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This paper basically explores a hypothetical scenario where scaling quantum memory ends up being cheaper than scaling computational qubits. The title (or abstra

by tbabej 6y ago

This paper basically explores a hypothetical scenario where scaling quantum memory ends up being cheaper than scaling computational qubits. The title (or abstract) unfortunately does not mention the quantum memory requirements at n=2048 explicitly. For factoring 2048 RSA integers, the technique proposed in the paper would require ~430 million memory qubits (see the table at top of page 16).

bunnie 6y ago
I'm trying to make sense of the 'memory qubits' e.g. 'spatial modes' in the paper. Do you know if they are they a sort of...quantum flip flop? Is this also a hypothetical structure, or have they been built and shown to be able to reliably store and retrieve quantum state in such spatial modes? 430 million is a much, much larger number than their headline 13436 qubits...
oldgradstudent 6y ago
So magic computer could be more effective with more magic memory than with more magic processing power, right?
- bdamm 6y ago
  Quantum computers exist today, they're just very low power, low gate counts, and extremely expensive. Don't discount it as magic just because of the claims. Scaling up QC would be like bringing mathematics to Mesopotamia. But it isn't "magic", it's physics.
  - gallerdude 6y ago
    ...would be like bringing mathematics to Mesopotamia. Can you expound on this? What sort of breakthroughs are bottlenecked by developments in quantum computing?
    - Craighead 6y ago
      Temperature
    - _hl_ 6y ago
      The hope is that quantum computers will give us an exponential speed up on some problems, which could (again, hopefully) allow us to solve some NP hard problems. This can be extremely useful for scientific computing (e.g. protein folding) and engineering, where computers have to solve complex NP-hard optimisation problems. It could also be possible to use the technology developed for the precise control and measurement of qubits to "rebuild" natural phenomena like the interaction of chemical molecules, something which is currently extremely hard to simulate.
      - syrrim 6y ago
        >which will allow us to solve some NP hard problems No it won't
        bollu 6y ago
        > No it won't We don't know either way. relationship between BPP, BQP, P, NP are all open.
        f6v 6y ago
        Could you elaborate?
  - azatris 6y ago
    I do not know much about quantum computing. But could you explain what makes these computers quantum? Is it the configuration of these transistors to invoke some quantum phenomena?
    - freeone3000 6y ago
      They're built off fundamentally different basic units. Contemporary computers use transistors, which occupy traditional physics and do logic with voltage thresholds. Quantum computers use a quantum phenomenon -- one easy-to-understand (and fairly easy to construct) quantum computer substrate uses the spin of electrons in superconducting loops. Electron spin is a quantum phenomenon, in that the spin isn't deterministically positive or negative, it's a probability distribution -- initially, equally likely to be positive or negative, but you can't tell what it is until you actually read it. It's not 0.0, it's either -0.5 or 0.5, both with a 50% chance. Equally importantly, you can perform (physical) operations on electrons singly or in pairs in order to manipulate these probabilities. Quantum computing is turning these probability fields and operations into useful computational results.
      - azatris 6y ago
        This is exactly what I wanted to hear! Awesome, thank you!
    - antepodius 6y ago
      In a classical computer, every bit of information in the system is in a definite state- 1 or 0. In a quantum system with such definite possible states, what you actually have most of the time of the system in some interpolation of the possible states- so in the quantum computer case each bit is usually in a state a1 + b0, where a and b are complex numbers such that |a^2|+|b^2| = 1. Most of the time, the 'weight' flows back and forth between a and b according to certain equations over time. When you measure the system- that is, when the bit interacts with the outside world, hopefully your measuring apparatus- you see a 1 or a 0, with probabilities |a^2| and |b^2| respectively. So what you can do is get a whole bunch of these quantum bits- qubits together, and set things up so that the time-evolution of their quantum state is correlated and probabilistically moves towards something you're interested in. Say you can set things up so the bit array- which, at first, will give you a mere perfectly random bit string on measurement- becomes more and more likely to give you, say, a prime factor, or the answer to some other question. So yes, the quantum phenomenon is that the bits of the computer are quantum objects as opposed to classical.
  - rnestler 6y ago
    > they're just very low power, I guess you meant to write high power as in high electrical power consumption? Or low power in the sense of low processing power? Anyway: Performance per Watt is probably pretty bad for current quantum computers ;)
    - RIMR 6y ago
      "Processing power" is colloquially the term most people I know use to describe computers of varying capabilities.
  - fractionalhare 6y ago
