Figure 4: Erasure process of a memory register. Stage 1: two wells with P(C=0)=P(C=1)=½. Stage 2: The potential barrier is lowered to allow thermal fluctuations between the wells. Stage 3: Tilt applied to force particle into well 1. Stage 4: Tilt removed and barrier raised back to initial height. The shape of the potential is the same as in 1 but now P(C=0)=0 and P(C=1)=1.
This sounds to me more like “setting” a memory register than “erasing” it?
That is, you go from
The register is in a state that you can measure. You don’t currently know what state. Could be that someone deliberately put it in that state so that you’d learn something when you measure it.
to
The register is in a state that you know because you put it there. Someone else, or yourself in future, can measure it to find what value you put there.
So yes, you’ve erased the previous value, but you’ve also put in your own.
And I think the “set” part of this feels more fundamental. For one, because you can’t set it without erasing but you presumably can erase without setting. (Skip stage 3, just lower the barrier and raise it again, you can’t predict what well it’s now in. Someone can measure, but you can’t transmit information to them.) And for two, because someone else might have already done that, so there might not have actually been a “useful bit” to erase in the first place.
Yea, I think you’re hitting on a weird duality between setting and erasing here. I think I agree that setting is more fundamental than erasing. I suppose when talking about energy expenditure of computation, each set bit must be erased in the long run, so they’re interchangeable in that sense.
This sounds to me more like “setting” a memory register than “erasing” it?
That is, you go from
The register is in a state that you can measure. You don’t currently know what state. Could be that someone deliberately put it in that state so that you’d learn something when you measure it.
to
The register is in a state that you know because you put it there. Someone else, or yourself in future, can measure it to find what value you put there.
So yes, you’ve erased the previous value, but you’ve also put in your own.
And I think the “set” part of this feels more fundamental. For one, because you can’t set it without erasing but you presumably can erase without setting. (Skip stage 3, just lower the barrier and raise it again, you can’t predict what well it’s now in. Someone can measure, but you can’t transmit information to them.) And for two, because someone else might have already done that, so there might not have actually been a “useful bit” to erase in the first place.
Am I understanding this correctly?
Yea, I think you’re hitting on a weird duality between setting and erasing here. I think I agree that setting is more fundamental than erasing. I suppose when talking about energy expenditure of computation, each set bit must be erased in the long run, so they’re interchangeable in that sense.