Current state of quantum computing (2019)

National Academies of Science, Engineering, Medicine (NAS, NAE, NAM) published a report on the progress and prospects of quantum computing as of 2019. This is a comprehensive (273 page) report. You can download the report from this link:

Click on the “Download Free PDF” button. When the next page comes up click on the “Download as Guest” button. Then follow the prompts (you have to provide your email).

In this study, the contributors did not have access to the classified information held by the nation-states and the proprietary knowhow held by private companies. But, this is not important for assessing the current status of quantum computing because the open (academic) information is huge.

Earlier this year I brought your attention to John Preskill’s review article on quantum computing. That article was very well written. The report by the National Academies of Science, Engineering, Medicine is also very readable. I highly recommend it. Here’s few quotes from the report:

“While the state of a classical computer is determined by the binary values of a collection
of bits, at any single point in time the state of a quantum computer with the same number of quantum bits can span all possible states of the corresponding classical computer, and thus works in an exponentially larger problem space. However, the ability to make use of this space requires that all of the qubits be intrinsically interconnected (“entangled”), well isolated from the outside environment, and very precisely controlled.”

“Given the unique characteristics and challenges of quantum computers, they are unlikely to be useful as a direct replacement for classical computers. In fact, they require a number of classical computers to control their operations and carry out computations needed to implement quantum error correction. Thus, they are currently being designed as special purpose devices operating in a complementary fashion with classical processors, analogous to a co-processor or an accelerator.”

“The first milestones of progress in QC were the demonstration of simple proof-of-principle analog and digital systems. Small digital NISQ computers became available in 2017, with tens of qubits with errors too high to be corrected. Work in quantum annealing began approximately a decade earlier using qubits built with a technology that had lower coherence times but that allowed them to scale more rapidly. Thus, by 2017 experimental quantum annealers had grown to machines with around 2,000 qubits. From this starting point, progress can be identified with the achievement of one of several possible milestones. Demonstration of “quantum supremacy”—that is, completing a task that is intractable on a classical computer, whether or not the task has practical utility—is one. While several teams have been focused on this goal, it has not yet been demonstrated (as of mid-2018). Another major milestone is creating a commercially useful quantum computer, which would require a QC that carries out at least one practical task more efficiently than any classical computer. While this milestone is in theory harder than achieving quantum supremacy—since the application in question must be better and more useful than available classical approaches—proving quantum supremacy could be difficult, especially for analog QC. Thus, it is possible that a useful application could arise before quantum supremacy is demonstrated. Deployment of QEC on a QC to create a logical qubit with a significant reduction in error rate is another major milestone and is the first step to creating fully error-corrected machines.”

  • Qubits cannot intrinsically reject noise
  • Error-free quantum computing requires quantum error correction
  • Large data inputs cannot be loaded into a quantum computer efficiently
  • Quantum algorithm design is challenging
  • Quantum computers will need a new software stack
  • The intermediate state of a quantum computer cannot be measured directly

“Current debugging methods for classical computers rely on memory, and the reading of intermediate machine states. Neither is possible in a quantum computer. A quantum state cannot simply be copied (per the so-called no-cloning theorem) for later examination, and any measurement of a quantum state collapses it to a set of classical bits, bringing computation to a halt. New approaches to debugging are essential for the development of large-scale quantum computers.”

“Although the physical qubit operations are sensitive to noise, it is possible to run a quantum error correction (QEC) algorithm on a physical quantum computer to emulate a noise-free, or “fully error corrected,” quantum computer. Without QEC, it is unlikely that a complex quantum program, such as one that implements Shor’s algorithm, would ever run correctly on a quantum computer. However, QEC incurs significant overheads in terms of both the number of physical qubits required to emulate a more robust and stable qubit, called a “logical qubit,” and the number of primitive qubit operations that must be performed on physical qubits to emulate a quantum operation on this logical qubit. While QECs will be essential to create error-free quantum computers in the future, they are too resource intensive to be used in the short term: quantum computers in the near term are likely to have errors. This class of machines is referred to as noisy intermediate-scale quantum (NISQ) computers.”

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