    The research in this field proceeds under the umbrella and framework of physics, yes. But it's not clear that it's possible to scale up to the number qubits required do anything nontrivial. It's plausible there are hard engineering limits on error correction which make it asymptotically more difficult with each order of magnitude more qubits involved. It might not look that way because there's a lot of (relatively) mainstream investment in quantum computing. However it's pretty common for speculative physics research to be pursued for years without ever coming to fruition. Especially when there are promising early results before it's shown that scaling the work reduces to an intractable problem.
    - reikonomusha 6y ago
      I think it’s reasonable to be more optimistic than you put it. Qubit counts and qubit fidelity have been increasing at a remarkable rate. Just five years ago we could barely eek out a handful of qubits, and when we did, they’d be bad. Google’s quantum supremacy result is a testament to that. (At this stage, whether they actually demonstrated the “supreme” part of supremacy is, imho, irrelevant. They’ve demonstrated a much larger, controllable, programmable quantum computer and measured its quality characteristics accurately.) Of course, I’m saying it’s reasonable to be more optimistic, not that we have a proof we will certainly be able to scale to enormous machine sizes. But it’s definitely more than “speculative physics”: real machines have been built and demonstrated to exhibit truly measurable quantum effects that allow for programmable computation. (To be sure, there is hype, there is a lot of cash sloshing around, and there are totally bogus claims some companies are publicly making.)
      - deleted 6y ago
        [deleted]
      - oldgradstudent 6y ago
        > They’ve demonstrated a much larger, controllable, programmable quantum computer No they didn't. Not in any meaningful sense. How exactly what that machine does can be called a "computation"? in what sense is it "programmable"?
      - 6y ago
  - jMyles 6y ago
    > would be like bringing mathematics to Mesopotamia I don't think I understand this metaphor.
  - ereli1 6y ago
    https://en.wikipedia.org/wiki/Babylonian_mathematics https://en.wikipedia.org/wiki/Babylonian_mathematics
- TheRealPomax 6y ago
  You know that qubit processors already exist, right? This is like going "yeah right, we can factor large primes if we had a 4.2GHz processor. So, magical processing power, right?" in the 386 era. We may not have 13k qubit systems today, but we do have qubit systems already. Expecting us to get better at them is pretty reasonable.
  - YZF 6y ago
    Not sure it is reasonable. Quantum systems don't scale that way. In order to get the quantum speedup you need to be able to maintain the larger quantum state which gets a lot harder the larger these systems get. This is like saying we already have 7nm process today for silicon so expecting us to get better, like 1nm, 10 angstrom... or we have a plane that goes 2000 miles per hour, expecting 10kmph, 100kmph, 2000kmph... physics doesn't work like that.
MichaelMoser123 6y ago
interesting qeustions that is not related to the feasibility of this idea: would this news item cause some more fluctuations in the rate of digital currencies?
- IncRnd 6y ago
  The results of this could, however this is simply theoretical at the current time.
Strilanc 6y ago
It's an interesting question whether, over the long term, quantum hardware will match the feature of classical hardware where memory is significantly cheaper than compute-capable bits in the CPU. Hard drives are 100x cheaper per bit than RAM which is 100x cheaper (I think?) per bit than a CPU register. How expensive is quantum memory relative to quantum compute, over the long term? Expectations about the answer to this question strongly affect whether or not you find this paper relevant. And the answer depends on the pieces you build your quantum computer out of, e.g. hypothetical photonic architectures have a bigger ratio than hypothetical superconducting qubit architectures. It is currently very much an open question whether or not quantum computers will have a memory hierarchy.
- thesz 6y ago
  You are off by orders of magnitude. The difference in memory density between DRAM and SRAM (register file) is not 100 times, more like 10x. Standalone "registers" - memory elements of pipelines, state machines etc, - are again not more than ten times less denser than SRAM. After DRAM goes SSD and after SSD goes disk. The difference in price per GB for SSD and disk is about four (4x) times, I looked for that numbers recently. The difference between tape and disk is, again, about 4-10 times (from memory).
  - Strilanc 6y ago
    Are you sure? I was just going off a quick search of Amazon, where 1 GB of ram cost ~30$, 1 TB of disk cost ~50$, and a CPU with ~1MB of L2 cache cost ~200$. Based on that I actually thought 100x was a comfortable underestimate, not an overestimate.
    - dcow 6y ago
      CPU cost is not dominated by the cache size is the problem, I think.
      - robocat 6y ago
        But I think that cache is roughly 50% of marginal production cost of the silicon chip die by mm2. https://cdn.wccftech.com/wp-content/uploads/2020/11/AMD-Ryzen-5000-Zen-3-Desktop-CPU_Vermeer_Die-Shot_1-scaled.jpg https://cdn.wccftech.com/wp-content/uploads/2020/11/AMD-Ryze... https://images.anandtech.com/doci/16214/Zen3_arch_19.jpg https://images.anandtech.com/doci/16214/Zen3_arch_19.jpg

by tbabej 6y ago

bunnie 6y ago

I'm trying to make sense of the 'memory qubits' e.g. 'spatial modes' in the paper. Do you know if they are they a sort of...quantum flip flop? Is this also a hypothetical structure, or have they been built and shown to be able to reliably store and retrieve quantum state in such spatial modes? 430 million is a much, much larger number than their headline 13436 qubits...

