No worries, my comment didn’t give much to go on. I did say “a typical thermal displacement of a rod during a cycle is going to be on the order of the 0.7nm error threshold for his proposed design”, which isn’t true if the mechanism works as described. It might have been better to frame it as—you’re in a bad situation when your thermal kinetic energy is on the order of the kinetic energy of the switching motion. There’s no clean win to be had.
If the positional uncertainty was close to the error limit, can we just bump up the logic element size(2x, 3x, 10x)? I’d assume scaling things up by some factor would reduce the relative effects of thermal noise and uncertainty.
That’s correct, although it increases power requirements and introduces low-frequency resonances to the logic elements.
Also, the expression ( ΔxΔF≥ℏ/2×bandwidth ) suggests the second concern might be clock rate?
In this design, the bandwidth requirement is set by how quickly a blocked rod will pass if the blocker fluctuates out of the way. If slowing the clock rate 10x includes reducing all forces by a factor of 100 to slow everything down proportionally, then yes, this lets you average away backaction noise like √10 while permitting more thermal motion. If you keep making everything both larger and slower, it will eventually work, yes. Will it be competitive with field-effect transistors? Practically, I doubt it, but it’s harder to find in-principle arguments at that level.
That noted, in this design, (I think) a blocked rod is tensioned with ~10x the switching drive force, so you’d want the response time of the restoring force to be ~10 ps. If your Δx is the same as the error threshold, then you’re admitting error rates of 10−1. Using (100 GHz, 0.07 nm [Drexler seems to claim 0.02nm in 12.3.7b]), the quantum-limited force noise spectral density is a few times less than the thermal force noise related to the claimed drag on the 1GHz cycle.
What I’m saying isn’t that the numbers in Nanosystems don’t keep the rod in place. These noise forces are connected with displacement noise by the stiffness of the mechanism, as you observe. What I’m saying is that these numbers are so close to quantum limits that they can’t be right, or even within a couple of orders of magnitude of right. As you say, quantum effects shouldn’t be relevant. By the same token, noise and dissipation should be far above quantum limits.
Yeah, transistor based designs also look promising. Insulation on the order of 2-3 nm suffices to prevent tunneling leakage and speeds are faster. Promises of quasi-reversibility, low power and the absurdly low element size made rod logic appealing if feasible. I’ll settle for clock speeds a factor of 100 higher even if you can’t fit a microcontroller in a microbe.
My instinct is to look for low hanging design optimizations to salvage performance (EG: drive system changes to make forces on rods at end of travel and blocked rods equal reducing speed of errors and removing most of that 10x penalty.) Maybe enough of those can cut the required scale-up to the point where it’s competitive in some areas with transistors.
But we won’t know any of this for sure unless it’s built. If thermal noise is 3OOM worse than Drexler’s figures it’s all pointless anyways.
I remain skeptical the system will move significant fractions of a bond length if a rod is held by a potential well formed by inter-atomic repulsion on one of the “alignment knobs” and mostly constant drive spring force. Stiffness and max force should be perhaps half that of a C-C bond and energy required to move the rod out of position would be 2-3x that to break a C-C bond since the spring can keep applying force over the error threshold distance. Alternatively the system *is* that aggressively built such that thermal noise is enough to break things in normal operation which is a big point against.
No worries, my comment didn’t give much to go on. I did say “a typical thermal displacement of a rod during a cycle is going to be on the order of the 0.7nm error threshold for his proposed design”, which isn’t true if the mechanism works as described. It might have been better to frame it as—you’re in a bad situation when your thermal kinetic energy is on the order of the kinetic energy of the switching motion. There’s no clean win to be had.
That’s correct, although it increases power requirements and introduces low-frequency resonances to the logic elements.
In this design, the bandwidth requirement is set by how quickly a blocked rod will pass if the blocker fluctuates out of the way. If slowing the clock rate 10x includes reducing all forces by a factor of 100 to slow everything down proportionally, then yes, this lets you average away backaction noise like √10 while permitting more thermal motion. If you keep making everything both larger and slower, it will eventually work, yes. Will it be competitive with field-effect transistors? Practically, I doubt it, but it’s harder to find in-principle arguments at that level.
That noted, in this design, (I think) a blocked rod is tensioned with ~10x the switching drive force, so you’d want the response time of the restoring force to be ~10 ps. If your Δx is the same as the error threshold, then you’re admitting error rates of 10−1. Using (100 GHz, 0.07 nm [Drexler seems to claim 0.02nm in 12.3.7b]), the quantum-limited force noise spectral density is a few times less than the thermal force noise related to the claimed drag on the 1GHz cycle.
What I’m saying isn’t that the numbers in Nanosystems don’t keep the rod in place. These noise forces are connected with displacement noise by the stiffness of the mechanism, as you observe. What I’m saying is that these numbers are so close to quantum limits that they can’t be right, or even within a couple of orders of magnitude of right. As you say, quantum effects shouldn’t be relevant. By the same token, noise and dissipation should be far above quantum limits.
Yeah, transistor based designs also look promising. Insulation on the order of 2-3 nm suffices to prevent tunneling leakage and speeds are faster. Promises of quasi-reversibility, low power and the absurdly low element size made rod logic appealing if feasible. I’ll settle for clock speeds a factor of 100 higher even if you can’t fit a microcontroller in a microbe.
My instinct is to look for low hanging design optimizations to salvage performance (EG: drive system changes to make forces on rods at end of travel and blocked rods equal reducing speed of errors and removing most of that 10x penalty.) Maybe enough of those can cut the required scale-up to the point where it’s competitive in some areas with transistors.
But we won’t know any of this for sure unless it’s built. If thermal noise is 3OOM worse than Drexler’s figures it’s all pointless anyways.
I remain skeptical the system will move significant fractions of a bond length if a rod is held by a potential well formed by inter-atomic repulsion on one of the “alignment knobs” and mostly constant drive spring force. Stiffness and max force should be perhaps half that of a C-C bond and energy required to move the rod out of position would be 2-3x that to break a C-C bond since the spring can keep applying force over the error threshold distance. Alternatively the system *is* that aggressively built such that thermal noise is enough to break things in normal operation which is a big point against.
Just to follow up, I spell out an argument for a lower bound on dissipation that’s 2-3 OOM higher in Appendix C here.