Characterization of entanglement on superconducting quantum computers of
  up to 414 qubits
As quantum technology advances and the size of quantum computers grow, it
becomes increasingly important to understand the extent of quality in the
devices. As large-scale entanglement is a quantum resource crucial for
achieving quantum advantage, the challenge in its generation makes it a
valuable benchmark for measuring the performance of universal quantum devices.
In this work, we study entanglement in Greenberger-Horne-Zeilinger (GHZ) and
graph states prepared on the range of IBM Quantum devices. We generate GHZ
states and investigate their coherence times with respect to state size and
dynamical decoupling techniques. A GHZ fidelity of $0.519 \pm 0.014$ is
measured on a 32-qubit GHZ state, certifying its genuine multipartite
entanglement (GME). We show a substantial improvement in GHZ decoherence rates
for a 7-qubit GHZ state after implementing dynamical decoupling, and observe a
linear trend in the decoherence rate of $\alpha=(7.13N+5.54)10^{-3}\mu s^{-1}$
for up to $N=15$ qubits, confirming the absence of superdecoherence.
Additionally, we prepare and characterize fully bipartite entangled native
graph states on 22 superconducting quantum devices with qubit counts as high as
414 qubits, all active qubits of the 433-qubit IBM Osprey device. Analysis of
the decay of 2-qubit entanglement within the prepared states shows suppression
of coherent noise signals with the implementation of dynamical decoupling
techniques. Additionally, we observe that the entanglement in some qubit pairs
oscillates over time, which is likely caused by residual ZZ-interactions.
Characterizing entanglement in native graph states, along with detecting
entanglement oscillations, can be an effective approach to low-level device
benchmarking that encapsulates 2-qubit error rates along with additional
sources of noise, with possible applications to quantum circuit compilation.
John F Kam Haiyue Kang Charles D Hill Gary J Mooney Lloyd C L Hollenberg