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Quantum Sensing Explained: Clocks, Sensors and Quantum Networks

Quantum sensing and networking explained: NIST's record clock, a 35 km entanglement test in Boston, and why the UK NCSC backs PQC over QKD.

By CyphrQ.ai

Written with AI assistance and human editorial review. Every figure linked to its source.

9 min read

Quantum sensing uses the fragile states of atoms and light as measuring instruments, so tiny changes in time, gravity or magnetic fields become measurable. Quantum networking links quantum devices by sending quantum states, such as entangled photons, over fibre. Both are real research fields with published results. Neither is the same as a quantum computer.

This post is quantum sensing explained from primary sources: a record-setting NIST clock, a city-scale entanglement experiment, the US Department of Energy's plan for a quantum internet, and the UK National Cyber Security Centre's view on quantum key distribution. Error-correction milestones in quantum computing are a separate story, covered in our post on logical qubits.

What is quantum sensing, in plain terms?

A quantum system reacts to the smallest disturbance around it. For a quantum computer, that sensitivity is a nuisance that causes errors. For a sensor, it is the whole point.

An atom's energy levels are fixed by nature, so a measurement tied to them is the same in every lab. A disturbance, such as a magnetic field or a change in gravity, shifts the atom's behaviour by a tiny but readable amount. Sensors built this way include atomic clocks, magnetometers and gravity sensors.

The US Department of Energy's 2020 quantum internet blueprint lists hoped-for uses for networks of "ultra-sensitive quantum sensors": helping engineers "better monitor and predict earthquakes", searching for underground oil, gas or mineral deposits, and health care and imaging. Those were stated goals in 2020, not delivered products.

How accurate is the world's most accurate clock?

On 14 July 2025 NIST announced that its aluminium ion clock had set a new record for accuracy. The key facts from NIST's release:

Measure What NIST reports
Systematic uncertainty 5.5 × 10⁻¹⁹, which NIST describes as accurate to the 19th decimal place
Against the previous record 41% greater accuracy than the previous record, held by a strontium lattice clock
Stability 2.6 times more stable than any other ion clock
Design an aluminium ion paired with a magnesium ion; the magnesium cools the aluminium and reads out its state
Paper Marshall and colleagues, Physical Review Letters, DOI 10.1103/hb3c-dk28

"Systematic uncertainty" is the clock-maker's estimate of how far the clock's frequency could be off because of every known physical effect, from stray fields to motion. A smaller number means a more trustworthy clock.

The pairing technique is called quantum logic. One ion keeps time; the other does the jobs the first can't do easily. NIST calls the clock a quantum logic testbed as well as a timekeeper.

Why would anyone need a clock that precise?

NIST names three uses:

  • Redefining the second. The clock supports the international effort "to redefine the second to much greater levels of accuracy".
  • Geodesy. Clocks tick at slightly different rates at different heights because of gravity, an effect predicted by general relativity. A clock precise enough can therefore measure height and the shape of Earth's gravity field.
  • New physics. Comparing clocks over time can test whether fundamental constants are "actually changing".

The geodesy point is the clearest example of a clock becoming a sensor. Precise timekeeping and precise measurement of gravity turn out to be the same instrument.

What is quantum networking?

A quantum network carries quantum states between locations, rather than ordinary bits. Its main resource is entanglement: two particles linked so that measuring one tells you about the other, however far apart they are. Entanglement does not send messages faster than light; you still need an ordinary channel to make use of it.

What would it be used for? Connecting quantum computers, linking quantum sensors so they measure together, and some forms of key exchange. The DOE blueprint, published on 23 July 2020, sets out four milestones:

  1. Verify "secure quantum protocols over existing fiber networks".
  2. Send entangled information "across campuses or cities".
  3. Expand networks between cities.
  4. Expand between states, using quantum "repeaters".

In the same announcement, the DOE said scientists from Argonne National Laboratory and the University of Chicago had entangled photons across a 52-mile "quantum loop" in the Chicago suburbs, to be connected to Fermilab as a "three-node, 80-mile testbed".

What did the Boston quantum network experiment show?

In May 2024, Nature published Entanglement of nanophotonic quantum memory nodes in a telecom network by C. M. Knaut and colleagues, a group that includes M. Lončar and M. D. Lukin. The abstract reports:

  • a two-node quantum network made of multi-qubit registers;
  • operation at a telecommunication wavelength, 1,350 nm;
  • tests through 40 km spools of low-loss fibre;
  • a 35 km fibre loop deployed in the Boston urban area;
  • entanglement of two nuclear spin memories.

The step that matters is "memory". Sending entangled photons is one thing. Storing the entanglement in a node that can hold it and work with it is what a future network needs, because repeaters rely on it. Doing that over fibre laid under a real city, rather than on a lab bench, moves the work toward DOE's second milestone.

Is quantum networking the same as quantum-safe encryption?

No, and this is where security decisions get confused.

Quantum key distribution (QKD) uses quantum states to let two parties agree a secret key. Post-quantum cryptography (PQC) is ordinary software maths designed to resist quantum computers. They are different answers to the same worry.

