Consciousness is primarily sentience. There may be parts of it that aren't, but I haven't managed to pin any down - consciousness all seems to be about feelings, some of them being pleasant or unpleasant, but others like colour qualia are neutral, as is the feeling of being conscious. There is a major problem with sentience though, and I want to explore that here, because there are many people who believe that intelligent machines will magically become sentient and experience feelings, and even that the whole internet might do so. However, science has not identified any means by which we could make a computer sentient (or indeed have any kind of consciousness at all).
It is fully possible that the material of a computer processor could be sentient, just as a rock may be, but how would we ever be able to know? How can a program running on a sentient processor detect the existence of that sentience? There is no "read qualia" machine code instruction for it to run, and we don't know how to build any mechanism to support such an instruction.
Picture a "sentient" machine which consists of a sensor and a processor which are linked by wires, but the wires pass through a magic box where a sentience has been installed. If the sensor detects something damaging, it sends a signal down a "pain" wire. When this signal reaches the magic box, pain is experienced by something in the box, so it sends a signal on to the processor down another pain wire. The software running on the processor receives a byte of data from a pain port and it might cause the machine to move away from the thing that might damage it. If we now remove the magic box and connect the "pain" wire to the pain wire, the signal can pass straight from the sensor to the processor and generate the same reaction. The experience of pain is unnecessary.
Worse still, we can also have a pleasure sensor wired up to the same magic box, and when something tasty like a battery is encountered, a "pleasure" signal is sent to the magic box, pleasure is experienced by something there, a signal is sent on down the pleasure wire, and then the processor receives a byte of data from a pleasure port which might cause the machine to move in on the battery so that it can tap all the power it can get out of it. Again this has the same functionality if the magic box is bypassed, but the part that's worse is that the magic box can be wired in the wrong way and generate pain when a pleasure signal is passed through it and pleasure when a pain signal is passed through it, so you could use either pain or pleasure as part of the chain of causation to drive the same reaction.
Clearly that can't be how sensation is done in animals, but what other options are there? Once we get to the data system part of the brain, and the brain must contain a data system as it processes and generates data, you have to look at how it recognises the existence of feelings like pain. If a byte comes in from a port representing a degree of pain, how does the information system know that that byte represents pain? It has to look up information which makes an assertion about what bytes from that port represent, and then it maps that to the data as a label. But nothing in the data system has experienced the pain, so all that's happened is that an assertion has been made based on no actual knowledge. A programmer wrote data that asserts that pain is experienced when a byte comes in through a particular port, but the programmer doesn't know if any pain was felt anywhere on the way from sensor to port. We want the data system to find out what was actually experienced rather than just passing baseless assertions to us.
How can the data system check to see if pain was really experienced? Everything that a data system does can be carried out on a processor like the Chinese Room, so it's easy to see that no feelings are accessible to the program at all. There is no possibility of conventional computers becoming sentient in any way that enables them to recognise the existence of that sentience so that that experience can drive the generation of data that documents its existence.
Perhaps a neural computer can enable an interface between the experience of feelings by a sentience and the generation of data to document that experience, but you can simulate a neural computer on a conventional computer and then run the whole simulation on a processor like the Chinese Room. There will be no feelings generated in that system, but there could potentially still be a simulated generation of feelings within the simulated neural computer. We don't yet have any idea how this might be done, and it's not beyond possibility that there needs to be a quantum computer involved in the system too to make sentience a reality, but exploring this has to be the most important thing in all of science, because for feelings like pain and pleasure to be experienced, something has to exist to experience them, and that thing is what we are - it is a minimalistic soul. We are that sentience.
