What Single Advance in Quantum Computing Is Most Needed in the Near Future?

  • Higher qubit fidelity
  1. Near future or near term — the next six months to a year or so
  2. In a nutshell
  3. Overview
  4. Higher qubit fidelity is the most needed advance in quantum computing in the near term
  5. Personal historical perspective
  6. Advance vs. capability or feature or characteristic
  7. Urgent need, importance, and priority
  8. Criteria for selecting the single most important near-term advance in quantum computing
  9. Different audiences may have different needs and priorities
  10. The audience focus here is on quantum algorithm designers and quantum application developers
  11. Researchers as an audience: what do they need in the near-term?
  12. What is a near-perfect qubit?
  13. What are nines of qubit fidelity?
  14. Qubit fidelity includes coherence, gate errors, and measurement errors
  15. Higher qubit fidelity not likely to achieve near-perfect qubits in the near term
  16. Coherence time will limit the degree to which SWAP networks can be used to simulate connectivity
  17. Two paths to greater circuit depth — longer coherence time or faster gate execution time
  18. The IBM 127-qubit Eagle didn’t address the qubit fidelity issue
  19. Preview of the IBM 433-qubit Osprey
  20. Further improvement to qubit fidelity in the IBM 27-qubit Falcon?
  21. Is 27 qubits the best we can do for the near term?
  22. Where are all of the 40-qubit algorithms?
  23. Need for automatically scalable quantum algorithms
  24. Limited connectivity is more of an absolute barrier — all or nothing, incremental advances are not really possible
  25. Advances not likely in the near term
  26. Quantum error correction (QEC) is a critical priority, but not in the near term, except for research
  27. Advances and capabilities not considered as critical gating or limiting factors in the near term
  28. An alternative to the Quantum Volume metric is not essential for the near term
  29. Advances in fine granularity of phase and probability amplitude not so likely in the near term
  30. A more advanced quantum programming model is not likely in the near term
  31. Advances in qubit fidelity have the added benefit of enabling other advances
  32. Many advances will eventually bump into limitations in other capabilities
  33. Ordering of advances — not so easy to predict or plan
  34. Possible that some qubit technologies might do better than others in the near term
  35. Limiting and critical gating factors may be algorithm-specific or application-specific
  36. Special needs for variational methods
  37. More capable simulators really are needed, but…
  38. Use simulation to find limits for benefits
  39. Any dispute as to the most urgent advance?
  40. What would be the second most needed advance?
  41. My original proposal for this topic
  42. Summary and conclusions

Near future or near term — the next six months to a year or so

In a nutshell

  1. More qubits.
  2. Higher qubit fidelity.
  3. Greater qubit connectivity.
  4. Longer coherence time.
  5. Faster gate execution time.
  6. Greater circuit depth.
  7. Finer granularity for phase and probability amplitude.
  8. Support for nontrivial quantum Fourier transform and quantum phase estimation.
  9. Richer collection of algorithmic building blocks.
  10. More capable simulators.
  • Higher qubit fidelity.
  1. Urgent need. People are struggling without it.
  2. Technical benefit is very high.
  3. Delivers the most bang for the buck.
  4. Applies to all algorithms and applications.
  5. Doesn’t rely on other advances to get started or to make progress.
  6. Incremental progress is possible.
  7. Enables other advances.
  8. It would give the field a boost in momentum.
  • More qubits.

Overview

  1. More qubits.
  2. Higher qubit fidelity.
  3. Higher gate fidelity.
  4. Higher qubit measurement fidelity.
  5. Near-perfect qubits.
  6. Greater qubit connectivity.
  7. Longer coherence time.
  8. Faster gate execution time.
  9. Greater circuit depth.
  10. Finer granularity for phase and probability amplitude.
  11. Support for nontrivial quantum Fourier transform and quantum phase estimation.
  12. Richer collection of algorithmic building blocks.
  13. Scalable quantum algorithms.
  14. Algorithms using 32 to 40 qubits.
  15. More capable simulators.
  16. New qubit technologies.
  17. More advanced qubit technologies.
  18. Modular quantum processor architectures.
  19. Research for advances two to five years from now.
  1. Raw qubit count is not a current limiting factor for most use cases.
  2. Quantum error correction is far over the horizon, so not a near-term priority. Research for two to five years, yes, but not for practical use in the coming months to a year — or even two years.

