It’s been some time since our original hybrid quantum-classical algorithm, the variational quantum eigensolver (VQE), made its debut on a photonic quantum device.  Since that time there have been many developments, some of which I outlined in a previous post here.  I’m happy to say we’ve finally developed a reasonable way to access excited states without requiring more coherence time and at the same time we managed to mitigate the effects of detrimental quantum channels with only an efficient amount of additional Pauli measurements and classical computation (all with no formal quantum error correction).  Overall, I think our new paper, entitled “Hybrid Quantum-Classical Hierarchy for Mitigation of Decoherence and Determination of Excited States”, contains a lot of interesting new insights into both variational quantum algorithms and ways to reduce the effects of noise, and I hope that you will check it out!