Three Quantum party tricks

Previously, Steve started hoarding encrypted gossip like a raccoon hoarding shiny objects. Today, we meet the three quantum rules that stop him hoarding quietly.

 

These are the bits of quantum mechanics you actually need for QKD. Not the whole quantum mechanics. Nobody needs the whole quantum mechanics. That way lies madness and an unfortunate number of chalkboards.

 

The Collapse of the Wavefunction

Imagine a beam of light. It’s hurtling along, not a care in the world, because there’s strength in numbers. Billions of photons, all on the same page, all with one common goal: to be a beam of light.

 

But one of these photons missed lunch. It sneaks off, out of the beam and away from the others. It finds a food truck and slips into the queue. Paranoid, it checks over its shoulder. It got away with it, none of the others noticed.

 

As the queue shuffles forward, it squints at the menu. What should it have with its mashed potatoes? Beans or gravy? It looks around, to confer, to assess the consensus, but, of course, there’s nobody there. It is alone, and alone it must decide.

 

It grins. Oh, the freedom of it, the power! Giddy with possibility, the photon imagines potatoes piled high with beans or smothered in gravy or - oh my - engulfed by a kaleidoscopic swirl of beans and gravy. It feels wild, it feels unleashed, like it’s simultaneously consuming all these potato wonders at once. It feels… like a wavefunction.

 

It gets to the front of the queue. Oh no. The server is looking at it - observing, in fact - the question in their raised eyebrows clear: what will it be? The photon panics, all excitement gone. What was it thinking? Decisions are terrible. It can’t do this alone.

 

Its heart sinks. Its shoulders slump in defeat. One could almost say it collapses. Embroiled in self-conscious horror, it opens its mouth and lets words fall out. What are they? In its state of utter panic, it can’t tell - but it has a sense that there is some sort of probability distribution function at play.

 

As it walks away, scowling at its cheese toasty, it silently vows to immediately join the next beam of light to hurtle past.

 

All that to say: if you observe a photon in a superposition state - which here means a photon that is simultaneously and outrageously occupying a bunch of states at once - by, say, measuring it, you collapse the wavefunction and force the photon to be sensible and do only one thing at a time.

 

In QKD terms: ‘peeking’ is a kind of measurement, and measurement leaves traces. Steve cannot look without, at least sometimes, changing what Bertha later sees.

 

Entanglement

A photon is minding her own business. She is moving through life, carefree and heedless, trusting in the universe. Suddenly, she finds herself spinning anticlockwise. Ah, she thinks to herself. Her sister, wherever she may be, far out there in the void, must be spinning clockwise.

 

Their mother, you see, never spun. Not once, not for anyone — she was known for it, in fact. It was as if, through her stubbornness, she compelled her daughters to collectively inherit this quirk. To conserve it, if you will.

 

So, whenever one daughter spun one way, the other was obliged to do the opposite, whether she willed it or no. Whatever the hour or the place, however far apart they were, one daughter’s twirl must negate the other’s; no net spin could be permitted.

 

The photon didn’t mind. She had come to terms with it. It wasn’t that intrusive really, and it was nice to still feel connected, to be entangled, with her sister, no matter the distance between them. She just wished she hadn’t eaten quite so many eggs for breakfast.

 

All clear? No? Strange. Basically: if two photons are entangled, they share a quantum state. That means that, if you measure one, you instantly learn something about what you will find when you measure the other (with correlations that classical physics can’t imitate). If you think that’s spooky, you’re in good company.

 

In QKD terms: entanglement lets you treat correlation itself as the tamper-evident seal. If Steve interferes, the correlations get worse, and you can detect that statistically.

 

No Cloning

Imagine you have a cake. Maybe it’s your birthday, or maybe you just found it sitting on your desk one morning. The cake is magnificent. It’s beautiful and you can tell it’s going to taste amazing. But that poses a conundrum: if you eat the cake, you won’t be able to gaze at it anymore, and if you preserve the cake, you’ll never taste its sugary goodness. What you really want… is to have your cake and eat it.

 

You know they say you can’t, but what do they know? You’ll just make a perfect copy of it so that you can eat one and keep the other. Easy. All you need to do is work out what it’s made from. The sharpness of the frosting, the bounciness of the sponge, the moistness - it’s in an unknown state right now, but you can work it out.

 

You stare at it. You walk around to the other side of the desk and stare some more. You shine a torch at it and squint like a mole emerging from its burrow, but it’s no use. You’re going to have to taste it.

 

With your pinky finger, you scoop out the smallest splodge of frosting and pop it on your tongue. Hmm. Is that citrus? It could be lemon, or lime, or even… raspberry? You go for another scoop, but it gets on your sleeve and you’ve smudged the sprinkles. Well, you’ve done it now, so you might as well get serious. You fetch your chemistry set.

 

Three hours later, you’ve titrated the icing, centrifuged the jam and Bunsen-burnered the sponge. It’s down your front, in your hair and on your nose. Everyone else in the office thinks you’re crazy and your boss has set up a meeting with HR, but you don’t care because you’ve done it. You’ve discovered all the cake’s secrets, you know its every molecule so well you could remake it in your sleep (you won’t though, because you know about oven safety).

 

Then, as you stare at the sticky mess of cakey innards covering your desk, yourself and half the office, your triumph begins to fade. You’ve destroyed it. Worse, you’ve forgotten what it looked like. Did you take a photo? No, you did not.

 

As desolation overcomes you, Susan from accounts pats you on the back. ‘Never mind,’ she says. ‘You can’t have your cake and eat it.’

 

The moral of the story is: if a photon is in an unknown quantum state, you can’t create a perfect copy of that state without disturbing the original.

In QKD terms: Steve can’t silently copy the ‘interesting’ photons, keep his copies for later, and forward the originals untouched. The universe has, irritatingly, vetoed his entire business model.

 

Right. So what?

Put those three together and you get a delicious, Steve-resistant recipe:

• Measuring unknown states disturbs them (collapse).
• You can’t make flawless backup copies (no cloning).
• Correlations can act as tripwires (entanglement).

 

Next time: we turn this into an actual protocol. You will fling photons at Bertha, you will publicly compare notes like reckless extroverts, and you will still end up with a secret key. Steve will be furious.

 

For more information, please contact Tom Mahon. 

 

This briefing is for general information purposes only and should not be used as a substitute for legal advice relating to your particular circumstances. We can discuss specific issues and facts on an individual basis. Please note that the law may have changed since the day this was first published in June 2026.