Higher qubit fidelity is the most needed advance in quantum computing in the near term

Personal historical perspective

Advance vs. capability or feature or characteristic

  1. The introduction or addition of a new capability or feature or characteristic.
  2. The improvement or enhancement of an existing capability or feature or characteristic.
  1. Function. What it does.
  2. Performance. How fast it does it. Units of processing per unit of time. Or units of time per unit of processing.
  3. Capacity. How large or how many units of information can be processed.

Urgent need, importance, and priority

Criteria for selecting the single most important near-term advance in quantum computing

  1. Urgent need. People are struggling without it.
  2. Technical benefit is very high.
  3. Delivers the most bang for the buck.
  4. It’s essential. Real progress is not possible without it.
  5. It would give the field a boost in momentum. Accelerate progress beyond the raw technical benefit of the advance itself.
  6. It’s a place to start. It requires nothing else. Helps to start building momentum.
  7. It’s an easy place to start. It requires little effort to get started.
  8. Doesn’t rely on other advances to get started or to make progress.
  9. It’s easy to implement. Minimal effort to complete.
  10. Incremental progress is possible. It can be implemented and used incrementally. It’s not an all or nothing proposition. Subsets of the full capability are reasonably useful.
  11. Already in progress. Needs to be finished, but risk is low.
  12. Immediate use without any change to algorithms or applications. A wide range of quantum algorithms and quantum applications could immediately use it without any change to the algorithms or application source code.
  13. It applies to a moderate range of algorithms and applications.
  14. It applies to a very wide range of algorithms and applications. Wider range makes it more valuable.
  15. It applies to all algorithms and applications. Universal benefit makes it extremely valuable.
  16. Research is ripe to pursue. No major open issues which might take years to resolve. Completed research is sitting on the shelf and published.
  17. Science and tech is ready. No research is needed. Everything that is needed is sitting on the shelf and published.
  18. Technical feasibility. Can be implemented within a few months to a year.
  19. Feasible in the near term. Everything that it requires is already in place — or will be completed as part of the task.
  20. People are clamoring for it. Satisfies a market demand.
  21. Needed for the two-year horizon. Foundation for the next stage.
  22. It enables other advances.
  23. It enables higher-priority capabilities.
  24. Helps to enable quantum error correction. A longer-term goal, but all of the pieces need to be put in place over an extended period of time.
  25. Helps to enable quantum parallelism. This is the whole point of quantum computing.
  26. Helps to achieve quantum advantage. Also the whole point of quantum computing.
  27. Simplifies quantum algorithm design.
  28. Simplifies quantum application development.
  29. Makes quantum algorithms more efficient.
  30. Makes quantum applications more efficient.

Different audiences may have different needs and priorities

  1. Researchers. They’re developing the capabilities described in this paper rather than using them. What they need is more money, talent, management support, and time. And access to underlying research.
  2. Algorithm designers.
  3. Application developers.
  4. Business customers.
  5. Science customers.
  6. Engineering customers.
  7. End users.
  8. IT staff.
  9. Management.
  10. Executives.
  11. Each application category.

The audience focus here is on quantum algorithm designers and quantum application developers

Researchers as an audience: what do they need in the near-term?

  1. Priority on research.
  2. Funding for research. And engineering.
  3. Talent pool for research. And engineering.
  4. Management support.
  5. Time.
  6. Access to underlying research. Building on the work of other researchers. Hopefully not hidden or protected by intellectual property (IP) protections.
  7. Feedback from users, algorithm designers, and application developers to develop better technical capabilities.

What is a near-perfect qubit?

What are nines of qubit fidelity?

  1. 90% = one nine.
  2. 95% = 1.5 nines.
  3. 98% = 1.8 nines.
  4. 98.5% = 1.85 nines.
  5. 99% = two nines.
  6. 99.5% = 2.5 nines.
  7. 99.9% = three nines.
  8. 99.95% = 3.5 nines.
  9. 99.99% = four nines.

