Entanglement-enhanced quantum sensing: fact or fiction?

Quantum sensing, the use of quantum systems to perform precision measurements, attracts enormous interest as an potential application of engineered quantum systems being developed. According to this review article, there are three classes of quantum sensing:

1. Sensing based on systems with quantized energy levels such as superconducting qubits, including SQUID magnetometers (already commercialized).

2. Sensing based on quantum coherence or wave-like properties, including noise suppression using squeezing (e.g. in gravitational wave detectors) and macroscopic quantum states of atoms for precision gravimetry and inertial navigation (under development / being commercialized).

3. Entanglement-enhanced sensing to achieve precision beyond what is attainable classically (research in progress). 

It is believed that only entanglement-enhanced sensing makes use of the full power of quantum mechanics (i.e. many-body entangled states intractable for classical computers). 

The growing availability of large controlled quantum systems has led to huge interest in entanglement-enhanced sensing schemes, with many publications in high impact journals, but not everyone is convinced.

Critiques of recent high-profile experiments on entanglement-enhanced sensing published in Nature and Nature Physics have been posted to arXiv: arXiv:2208:14816, arXiv:2301.04396, and (today) arXiv:2302.00733. The first is a particularly interesting read, since it has been updated to include correspondence with the paper authors and Nature Physics editors, who declined to publish it.

I do not work in this field. I do not have the expertise to judge whether the criticism is valid or not. But it seems to me that the comments come from a knowledgeable expert, are written in a scientific style, and are of a reviewable standard. Moreover, the claims in the critiqued articles (unprecedented sensitivity at measuring some quantity) are quantitative, and can thus be unambiguously proved or disproved. Thus, it should be concerning that while one of the articles claiming entanglement-enhanced sensitivity has already been cited 30 times according to Google Scholar, the criticism seems to be ignored - not cited, not responded to, not even upvoted on scirate.

Several years ago there was a similar controversy in photonics, with many researchers racing to be the first to claim to demonstrate lasing in a variety of exotic materials. In response to this, Nature Photonics introduced a "laser checklist" to ensure that all submissions reporting claims of lasing provide a standardized set of measurements and experimental details which can be scrutinized and easily compared between different platforms and research groups. Perhaps something similar can be done for entanglement-enhanced sensing papers?

Quantum computing debated in The Financial Times

Criticism of quantum computing hype and a rebuttal recently appeared in The Financial Times. The first article argues that even "well-established" applications of future quantum computers - breaking encryption and efficient quantum chemistry calculations - may not be useful in practice. The second article notes that even though there is tremendous hype, there is also slow but steady progress in scaling up quantum processors and understanding which quantum algorithms might provide value and which will not.

It is worth emphasizing that quantum technologies are much broader than quantum computing. For example, quantum research in Singapore are also encompasses quantum communications and quantum sensing. While these areas a seen as being closer to useful commercial applications, there are still some important caveats:

Quantum communication technologies are often marketed as the solution to the problem of future quantum computers being able to break widely-used public key cryptography schemes, with quantum key distribution providing unbreakable encryption protected by the laws of physics. The reality is that sharing of encryption keys is just one part of a secure communications network; a far bigger problem is authentication - how can you prove the other party is who they claim to be? Indeed, the vast majority of data breaches or online scams are not due to encryption protocols being broken or passwords being hacked, but rather are a result of phishing attacks where the victim is tricked into believing the attacker is someone else. The UK's National Cyber Security Centre's position on quantum communication technologies is:

"Given the specialised hardware requirements of QKD over classical cryptographic key agreement mechanisms and the requirement for authentication in all use cases, the NCSC does not endorse the use of QKD for any government or military applications, and cautions against sole reliance on QKD for business-critical networks, especially in Critical National Infrastructure sectors.

In addition, we advise that any other organisations considering the use of QKD as a key agreement mechanism ensure that robust quantum-safe cryptographic mechanisms for authentication are implemented alongside them."

Quantum sensing promises the ability to perform measurements with precision unattainable using classical devices. This encompasses many well-established approaches based on quantum coherence, including SQUIDs, atomic clocks, atomic gravimeters, and squeezed light interferometers, and more speculative ideas based on large-scale quantum entanglement. The latter entanglement-based approaches have however attracted criticism (see for example this preprint).

In all these examples - quantum computing, quantum communications, and quantum sensing - useful technologies will not emerge from quantum researchers working in isolation. Collaboration with researchers working in other disciplines and industry is essential to keep quantum "solutions" honest and ensure that we are solving problems that need to be solved, and to establish that quantum techniques provide a better solution than well-established classical methods.