oldgradstudent 6y ago

So magic computer could be more effective with more magic memory than with more magic processing power, right?

bdamm 6y ago
Quantum computers exist today, they're just very low power, low gate counts, and extremely expensive. Don't discount it as magic just because of the claims. Scaling up QC would be like bringing mathematics to Mesopotamia. But it isn't "magic", it's physics.
- gallerdude 6y ago
  ...would be like bringing mathematics to Mesopotamia. Can you expound on this? What sort of breakthroughs are bottlenecked by developments in quantum computing?
  - Craighead 6y ago
    Temperature
  - _hl_ 6y ago
    The hope is that quantum computers will give us an exponential speed up on some problems, which could (again, hopefully) allow us to solve some NP hard problems. This can be extremely useful for scientific computing (e.g. protein folding) and engineering, where computers have to solve complex NP-hard optimisation problems. It could also be possible to use the technology developed for the precise control and measurement of qubits to "rebuild" natural phenomena like the interaction of chemical molecules, something which is currently extremely hard to simulate.
    - syrrim 6y ago
      >which will allow us to solve some NP hard problems No it won't
      - bollu 6y ago
        > No it won't We don't know either way. relationship between BPP, BQP, P, NP are all open.
      - f6v 6y ago
        Could you elaborate?
- azatris 6y ago
  I do not know much about quantum computing. But could you explain what makes these computers quantum? Is it the configuration of these transistors to invoke some quantum phenomena?
  - freeone3000 6y ago
    They're built off fundamentally different basic units. Contemporary computers use transistors, which occupy traditional physics and do logic with voltage thresholds. Quantum computers use a quantum phenomenon -- one easy-to-understand (and fairly easy to construct) quantum computer substrate uses the spin of electrons in superconducting loops. Electron spin is a quantum phenomenon, in that the spin isn't deterministically positive or negative, it's a probability distribution -- initially, equally likely to be positive or negative, but you can't tell what it is until you actually read it. It's not 0.0, it's either -0.5 or 0.5, both with a 50% chance. Equally importantly, you can perform (physical) operations on electrons singly or in pairs in order to manipulate these probabilities. Quantum computing is turning these probability fields and operations into useful computational results.
    - azatris 6y ago
      This is exactly what I wanted to hear! Awesome, thank you!
  - antepodius 6y ago
    In a classical computer, every bit of information in the system is in a definite state- 1 or 0. In a quantum system with such definite possible states, what you actually have most of the time of the system in some interpolation of the possible states- so in the quantum computer case each bit is usually in a state a1 + b0, where a and b are complex numbers such that |a^2|+|b^2| = 1. Most of the time, the 'weight' flows back and forth between a and b according to certain equations over time. When you measure the system- that is, when the bit interacts with the outside world, hopefully your measuring apparatus- you see a 1 or a 0, with probabilities |a^2| and |b^2| respectively. So what you can do is get a whole bunch of these quantum bits- qubits together, and set things up so that the time-evolution of their quantum state is correlated and probabilistically moves towards something you're interested in. Say you can set things up so the bit array- which, at first, will give you a mere perfectly random bit string on measurement- becomes more and more likely to give you, say, a prime factor, or the answer to some other question. So yes, the quantum phenomenon is that the bits of the computer are quantum objects as opposed to classical.
- rnestler 6y ago
  > they're just very low power, I guess you meant to write high power as in high electrical power consumption? Or low power in the sense of low processing power? Anyway: Performance per Watt is probably pretty bad for current quantum computers ;)
  - RIMR 6y ago
    "Processing power" is colloquially the term most people I know use to describe computers of varying capabilities.
- fractionalhare 6y ago
  The research in this field proceeds under the umbrella and framework of physics, yes. But it's not clear that it's possible to scale up to the number qubits required do anything nontrivial. It's plausible there are hard engineering limits on error correction which make it asymptotically more difficult with each order of magnitude more qubits involved. It might not look that way because there's a lot of (relatively) mainstream investment in quantum computing. However it's pretty common for speculative physics research to be pursued for years without ever coming to fruition. Especially when there are promising early results before it's shown that scaling the work reduces to an intractable problem.
  - reikonomusha 6y ago
    I think it’s reasonable to be more optimistic than you put it. Qubit counts and qubit fidelity have been increasing at a remarkable rate. Just five years ago we could barely eek out a handful of qubits, and when we did, they’d be bad. Google’s quantum supremacy result is a testament to that. (At this stage, whether they actually demonstrated the “supreme” part of supremacy is, imho, irrelevant. They’ve demonstrated a much larger, controllable, programmable quantum computer and measured its quality characteristics accurately.) Of course, I’m saying it’s reasonable to be more optimistic, not that we have a proof we will certainly be able to scale to enormous machine sizes. But it’s definitely more than “speculative physics”: real machines have been built and demonstrated to exhibit truly measurable quantum effects that allow for programmable computation. (To be sure, there is hype, there is a lot of cash sloshing around, and there are totally bogus claims some companies are publicly making.)
    - deleted 6y ago
      [deleted]
    - oldgradstudent 6y ago
      > They’ve demonstrated a much larger, controllable, programmable quantum computer No they didn't. Not in any meaningful sense. How exactly what that machine does can be called a "computation"? in what sense is it "programmable"?
    - 6y ago
- jMyles 6y ago
  > would be like bringing mathematics to Mesopotamia I don't think I understand this metaphor.
- ereli1 6y ago
  https://en.wikipedia.org/wiki/Babylonian_mathematics https://en.wikipedia.org/wiki/Babylonian_mathematics
TheRealPomax 6y ago
You know that qubit processors already exist, right? This is like going "yeah right, we can factor large primes if we had a 4.2GHz processor. So, magical processing power, right?" in the 386 era. We may not have 13k qubit systems today, but we do have qubit systems already. Expecting us to get better at them is pretty reasonable.
- YZF 6y ago
  Not sure it is reasonable. Quantum systems don't scale that way. In order to get the quantum speedup you need to be able to maintain the larger quantum state which gets a lot harder the larger these systems get. This is like saying we already have 7nm process today for silicon so expecting us to get better, like 1nm, 10 angstrom... or we have a plane that goes 2000 miles per hour, expecting 10kmph, 100kmph, 2000kmph... physics doesn't work like that.