Qubit fidelity includes coherence, gate errors, and measurement errors

  1. Coherence. Quantum state can decay or decohere over time, the coherence time.
  2. Single-qubit gate execution errors. Potential for errors executing a quantum logic gate on even a single qubit.
  3. Two-qubit gate execution errors. Potential for errors executing a quantum logic gate on two qubits. This is generally the limiting factor which determines overall qubit fidelity.
  4. Measurement errors. Even measurement of a qubit is not 100% reliable. And it tends to be less reliable than even two-qubit gate execution.
  5. Variations between qubits. Not all qubits in a given quantum processor have the same fidelity. Even different pairs of qubits can have different two-qubit gate execution errors.

Higher qubit fidelity not likely to achieve near-perfect qubits in the near term

  1. Full near-perfect qubit fidelity. A full four nines or better. Rather unlikely in the near term. The primary goal for the two-year timeframe. Good enough for the vast majority of quantum algorithms and quantum applications.
  2. 3.75 nines. May be close enough to near-perfect for many quantum algorithms and quantum applications. But not so likely in the near term. Good enough for a sizable majority of quantum algorithms and quantum applications.
  3. 3.50 nines. Perfectly reasonable goal for the near term. May be the best to hope for in the near term. Reasonably acceptable minimum achievement for the two-year timeframe. Good enough for many or even most quantum algorithms and quantum applications.
  4. 3.25 nines. Reasonable goal for the near term. Marginally acceptable achievement for the two-year timeframe. Good enough for a significant fraction of quantum algorithms and quantum applications.
  5. Three nines. Marginally reasonable goal for the near term. Bare minimum achievement for the two-year timeframe. Good enough for some quantum algorithms and quantum applications.
  6. 2.75 nines. Minimal reasonable goal for the near term. Disappointing achievement for the two-year timeframe. Good enough for some niche quantum algorithms and quantum applications.
  7. 2.50 nines. Disappointing achievement for the near term. Dismal failure for the two-year timeframe. Generally not good enough for any quantum algorithms and quantum applications.
  8. 2.25 nines. Only acceptable as a stepping stone, a milestone on the path to higher qubit fidelity. But not really usable in any meaningful manner.
  9. Under two nines. Fairly dismal failure, even in the near term.

Coherence time will limit the degree to which SWAP networks can be used to simulate connectivity

  1. Deeper circuits.
  2. Larger circuits.
  3. Improved effective connectivity — if relying on SWAP networks.

Two paths to greater circuit depth — longer coherence time or faster gate execution time

The IBM 127-qubit Eagle didn’t address the qubit fidelity issue

Preview of the IBM 433-qubit Osprey

Further improvement to qubit fidelity in the IBM 27-qubit Falcon?

Is 27 qubits the best we can do for the near term?

Where are all of the 40-qubit algorithms?

  1. Low qubit fidelity.
  2. Limited qubit connectivity.
  3. Limited coherence time.
  4. Limited circuit depth.
  5. Lack of rich algorithmic building blocks.
  6. Lack of experience designing complex algorithms.

Need for automatically scalable quantum algorithms

Limited connectivity is more of an absolute barrier — all or nothing, incremental advances are not really possible

Advances not likely in the near term

  1. Quantum error correction (QEC). Some number of years required.
  2. Larger quantum Fourier transform (QFT). Lucky if we can get even 12 or 16 qubits in the near term.
  3. Greater connectivity. Significant architectural changes required.
  4. Fine granularity of phase and probability amplitude. Unless it’s a quick fix in the firmware. But more sophisticated hardware and architectural changes are likely required.
  5. Quantum networking. Much research is needed.
  6. Quantum volume alternative. Unless somebody comes up with one shortly.
  7. Quantum-native programming languages. Depend on more advanced programming models.
  8. More advanced quantum programming models. Still too hard and too unknown, even if it is desperately needed. Much research is required. Not really needed until we have a moderate range of qubits with high fidelity anyway. Figure three to five years.

Quantum error correction (QEC) is a critical priority, but not in the near term, except for research

Advances and capabilities not considered as critical gating or limiting factors in the near term

  1. More qubits. We already have plenty for many use cases — they just don’t have sufficient fidelity, connectivity, or fine enough granularity of phase or probability amplitude.
  2. Support software and tools. They are important, but generally they can be designed and implemented relatively easily and with low technical risk so that they are not true and substantial advances per se. Generally they will make life easier, but quantum algorithm designers and quantum application developers can generally get along (or occasionally limp along) without them, or with only primitive support software and tools.
  3. An alternative to the Quantum Volume metric. Such as for more than 50 qubits, or even 40, 32, or 28, or 24 qubits.
  4. A more advanced programming model. This will be essential at some stage in the future, but there are much more pressing needs in the near term.
  5. A quantum-native programming language. Ditto.

