Quantum Experiment Confirms Four-Qubit Entanglement on Superconducting Processors, Despite Noise.

Multipartite entanglement, a cornerstone of quantum information science, underpins advances in computation, communication, and sensing. Tomis Prajapati, Harsh Mehta, Shreya Banerjee, Prasanta K. Panigrahi, and V. Narayanan demonstrate the experimental evaluation of an entanglement witness, a measurable quantity indicating entanglement, for a four-qubit Dicke state. They achieve this on two 127-qubit superconducting quantum processing units (QPUs), named _sherbrook and _brisbane, reporting maximum negative witness values of -0.28 and -0.35, respectively, dependent on the state preparation method. Furthermore, the research theoretically models the impact of realistic noise, including amplitude damping, depolarising noise, bit-phase flips, and readout errors, on the stability of this entanglement, establishing a temporal bound for successful Dicke state generation on superconducting hardware.

This research successfully demonstrates the experimental verification of genuine four-partite entanglement using the Dicke state and entanglement witnesses (EWs). The authors derive a specific EW for this state and evaluate its performance on two IBM quantum processing units, ibm_sherbrook and ibm_brisbane, achieving negative expectation values of −0.178 ± 0.009 and −0.169 ± 0.002, respectively, confirming the presence of multipartite entanglement. The analysis reveals how noise sources degrade the observed entanglement and establishes a bound on the thermal relaxation time (T1) required for successful Dicke state generation on superconducting quantum processors. This provides a practical constraint for future experimental designs and optimisation of qubit coherence.

The work highlights the utility of EWs as a robust tool for detecting and characterising multipartite entanglement in the presence of realistic noise, offering valuable insights for advancing quantum information processing technologies. Future research could focus on exploring more sophisticated error mitigation techniques further to enhance the resilience of entangled states against noise. Investigating the performance of these EWs with different types of multipartite states and on quantum processors with varying architectures would also broaden the applicability of this approach. Furthermore, extending the theoretical modelling to include correlated noise sources could provide a more accurate representation of the challenges faced in real-world quantum systems.