer that does not require encoding of the logical qubits into more physical qubits. Thus, it avoids the encoding overhead of standard quantum error correction codes. If the noise affecting the circuit is a Pauli channel (we can bias the noise with twirling), the ideal output distribution and noisy distribution in the standard basis are related by a stochastic matrix. We prove that this matrix has a circulant block structure. Thus, the ideal distribution can be retrieved from the noisy output distribution by applying the Fast Fourier Transform. Moreover, due to its circulant structure, characterization of this matrix can be achieved by sampling a single circuit. The results are corroborated with quantum hardware executions consisting of 20-qubit and 30-qubit GHZ state preparation, 5-qubit Grover, 6-qubit and 10-qubit quantum phase estimation, and 10-qubit and 20-qubit Dicke state preparation circuits. The correction process dramatically improves the accuracies of the output distributions. For 30-qubit GHZ state preparation, a corrected distribution fidelity of 97.7% is achieved from an initial raw fidelity of 23.2%.