An alternative to the Quantum Volume metric is not essential for the near term

Advances in fine granularity of phase and probability amplitude not so likely in the near term

  1. Reduction and limitation of noise and other interference.
  2. Precision of the digital to analog converters (DACs) used to convert digital data to analog form to be applied to the qubit hardware.
  1. Practical considerations. Availability and cost of fine granularity DACs. And practical limitations on precision of DACs. 8-bit, 16-bit, 18-bit, 20-bit, and 32-bit precision is available. What’s actually practical in the context of a quantum computer and qubit control is another matter, and unclear, and never documented.
  2. Theoretical considerations and limits of physics. What if an application really does need a 48-bit or 80-bit quantum Fourier transform (or Shor’s factoring algorithm needs 4096 or 8192-bit QFT)? What does the underlying physics support even if you had ideal digital and analog logic?
  1. 8 qubits. Hopefully a slam dunk. I would hope this could happen over the next year.
  2. 12 qubits. Hopefully a slam dunk. But when? Again, one can hope for the next year, but that would require a number of other advances, and possibly upgrades to the qubit control hardware.
  3. 16 qubits. Unlikely over the next year.
  4. 20 qubits. Should be feasible. But not over the next year.
  5. 24 qubits. May or may not be feasible.
  6. 28 qubits. May or may not be feasible.
  7. 32 qubits. May or may not be feasible.
  8. 40 qubits. Questionable feasibility.
  9. 48 qubits. Dubious feasibility.
  10. 56 qubits. Beyond speculation at this stage.
  11. 64 qubits. Ditto.
  12. 72 qubits. Ditto.
  13. 80 qubits. Ditto.
  14. 96 qubits. Ditto.
  15. 128 qubits. Ditto.
  16. And beyond 128 qubits. Ditto.

A more advanced quantum programming model is not likely in the near term

  1. It’s not the critical technical gating factor. Lack of high fidelity qubits would preclude its operation. Since only relatively short quantum circuits would be supported by the hardware, the advanced quantum programming model would focus on larger, more sophisticated quantum algorithms.
  2. The conceptualization of a more advanced quantum programming model is still an open research question. Could take at least a few years of conceptual development and experimentation before the conceptual model matures and serious, production-scale implementation can begin.
  3. A more advanced programming model would be more focused on larger algorithms with many more qubits. Limited utility for 32 to 40 or maybe even 80 qubits.

Advances in qubit fidelity have the added benefit of enabling other advances

  1. Greater simulated connectivity using SWAP networks. Critically limited by low qubit fidelity.
  2. Greater circuit size.
  3. Greater circuit depth.
  4. Nontrivial quantum Fourier transform (QFT).
  5. Nontrivial quantum phase estimation (QPE).
  6. Nontrivial quantum amplitude estimation (QAE).
  7. Nontrivial amplitude amplification.
  8. More advanced and more sophisticated quantum circuits.
  9. Much richer collection of algorithmic building blocks.

Many advances will eventually bump into limitations in other capabilities

  1. Limited connectivity. SWAP networks can dilute or consume too much of qubit fidelity.
  2. Coherence time. Even with very high fidelity, eventually qubits (or gates, actually) will bump up against limited coherence time.

Ordering of advances — not so easy to predict or plan

Possible that some qubit technologies might do better than others in the near term

  1. Longer coherence time.
  2. Generally better qubit fidelity. Although transmon qubits may be catching up.
  3. Greater connectivity. Full any to any connectivity.

Limiting and critical gating factors may be algorithm-specific or application-specific

  1. Qubit connectivity.
  2. Coherence time or circuit depth.
  3. Fine granularity of phase or probability amplitude.
  4. General lack of support for nontrivial quantum Fourier transform (QFT) or quantum phase estimation (QPE).

