How Quantum Entanglement Works – For Dummies

5. December, 2022

So you’ve heard about this “quantum entanglement” stuff and how Einstein was apparently worried it might break the speed of light.

It doesn’t and he wasn’t.

Here is the simple version: Take a piece of paper. Rip it apart once. Check the pieces that you got. Maybe scribble something on it. Mix the two pieces behind your back. Give one of them to a friend without looking. Send the friend to the end of the universe. If he refuses, find a real friend. Wait a few billion years. Open your hand. In that instant, no matter how far away your friend is, you will know what piece he has in his hands.

The complicated version: Quantum is weird but some stuff is actually easy to understand. There are just a few ways to create entangled particles. All of them have something in common: The pieces must add up exactly to what you put in. This isn’t magic or some badly written Star *beep* episode. Imagine you put in a piece of paper (or a photon – a blip of light). You can’t have more paper (or more light) after splitting it. In the case of the photon: If you add energy, you get more light but no entanglement.

So the entangled photon pairs are always half of the original in terms of energy (which roughly translates to “half as bright”). And they always go in exactly opposite directions (see conservation of momentum). Things like that. Which means you know everything about the two particles except one thing: You don’t know which is which unless you look.

If you keep one of them around and sent the other away, and then at some later point look at what you kept, you know exactly and instantly what the other must look like, no matter how far away it is now.

But you can’t change the far particle anymore. Same as you can’t add text to the paper which your friend at the edge of the universe is holding. So you can’t use this to beam information.


Be grateful for having friends like that. A pity that you sent him away.

Public Talk: Quantum Computing (2nd Try)

14. April, 2008

After my back is back, I’ll give the talk about quantum computing this week, Thursday 17th of April. See this page for details.

Public Talk: Quantum Computing [Update]

10. March, 2008

Because of major back pain, I can’t give this talk today. I’ll post a new announcement when I knew the new date. Sorry.

If you ever wanted to meet the mind behind the blog and you happen to be in Zurich on April, 4th, you can. I’ll be giving a public talk about quantum computing for the LUGS (Linux User Group Switzerland). The talk will be in German.

It’s not mandatory to be a member of the LUGS to attend the talk but of course, you’re welcome to become a member of Switzerland’s largest LUG, too.

Telepods of Doom 2

19. October, 2007

On Telepods of Doom, Mike P. argues:

We can only assume that a machine can reconstruct experience, consciousness and the human soul.

First of all, the machine maybe doesn’t have to reconstruct the soul of the being transported. Our everyday experience shows that the soul moves along with the body. There doesn’t seem to be a limitation on how fast the body can move (at least not up to the speed we can achieve) without losing contact to its soul. In fact, looking at the problem from a quantum physics view, there is no reason to believe that the soul has to care about the actual location of the body. This means that if the wave form which represents our body is teleported across the universe, the soul might just stick to it.

Of course, I might be wrong and the soul might loose contact the moment the body is teleported. On the positive side, this would be a final proof that a soul exists (or at least something beyond the sub-atomic level). On the negative side, this would open a whole new world of tools to people who are not prepared for such power.

When someone manages to prove the existence of the soul, people will start to work on way to measure it. To access it. To modify it. Area Denial Systems already offer convenient new ways of torturing anyone you happen to dislike without leaving traces. For the victims, this makes it impossible to prove the act in court, making their situation twice as bad.

Imagine machines which can access the soul.

Luckily, nature has laws which will make sure we become extinct unless we are able to handle the powers which we seize.

Telepods of Doom

26. September, 2007

On BeContrary is a discussion about Telepods of Doom. The question goes like this:

It is the year 2112. Telepods have been in use for a decade to instantly transport matter from one part of the universe to another. You are waiting in line with your family at a telepod station to go to Tau Ceti. In front of you in the queue you meet the inventor of the telepods. He tells you that the telepods only appear to move matter, what they actualy do is create an exact duplicate at the destination and destroy the original in the process.

Do you get in the telepod?

As my math teacher would say: You’re mixing up two frames of reference. In quantum physics, objects exist only once. There can be similar objects but these can never be exactly the same (they must differ in at least one attribute, for example in spin). Don’t use that argument when the MPAA comes after you. (“That music isn’t what was on CD! It must be different! Quantum theory says so!”)

One way to make exact copies is to destroy the original and transfer all attributes onto another object (thus destroying the other object and creating a new “original”). In the real (macro) world, this can lead to all kinds of problems: If the destroy happens before the “apply attributes”, you lose the object. If the destroy doesn’t happen at all, you suddenly have two copies. If only a part of the attributes are copied, you have an imperfect copy.

In the quantum world, none of these effects can happen. It’s either all or nothing because there is no state in between. Quantum particles can move through “solid” walls because they never spend any time inside the wall. In one moment, they are on one side, the next, they are on the other. The theory doesn’t ask for continuous movement. It just says “when you look several times, there is a certain chance that you’ll see the particle.” There is no explanation how it gets from one place to the other and how it spends the time when you don’t see it.

Since no one has found a flaw in the theory so far, it seems to be an accurate description of reality. That it contradicts our view of reality means that our view of reality is imperfect, not that quantum theory is wrong. Or as Douglas Adams put it:

“There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.”
— Douglas Adams

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