MichaelMoser123 6y ago

interesting qeustions that is not related to the feasibility of this idea: would this news item cause some more fluctuations in the rate of digital currencies?

IncRnd 6y ago
The results of this could, however this is simply theoretical at the current time.

Strilanc 6y ago

It's an interesting question whether, over the long term, quantum hardware will match the feature of classical hardware where memory is significantly cheaper than compute-capable bits in the CPU. Hard drives are 100x cheaper per bit than RAM which is 100x cheaper (I think?) per bit than a CPU register. How expensive is quantum memory relative to quantum compute, over the long term? Expectations about the answer to this question strongly affect whether or not you find this paper relevant. And the answer depends on the pieces you build your quantum computer out of, e.g. hypothetical photonic architectures have a bigger ratio than hypothetical superconducting qubit architectures. It is currently very much an open question whether or not quantum computers will have a memory hierarchy.

thesz 6y ago
You are off by orders of magnitude. The difference in memory density between DRAM and SRAM (register file) is not 100 times, more like 10x. Standalone "registers" - memory elements of pipelines, state machines etc, - are again not more than ten times less denser than SRAM. After DRAM goes SSD and after SSD goes disk. The difference in price per GB for SSD and disk is about four (4x) times, I looked for that numbers recently. The difference between tape and disk is, again, about 4-10 times (from memory).
- Strilanc 6y ago
  Are you sure? I was just going off a quick search of Amazon, where 1 GB of ram cost ~30$, 1 TB of disk cost ~50$, and a CPU with ~1MB of L2 cache cost ~200$. Based on that I actually thought 100x was a comfortable underestimate, not an overestimate.
  - dcow 6y ago
    CPU cost is not dominated by the cache size is the problem, I think.
    - robocat 6y ago
      But I think that cache is roughly 50% of marginal production cost of the silicon chip die by mm2. https://cdn.wccftech.com/wp-content/uploads/2020/11/AMD-Ryzen-5000-Zen-3-Desktop-CPU_Vermeer_Die-Shot_1-scaled.jpg https://cdn.wccftech.com/wp-content/uploads/2020/11/AMD-Ryze... https://images.anandtech.com/doci/16214/Zen3_arch_19.jpg https://images.anandtech.com/doci/16214/Zen3_arch_19.jpg