Special needs for variational methods

More capable simulators really are needed, but…

  1. Greater capacity.
  2. Higher performance.
  3. More qubits.
  4. Greater circuit depth.
  5. More analysis tools.
  6. More debugging tools.
  7. Better and more accurate noise models. Exactly match existing and proposed quantum computers, so that a simulation run is an accurate reflection of running on a real quantum computer.
  8. Exploit distributed computing. Much greater capacity and performance.
  9. In summary, deliver great simulation of 32 to 40-qubit quantum circuits.

Use simulation to find limits for benefits

Any dispute as to the most urgent advance?

What would be the second most needed advance?

  1. Greater qubit connectivity.
  2. Longer coherence time.
  3. Faster gate execution time.
  4. Greater circuit depth.
  5. Finer granularity for phase and probability amplitude. To enable quantum Fourier transform (QFT), quantum phase estimation (QPE), quantum amplitude estimation (QAE), and amplitude amplification.
  6. Support for nontrivial quantum Fourier transform and quantum phase estimation.
  7. Richer collection of algorithmic building blocks.
  8. More capable simulators.
  1. Focus on connectivity for transmon qubits.
  2. Focus on finer phase granularity for trapped-ion and neutral-atom qubits. And possibly for transmon qubits as well if greater connectivity is not feasible in the near term.

My original proposal for this topic

  • What single advance in quantum computing is most needed in the near future? There are so many! What criteria to use. How near-term — three months, six months, nine months, one year? Maybe qubit fidelity, or maybe qubit connectivity, or…?

Summary and conclusions

  1. Higher qubit fidelity is the most needed advance in the near term — the next year or so.
  2. Higher qubit fidelity will deliver the most bang for the buck.
  3. Higher qubit fidelity enables numerous other advances, including quantum Fourier transform (QFT) and quantum phase estimation (QPE), use of SWAP networks to simulate qubit connectivity for transmon qubits, and deeper quantum circuits.
  4. Greater qubit connectivity is very important, but will likely require architectural changes for transmon qubits.
  5. Quantum error correction (QEC) is very important for the longer term and certainly a research priority in the near term, but not a practical priority for the near term.
  6. Higher qubit fidelity is not likely or guaranteed to achieve true near-perfect qubits in the near term — over the next year. It might take two years to get there.
  7. Greater coherence time and circuit depth are important for the medium term, but are not critical until qubit fidelity and qubit connectivity are addressed — only relatively shallow circuits can be executed reliably with low qubit fidelity and limited qubit connectivity.
  8. Different audiences may have different needs and priorities relative to each possible advance. This paper focuses on quantum algorithm designers in general as well as quantum application developers in general. Specific niche categories may have different needs and priorities than discussed here.
  9. Different algorithm and application categories may have different requirements for which advances should have top priority.
  10. There are a wide range of criteria that can be used to judge which advances should have higher priority, including: urgent need — people are struggling without it, technical benefit is very high, applies to all algorithms and applications, doesn’t rely on other advances to get started or to make progress, incremental progress is possible, enables other advances, and it would give the field a boost in momentum. And many other criteria.
  11. What should be the next priority after higher qubit fidelity? That’s too difficult to say — there are so many urgent priorities. But once qubit fidelity is no longer the big holdup for most algorithms, the next big holdup will quickly become obvious. I suspect it will be limited connectivity.
  12. The one advance that isn’t a candidate for top priority in the near term is actually the advance that gets so much of the attention in recent months and years: more qubits. We already have enough qubits for many or even most algorithms, but low qubit fidelity and limited qubit connectivity make it very difficult for many algorithms to utilize any significant fraction of those qubits.
  13. I do think we definitely need a much richer collection of algorithmic building blocks as soon as possible, but once again low qubit fidelity and limited qubit connectivity render this goal unachievable in the near term.
  14. Where are all of the 40-qubit algorithms? There are a number of limiting factors, with low qubit fidelity being at the top of the list. Higher qubit fidelity alone may not be enough to open the floodgates for 40-qubit algorithms, but it’s the top priority step to take.
  15. More capable simulators would be a big win in the near term, but I’d rather keep the priority focus on qubit fidelity for now. That said, I’d push hard for further research in simulators, near term, medium term, and longer term.

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Freelance Consultant

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Jack Krupansky

Jack Krupansky

Freelance Consultant

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