Quantum Repeaters: Overcoming Loss for Long-Distance Entanglement

Just think of it like never falling out of love no matter far apart you are.

I recently read Robert Malone’s recent article called The Unbreakable Message about quantum communication not being a physics thought experiment anymore. He says therein “it’s being deployed right now, and it’s going to change who controls secrets, who wins wars, and who you can trust online”.

I agree.

Before I try my best to explain quantum repeater technology, just think of it like in the context of that old saying: “Distance makes the heart grow fonder”. In quantum repeater world however, this might be better expressed as: “Distance doesn’t make love dissipate”. Alice and Bob are linked forever.

Figure 1: Figure 6: Quantum Repeaters and Quantum Networks. Quantum repeaters can extend the range of a quantum network by replacing a long fiber-optic link, where entangled photons are likely to be lost, with a series of shorter links. Each shorter link can attempt to distribute entanglement between repeaters multiple times, while successfully transmitted entanglement is stored in quantum memories. When entanglement is established on every link, the quantum repeaters can perform entanglement swaps to distribute it to the end users. David Levonian: https://aws.amazon.com/blogs/quantum-computing/an-illustrated-introduction-to-quantum-networks-and-quantum-repeaters/.

What is repeater technology?

Context

Imagine you’re sitting at the front of a math class. Nerdy, eager to learn. Phones aren’t allowed in this class but you want to send a message to your classmate at the back of the classroom that contains the solution to the Navier Stokes equations. You decide to write the solution on a piece of paper and put it into an envelope. To get to the back of the classroom, the envelope will have to pass through many hands.

Students as classical repeaters

The other students naturally might want to know what’s in the envelope. Pretty understandable given that they are in math class and the Navier-Stokes solution comes with a million-dollar prize. Let’s assume that perhaps they read the message and decided to rewrite a perfect copy and stuff it in a new sealed envelope. Then, they passed that new sealed envelope to the next student who does the same thing. And on and on. Not a very privacy-abiding class, is it? There are now multiple copies of the Navier-Stokes solutions floating around having been “repeated”. The students act as classical repeaters whereby each one has a little look-see at the message being sent.

But what if the envelope must arrive to this classmate sitting at the back of the classroom without being opened, read or copied, lest the message therein will vanish?

Students as quantum repeaters

Let’s imagine our students are like magical privacy-abiding mules who never open or even peek inside the envelope to read the secret message. Instead, each one uses some invisible quantum threads attached in a specific way to pass the envelope. Each mule performs some secret-relay tricks on their local thread ends, without ever learning the contents, and pass along just some necessary stuff in order to coordinate to passage of the envelope’s contents. In this way, the final mule can pull on their thread and have the exact original Navier-Stokes equations suddenly appear in their hands exactly as originally written.

Thus, in what is a quantum-mechanically enforced way, no mule has read, copied, or otherwise accessed the secret message contents in between.

Here’s an anthropomorphized analogy for any Cartel members reading this. Good drug mules who don’t cut your product are quantum repeaters; bad drug mules who do are classical repeaters.

People are indeed working on quantum repeater technology right now. What that means is that we’re closer than anyone thinks to distributed mega-computing (think global communication) that is unconditionally secure and immune to eavesdropping.

Solving the quantum repeater problem would move spooky action at a distance (quantum entanglement) from the lab setting to global utility to create an unhackable, distributed quantum layer over our existing internet that connects quantum computers, sensors, and secure channels worldwide, much like how optical amplifiers enabled today’s worldwide classical web.

So, yeah. Think of it like when engineers invented optical amplifiers in the 1980s–90s to enable direct amplification using only light, as opposed to the earlier electronic regenerators that required converting the light to electricity for amplification and back to light. Optical amplifiers take the need to convert information (data transmissions) to an electrical signal out of the equation, resulting in better maintenance of information integrity and the ability to transmit said information across much farther distances.

Before those amps, internet signals (for example) traveling through undersea fiber cables weakened so much after just a few tens to hundreds of kilometers that you couldn’t have a truly global web - you’d need to stop, regenerate, and retransmit everything at specific points along the way. So optical amplifiers make the signal strong again without destroying the data, turning choppy long-distance seas (connections) into glassy, gorgeous swells (ie: seamless worldwide internet).

So, much in the same way, quantum repeaters would take those glassy, gorgeous swells and put them on a break that nobody knows about. (I think I took the surf analogy too far here.) Quantum repeaters are like the quantum analog of classical optical amplifiers/repeaters (like Erbium-Doped Fiber Amplifiers (EDFAs). Where optical amps enabled the world wide web by enabling light signals to travel worldwide with minimal added degradation (no electrical conversion, low extra latency, high integrity for bits), quantum repeaters are designed to enable a scalable quantum internet by distributing entanglement over long distances, despite exponential photon loss in fibers or free space.

The result is a unified, upgraded internet where the quantum layer augments the classical one - like adding a secure VIP lane to the existing highway - without tearing up the roads or building a whole new global network from scratch. This coexistence on shared fiber is key to making quantum tech practical and scalable soon.

When I say the quantum internet would run “right next to” our regular (classical) internet, I mean they would share the same physical infrastructure - especially the existing fiber-optic cables that already carry all our emails, videos, Zoom calls, and web traffic - without one interfering with the other.

