I had a conversation with someone last night that has disrupted my thinking on this. This friend approaches it all from a similar place as me - philosophy and physics combined. Depending on the set of assumptions you make, you can come to the conclusion that consciousness only requires that the right calculations are made. A human brain is a quantum mechanical device that makes very sophisticated calculations and manipulates quantum information in a particular way. From a physical standpoint, there’s no reason that something else cannot also make those calculations.
This friend of mine then extrapolates this to say that a rock must therefore also have some kind of experience as it is also a quantum mechanical system that is performing calculations. Its subjective experience would necessarily be unrecognizable to us. As would a tree’s, or a swamp’s, or even perhaps a star or a nebula. But the point is that, so long as an information system is performing the correct set of calculations, and the information is being handled in the correct manner, it could qualify as conscious or sentient.
However
This is a purely materialist interpretation. All of these assumptions are predicated on the physical world being real.
Perhaps, though, this is not the fundamental nature of the universe. When we examine phenomena such as Near Death Experiences (NDE) we have bountiful evidence that human consciousness is not always attached to a body. Indeed, there are thousands of reports every year of people who were clinically dead and yet had meaningful, verifiable experiences. These events are (currently) beyond the ken of materialism. Here’s a great lecture on what I mean. University of California, Riverside lecture on Near Death Experiences and their interpretation.
If we accept that consciousness is not necessarily attached to a body at all times, this necessitates an entirely different worldview. Perhaps there is something quite special and unique about being a conscious entity, something that cannot simply be explained by patterns of quantum information. It calls into question everything we assume about physical reality. Do we have souls? Perhaps animism is a more valid model of the universe? Perhaps, as one internet denizen comically put it on Tumblr years ago, we are all ghosts piloting meat-robots.
The notion that consciousness may be separated from a body creates infinitely more questions than it answers, which is why I began by saying the debate over machine consciousness and sentience may take decades or centuries to be settled. But it may also eventually just require a leap of faith. We have all miraculously found ourselves awake in a mystifying universe and the very nature of our existence defies explanation, and yet we cannot ignore the obviousness of our existence.
I will still stand by the conclusion I come to in Benevolent By Design - whether or not a machine is truly conscious may ultimately be irrelevant if (1) it cannot suffer and (2) it does not fear death. In her book Braintrust, Patricia Churchland elucidates the evolution of both suffering and the fear of death. These are purely to promote survival of biological forms. If we do not imbue a machine with these two abilities, I doubt there will be much moral ambiguity about, say, building kill-switches into all our machines.
To refer back to the original question about LaMDA - is it sentient? Probably not, certainly not in the way that we are. While materialism cannot satisfactorily or completely explain consciousness, neuroscience must still be given its due. Brain injuries, such as strokes, or disorders such as schizophrenia and autism, all demonstrate that our sense of self and reality can be modified simply by changing our brains. This indicates that consciousness is often greatly influenced by the physical world, so perhaps a dualism model is more accurate. Interestingly, during NDE many people report that they are whole and perfect. Even people who have been blind their entire life report having the ability to see during NDE. How are we to interpret this? Does this imply that we are incorporeal beings that are temporarily limited by corporeal constraints? If this is all true, it begs the ultimate question of why?
As with all good science, we are left with far more questions than answers.
EDIT: Then you throw in things like quantum nonlocality and nonrealism into the mix along with the strong anthropic principle and the implications of delayed choice experiments and the universe seems far stranger than ever before.