Werner Heisenberg: Uncertainty into Certainty

In the late 1920s the world was introduced to Werner Heisenberg’s gamma-ray thought experiment. This sought to convey a fundamental conundrum within quantum mechanics: that it is impossible to simultaneously measure the position of a particle and its momentum. The more one accurately measures position, the less is known about momentum, and vice versa. Our understanding of quantum mechanics (including the uncertainty principle) has had enormous implications for the development of all quantum related technologies, from the 1930s to 2030s; from atomic power to compute power. The latter requires a new 6G communications infrastructure which will, over the next decade, require numerous global quantum links utilizing both fiber and free-space optics.

If much is clear at a theoretical level, the practical construction of this infrastructure faces many challenges that scientists and engineers are currently—and increasingly successfully—engaged in tackling the following challenges:

Photon Loss — This refers to exponential signal degradation concomitant to the distance travelled, that is, the further the photon travels the smaller the number of photons that successfully reach the receiver. In practical terms and relating to optical cables, this means that after the 120 to 130 km range, almost none arrive. This is also true of free space optics, but for very different reasons; atmospheric factors (such as moisture) that cause beam divergence, absorption, or scattering.

Decoherence — Refers to the classic quantum states of superposition and entanglement. These are inherently fragile and very sensitive to the deleterious effects coming from temperature, vibration, electromagnetic interference, and the physical properties of optical cables, among other environmental factors. These can rapidly degrade both the quality and luminosity of the quantum information.

Quantum Repeaters — If quantum communication is to be workable over continental and intercontinental distances (which is the whole idea) then this requires the use of quantum repeaters, or nodes, set at short distances of around 100 km. As the amplification or copying of quantum states is physically impossible (since classical approaches destroy the quantum state), the solution is to regenerate entanglement locally, before sending to the next node, where that same process is repeated.

Hardware and Component Limitations — Building a practical global quantum network depends on key hardware components that are still far from either technically mature, commercially viable, or impractical (or cost ineffective) to package at the volumes needed. Given these will be of particular interest to our readers, we further elaborate:

• Low-noise single-photon detectors — By definition, the quantum signal is faint (remember we mostly are dealing with individual photons) which throws up challenges if they are to be accurately registered. The current preferred solution, cryogenic cooled superconducting nanowire single-photon detectors (SNSPDs) face daunting challenges in terms of complexity, power consumption, and the cost of packaging.
• High-quality entangled photon sources — Efficiently generating entangled photon sources to the exacting standard required, is one of the more technically disruptive challenges in building a truly global quantum network. With so many elements needing to be simultaneously aligned and with such precision, the whole forms a particularly complex puzzle—think Rubik’s cube.
• Quantum memory — Only by successfully mapping/storing quantum states at “repeating station” can we enable the necessary entanglement and swapping operations needed for global reach. This is achieved by transferring photon's quantum state into another quantum system (such as a cloud of atoms or a trapped ion). Microseconds to seconds later, the quantum state is retrieved, then re-emitted as a photon to the next node, where the process is repeated.
• Ultra-low-loss components — These include low-loss optical fibers, switches, frequency converters, and integrated photonic circuits. In the quantum environment, even losses that would from the standpoint of classical physics be considered marginal, in the quantum world work to severely compound the exponential photon loss problem. Thus, even fractional weaknesses dramatically increase the stress placed on resource overheads (see below).

Atmospheric Interference and other Challenges in Free-Space Optics — It is often forgotten that even under the most perfect conditions free space optics faces the significant impediments of path-loss, atmospheric absorption, and physical obstacles, all adding to resolution needs beyond “simple” degradation. Collectively these will be met through hardware and software solutions—ironically much of it depending on AI.

Scalability, Integration, and Resource Overheads — The question of whether a quantum network can globally scale remains very much to the fore. From one perspective evolution seems painfully slow since the first DARPA network (2003-2007) (though we will qualify this in the concluding part of this blog). Integration with emerging, first generation classical 6G infrastructures, faces several impediments; not least, quantum channels will need to coexist on the same fiber, or spectrum, without mutual interference, yet still adhere to many common operating protocols (such as precise timing references). Compounding these difficulties are the substantial pressures placed on both quantum and classical resources.

Practical Engineering Issues — Slow QKD key generation rates are currently frustrating the wider adoption of quantum communication technology. This is because most individual photons sent through fiber get lost, undermining the very foundation of the encryption process. By way of an analogy, imagine the inefficiency caused by an authentication code being sent one digit at a time, the majority subject to re-send requests.

Since 2022 there have been significant breakthroughs putting us well on the road to resolving many of the critical issues noted above, several more became apparent in 2025, but there have been some exceptional break-throughs in the first few months of this year. These will be the subject of our third blog in this series.

Read the first blog here.

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Dr. Anthony O'Sullivan
Palomar Technologies
Senior Director of Strategic Marketing