Think of the fiber-optic cable as a big multi-lane highway where light beams (different “colors” or wavelengths of light) zoom along side by side in separate lanes, like cars on a road. (Sorry, I couldn’t think of a surfing analogy.) Our regular internet uses certain lanes (mostly in the C-band wavelengths) to send massive amounts of classical data at high power. The quantum internet would use quieter, separate lanes (often in the O-band or carefully filtered parts of the same band) to send fragile quantum signals (ie: entangled photons) at very low power.

These lanes co-exist happily in the same cable because engineers use savvy tricks like wavelength division multiplexing (WDM) - basically assigning different light colors to different jobs - and special filters to block noise from the loud classical traffic from drowning out the quiet quantum signals.

You can also think of it like multiple parallel pipes of differently colored water merging into one big pipe (via wavelength-selective junctions), traveling together without the colors mixing, then separated again at the end by color filters into their original pipes. So it’s like a dual-purpose carrier system in one pipe.

Another example. If your city’s water pipes carry both regular drinking water (classical data: high volume, robust) and a tiny, ultra-pure stream of drug (quantum data: low volume, extremely sensitive). They flow through the same pipes but in separate channels so the drug never mixes with or gets contaminated by the main water flow (ahem). The drug delivery is guided and routed using info from the regular water system (like pressure readings or address labels), but the drug itself travels untouched and secure.

I bet they’re actually doing this, by the way, based on newly-released declassified CIA files, by the way. But I digress.

So what you get in the end is seamless harmony of information exchange: classical signals handle routing, addressing, error correction signals, and most of the heavy lifting (ie: sending the actual encrypted message after quantum keys are shared). Quantum parts handle only the special tasks they excel at - like generating unbreakable encryption keys or linking distant quantum computers.

Hybrid packets are emerging too. Recent demos (as of 2025–2026) show quantum info bundled with classical headers (like IP addresses) so the whole thing routes using today’s internet protocols - quantum data rides along invisibly, guided by classical rules.

Quantum would never be used as a replacement for classical systems, it would be used to boost security and enable new stuff like if a bank sent classical transaction data using quantum-secure keys generated over the same fiber.

So solving the quantum repeater problem requires creating entanglement - as in, forcing two quantum particles to be born from the same event, or to interact in a way, that ties their properties together inseparably. No classical copying or communication is involved; the link is baked into their shared quantum origin or interaction from the start. You can think of it like twins being born on the same day and getting a slice of birthday cake where one twin gets white cake and the other gets chocolate: foreverrrrrrrrrr.

Some stuff on photons as linky-doodles

The link between entangled particles is primarily created using photons. You have a single super-energetic parent photon that acts as a high-energy light bullet fired from a powerful laser. You shine this laser into a special crystal (like beta-barium borate or periodically poled lithium niobate - often just called a nonlinear crystal where the intensity of the light influences how the material interacts with it)¹. Very rarely (but predictably), the crystal’s weird quantum rules make the parent photon split into two lower-energy daughter photons (called the signal and idler photons).

Because the split happens from one single particle obeying conservation laws (energy, momentum, and usually polarization or other properties must add up exactly right), the two daughters are forced to be perfectly matched opposites in certain traits - like if one photon ends up vibrating horizontally, the other must vibrate vertically, or their paths/colors are correlated in specific ways.

So they emerge entangled right from creation: their quantum states are no longer independent; measuring one instantly determines the other’s property, even if you separate them by kilometers.This process is called Spontaneous Parametric Down-Conversion (SPDC) - it’s the method used in almost every quantum optics lab, quantum key distribution demos, and early quantum internet experiments. It’s probabilistic (only a tiny fraction of pump photons split this way), but modern setups make it efficient enough for practical use.

So how far along are we in the journey to quantum interwebs? Currently metropolitan and regional quantum networks exist (e.g., ~2,000 km Beijing–Shanghai backbone stable for years, U.S. testbeds ~400+ km) as well as long-distance demos: satellite–ground links >7,600 km achieved. Quantum repeaters (critical for scaling beyond ~100 km without loss) exist as research prototypes (tens of km range), with commercial viability expected ~2027+.

Also, commercial quantum key distribution (QKD) links and early entanglement-based networks have been deployed in pilots (finance, government, utilities) and photonic components and memory tech are advancing rapidly; projects like EuroQCI (European Union), U.S. national testbeds, and companies (e.g., Nu Quantum (UK), IonQ via acquisitions (IonQ are partnered with DARPA and Astrazeneca) building infrastructure.

Quantum machine learning (QML) in the context of LLMs is also a thing and aims to out-do classical machine learning algorithms by improving space and time complexity. For example, why not outsource computationally difficult subroutines to a quantum device?

So that’s it in a nutshell. The mules are gonna mule but it seems that in the very near future, they’re going to be the ones that don’t get offed by Cartel leaders.

And to return to our “Distance doesn’t make love dissipate” saying, it’s true and describes this “state” of quantum transfer of information, where a pair is a single, inseparable system.

How romantic.

Oh, I forgot to add this song by Fugazi, for the punks out there who didn’t sell out. Lookin’ at you Lou. :D

1. https://www.coherent.com/news/glossary/nonlinear-crystals