January 29, 2024

Entangling Quantum-ly Redux

Science continues to ruin faster-than-light communication.

Entangling Quantum-ly Redux

Previously, on The Info Dump..._

Particles with correlated wave functions!

...you can arrange for two or more particles to have wave functions that are correlated. The wave function of one is related to another....if we measure the first electron and find that its spin is up, then we know the other one's is down without measuring it.

Spooky action at a distance!

You can measure one particle and the other particle's measurement matches? Instantaneously? Faster-than-light instantaneously? That can't be right, can it?

Wild speculation about what quantum entanglement might mean!

If quantum entanglement means measuring a particle in one location immediately determines what will be measured for a second particle in another location, no matter how far separated, then we've done it! Faster than light communication, everyone!

Is it actually FTL communication? Don't make me tap the sign, people.

Why Quantum Entanglement Won't Give Us FTL Comms

You're about to meet the leader of the alien space fleet that is hovering around Saturn. Maybe they come in peace. Maybe they come in war. I'm back on Earth, anxiously awaiting word from you about whether we should celebrate or panic. This looks like a job for entangled particles!

How would we use entangled particles to communicate? Let's say we entangle two ions whose spin is in a superposition of up and down. We agree that spin up means peace and spin down means war. You jet off to Jupiter with your ion while I stay home. When you get to Jupiter, hooray, they come in peace! But if you measure your ion's spin, it'll be randomly up or down. Whoops! Sure, when I measure my ion's spin I'll get the same answer as you, but that doesn't tell me anything, because the answer was randomly up or down. You haven't passed me any information.

Ugh, that's not what we wanted to hear. Let's have another stock illustration of quantum entanglement to help us feel better.

Can you force your ion to be spin up before you measure it? That is, can you interact with it without measuring it to make it be in a specific state? In this case, you need to flip the ion's spin if and only if it's down, so that it's guaranteed to be up when you measure it.

In fact, there are ways to do that! You can set things up with ions and a laser so that the ions only interact with the laser if their spin is down, and doing so flips their spin to up. But this breaks the entanglement between your and my ions. Sure, your ion is now guaranteed to be spin up. Mine, though, is in a superposition of up and down and will be randomly one or the other when I measure it.

This turns out to be a fundamental problem. For reasons that are both deep and involve a lot of linear algebra, there's nothing you can do to your ion that I'll be able to detect. This is called the no-communication theorem. You can't pass me information via entanglement.

Double ugh. Stock illustration time.

Okay, let's try something different. You're going to change how you measure your ion. If the aliens are peaceful, then you'll measure whether your ion's spin is up or down. But if they're here for war, you'll measure whether its spin is left or right. In quantum mechanics speak, you're going to change your measurement basis. If you measure whether your ion's spin is up or down, then when I measure whether my ion's spin is up or down, my result will match yours. But if you measure whether your ion's spin is left or right, regardless of what answer you get, when I measure whether mine's is up or down, I've got a 50% chance of it being up and 50% chance of it being down¹.

That doesn't sound that useful until you talk about a bunch of atoms. If I can copy my ion a thousand times² and then measure each copy's spin separately, I can see if my measurement had any randomness or not. If you measured up or down, then my thousand ions will all have spin up or spin down. If you measured left or right, then 50% of them will have spin up and 50% will have spin down.

We've done it! FTL comms! Only, whoops again, you can't copy a particle's quantum state unless you know what it is. And the only way I can know my particle's state is by measuring it, which—you guessed it—breaks our entanglement. We've run afoul of the no-cloning theorem³.

I'm not sure even stock illustrations will cheer me up, but I might as well try.

As far as we've been able to determine, you can't send information faster than light. Entangled particles don't give us a get-out-of-relativity-free card. This is, in scientific terms, a major bummer.

But, hey, if you're an author wanting to pretend it does, I won't stop you! Go ahead and use faster than light communication. You'll be joining a long and distinguished history of ansible-users.

What's Up With Stephen?

A poem we published in Small Wonders, Myna Chang's fabulous "Let Us Dream", has been nominated for a Rhysling Award, the Science Fiction and Fantasy Poetry Association's annual award. I never dreamed our tiny new magazine would get this kind of attention, and I'm super excited that Myna's poem was nominated.

What I'm Vibing With

• The Ingenuity helicopter has made its last flight on Mars. Godspeed, little robot helicopter. You were a very good copter.

• The best AO3 note ever.

• After months, NASA finally got the lid off and can touch the forbidden asteroid rocks.

• At LitHub, Kristen Arnett will tell you if you're the literary asshole.

• I was unprepared to be as owned as I was by this list of types of science fiction. Tag yourself, I am a "space pirate, scientist, and award-winning Virgo".

• Last week was a bloodbath for folks in the news and video game industries. If you were one of the affected folks, I'm so sorry.

Boy, that last one sure was a bummer. Let's end on a higher note: appreciating Feferi's new best friend.