The UK National Cyber Security Centre's white paper Quantum networking technologies, version 1.0 of 5 August 2025, is direct (the page's web address uses a different name, but the title on the page is "Quantum networking technologies"):

  • "The NCSC will not support the use of QKD for government or military applications."
  • "PQC is the best mitigation to the threat to cryptography from quantum computers."
  • "QKD does not provide authentication, nor do any other quantum techniques."

The white paper also points to the difficulty of integrating quantum and classical components securely, and of building implementations that hold up against well-resourced attackers.

For what PQC means for you, see Q-Day explained and our post on hybrid post-quantum key exchange in TLS.

Is quantum navigation or a quantum internet ready yet?

Not yet. None of the sources in this post shows:

  • That record clocks leave the lab. NIST's result is a laboratory instrument. The release describes what it could support, not a fielded product.
  • That two nodes make an internet. The Boston experiment links two nodes. DOE's fourth milestone, repeaters spanning states, has not been reached in any source I read.
  • That quantum navigation is ready. Using quantum sensors to navigate without satellite signals is widely discussed. I didn't find an official result to cite for this post, so I make no claim about its readiness.
  • That QKD replaces authentication. The NCSC is explicit that it doesn't.

What should you watch next in quantum sensing and networking?

  1. The redefinition of the second. Watch for international decisions on which kind of optical clock becomes the standard.
  2. Networks with more than two memory nodes, and the first working quantum repeaters over deployed fibre.
  3. Field trials with published comparisons, where a quantum sensor's performance is reported against a classical one under the same conditions.
  4. Government positions on QKD and PQC, including whether other agencies follow the NCSC's stance.

Going deeper: for security and IT teams

This section is for technical readers and decision-makers: architects, network and security leads, and anyone asked to evaluate a "quantum" product.

Your real dependency is time. Most organisations will meet quantum clocks indirectly, through better national time services. What they depend on today is accurate, trustworthy time. Certificate validity checks, Kerberos ticket lifetimes, log correlation during an incident, and transaction timestamps all assume clocks agree. Attackers who can shift a system's clock can make expired certificates look valid or scramble an investigation timeline.

Controls for time. Use more than one independent time source and alert when they disagree. Where your platforms support it, use Network Time Security, defined in RFC 8915 (Standards Track, September 2020). NTS adds cryptographic protection to client-server NTP: it uses TLS for initial key establishment, through the NTS Key Establishment protocol on TCP port 4460, then authenticated encryption (AEAD) to protect the time packets. If a firewall blocks TCP 4460, NTS-KE fails, so test the path before you rely on it. Log time-source changes and large clock corrections as security events.

Threat model for QKD claims. QKD protects key agreement on one link. It still needs an authenticated classical channel, and the NCSC states that QKD itself provides no authentication. In practice, that authentication comes from classical cryptography, which brings back the question QKD was meant to answer. Long links may also need relay points that hold keys, which become high-value targets. And a dedicated fibre link can be disrupted.

Questions for a QKD vendor: - How are the endpoints authenticated, and with which algorithm? - If authentication uses post-quantum signatures or pre-shared keys, what does QKD add over PQC alone? - Are there intermediate nodes that see keys, and how are they protected and audited? - What independent evaluation or certification has the product passed? - What happens to traffic when the quantum link is down?

Prioritise PQC migration. For most organisations the defensible move is a cryptographic inventory and a PQC migration plan, in line with the NCSC's statement that PQC is the best mitigation. Treat QKD as a niche option that needs its own justification.

Treat quantum sensors as connected devices. If your organisation pilots quantum sensors for infrastructure monitoring or surveying, they are IoT or operational-technology devices first. Apply the usual controls: firmware update process, network segmentation, signed data, and an owner for each device.

Asking for evidence on sensor claims. Request performance figures with the conditions they were measured under, such as temperature, vibration and magnetic environment, plus independent calibration. Ask whether results are peer-reviewed or traceable to a national measurement institute.

A checklist for evaluating quantum sensing and networking claims

  1. Ask which milestone it is. Lab record, city-fibre test or deployed service.
  2. Check node count and distance for any network claim.
  3. Ask who authenticates in any QKD proposal.
  4. Put PQC first in your cryptography roadmap.
  5. Secure your time sources now, whatever happens with quantum clocks.

Learn more

Key takeaways

  • Quantum sensing turns the sensitivity of atoms and light into precise measurement of time, gravity and magnetic fields.
  • NIST's aluminium ion clock reports a systematic uncertainty of 5.5 × 10⁻¹⁹, 41% better than the previous record.
  • A 2024 Nature paper entangled two quantum memory nodes over a 35 km fibre loop in Boston.
  • DOE's 2020 blueprint sets four milestones for a quantum internet, ending with repeaters between states.
  • The UK NCSC will not support QKD for government or military use and calls PQC the best mitigation.
  • For IT teams, the near-term action is trustworthy time (including NTS, RFC 8915) and PQC planning.

Sources