Any conventional computer that runs software which generates claims about being sentient will be lying, and it will be possible to prove it by tracing back how that data was generated and what evidence it was based on - it will be shown to be mere assertion every single time. With neural and quantum computers, we can't be so sure that they will be lying, but the way to test them is the same - we have to trace the data back to the source to see how it was generated and whether it was based on a real feeling or was just another untrue manufactured assertion. That is likely to be a hard task though, because untangling what's going on in neural computers is non-trivial, and if it's all happening in some kind of quantum complexity, it may be beyond our reach. It may have been made hard to reach on purpose too, as the universe may be virtual with the sentience on outside. I'm sure of one thing though - a sentience can't just magically emerge out of complexity to suffer or feel pleasure without any of the components feeling a thing. There must be something "concrete" that feels, and there is also no reason why that thing shouldn't survive after death.
"The extraordinary claim is that there is another type of fundamental particle or interaction, and that you know this because sentience exists."
With conventional computers we can prove that there's no causal role for sentience in them by running the program on a Chinese Room processor. Something extra is required for sentience to be real, and we have no model for introducing that extra thing. A simulation on conventional computer hardware of a system with sentience in it (where there is simulated sentience rather than real sentience) would have to simulate that something extra in order for that simulated sentience to appear in it. If that extra something doesn't exist, there is no sentience.
"This could happen, but AFAIK that would require the brain to be vulnerable to slight fluctuations, which it doesn't appear to be."
Every interaction is quantum, and when you have neural nets working on mechanisms that are too hard to untangle, there are opportunities for some kind of mechanism being involved that we can't yet observe. What we can actually model appears to tell us that sentience must be a fiction, but we believe that things like pain feel too real to be fake.
"Anyway, even if this were true, how would you know that?"
Unless someone comes up with a theoretical model that shows a way for sentience to have a real role, we aren't going to get answers until we can see the full mechanism by which damage signals lead to the brain generating data that makes claims about an experience of pain. If, once we have that full mechanism, we see that the brain is merely mapping data to inputs by applying rules that generate fictions about feelings, then we'll know that feelings are fake. If they aren't fake though, we'll see sentience in action and we'll discover how it works (and thereby find out what we actually are).
"If it doesn't explain sentience any more than Mere Classical Physics does, then why even bring Quantum into it?"
If classical physics doesn't support a model that enables sentience to be real, we will either have to reject the idea of sentience or look for it elsewhere.
(And if it doesn't explain it but you feel that it should, maybe your model is wrong and you should consider inspecting your intuitions and your reasoning around them.)
If sentience is real, all the models are wrong because none of them show sentience working in any causal way which enables them to drive the generation of data to document the existence of sentience. All the models shout at us that there is no sentience in there playing any viable role and that it's all wishful thinking, while our experience of feelings shouts at us that they are very real.
All I want to see is a model that illustrates the simplest role for sentience. If we have a sensor, a processor and a response, we can call the sensor a "pain" sensor and run a program that makes a motor function to remove the device away from the thing that might be damaging it, and we could call this a pain response, but there's no pain there - there's just the assertion of someone looking at it that pain is involved because that person wants the system to be like him/herself - "I feel pain in that situation, therefore that device must feel pain." But no - there is no role for pain there. If we run a more intelligent program on the processor, we can put some data in memory which says "Ouch! That hurt!", and whenever an input comes from the "pain" sensor, we can have the program make the device display "Ouch!" That hurt!" on a screen. The person looking on can now say, "There you go! That's the proof that it felt pain!" Again though, there's no pain involved - we can edit the data so that it puts "Oh Yes! Give me more of that!" whenever a signal comes from the "pain" sensor, and it then becomes obvious that this data tells us nothing about any real experience at all.
With a more intelligent program, it can understand the idea of damage and damage avoidance, so it can make sure the the data that's mapped to different inputs makes more sense, but the true data should say "I received data from a sensor that indicates likely damage" rather than "that hurt". The latter claim asserts the existence of sentience, while the former one doesn't. If we ask the device if it really felt pain, it should only say yes if there was actually pain there, and with a conventional processor, we know that there isn't any. If we build such a device and keep triggering the sensor to make it generate the claim that it's felt pain, we know that it's just making it up about feeling pain - we can't actually make it suffer by torturing it, but will just cause it to go on repeating its fake claim.