Could it be possible to quantum entangle particles on a mass scale?

Question

So in the world I'm building, I'm considering to give one of my characters the ability to quantum entangle specific particles on demand. Now although for many years we thought quantum entanglement was a random and uncontrollable phenomenon, it actually turns out that scientists have been able to do this through their own initiative, to a certain degree: https://www.sciencealert.com/new-production-line-method-for-quantum-entanglement-on-demand.

Sadly however, I have a tendency to misinterpret the science from these sort of articles, but I'll try my best here. So essentially what these scientists have done is somehow entangled photons with electrons in a method that can generate 40 entanglements on demand in a single second.

Of course, I'm no physicist, let alone quantum physicist, so I'm not going to pretend I can fully understand this. This, of course, is where you guys come in. So overall, my question is What kind of ability or device would my character need to do this sort of thing and what would be the limitations of this quantum entanglement?

Answer 1

Reduce the rate at which you lose entanglement

(The paper, for anyone wanting to read it, is Humphreys et al. 2018.)

The hey problem here isn't entangling particles, per se - the problem is keeping them entangled. The authors make the point that what we're interested in isn't just the rate at which we entangle particles $r_{\text{ent}}$, but also the decoherence rate $r_{\text{dec}}$, the rate at which particles decohere, often due to interactions with their immediate environment. This breaks the entanglement and is a huge problem in quantum computing, for a number of reasons. Now, you need to have $r_{\text{ent}}>r_{\text{dec}}$ to have a net positive gain of entangled pairs, or, as they put it, a quantum link efficiency of $\eta\equiv r_{\text{ent}}/r_{\text{dec}}>1$. If $\eta<1$, the number of entangled particles is decreasing.

The study in question produces entanglement rates of $r_{\text{ent}}=39\text{ Hz}$ (where our units refer to particles entangled per second). Previous work (Stockhill et al. 2017) produced entanglement rates as high as $r_{\text{ent}}\sim1000\text{ Hz}$ using objects called quantum dots - but at the cost of $r_{\text{dec}}\sim10^7\text{ Hz}$, for an efficiency of only $\eta\sim10^{-4}$, a net loss. The big jump here was producing a significantly large $\eta$ by producing low decoherence rates. With $r_{\text{dec}}=5\text{ Hz}$, Humphreys et al. achieved an efficiency of $\eta=39/5\approx8$. Quantum link efficiencies significantly greater than $\eta=1$ are possible!

Let's go back to quantum dots. Stockhill et al. were able to entangle qubits at a rate of $r_{\text{ent}}=7300\text{ Hz}$ - almost 200 times as many each second! But there are limits to how quickly you can entangle particles, and those limits are set by your experimental setup. For instance, the quantum dot experiment required lasers, detectors, a magnetic field, and plenty of additional equipment. So you're going to have to improve your experimental setup to increase $r_{\text{ent}}$. The authors speculated that they could reach $r_{\text{ent}}\approx130000\text{ Hz}$ with particular upgrades.

Again, though, you'd still have to deal with decoherence rates. But raising $r_{\text{ent}}$ by that much - a factor of 20 - would make the quantum dot method potentially more feasible, in terms of efficiency $\eta$. You just have to deal with decoherence.

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Folks have pointed out that, compared the number of particles we interact with on macroscopic scales, you could only entangle small numbers of particles within reasonably timescales. This is true, but it overlooks the fact that you probably don't need absurdly large numbers of entangled pairs. For example, quantum computers can perform pretty powerful computations with a few thousand qubits - and even dozens of qubits would allow for excellent performance for some tasks. So I suspect this isn't really an issue at all.

If we produced qubits using the methods of Humphreys et al., the net gain of entangled particles would be $$\Delta=r_{\text{ent}}-r_{\text{dec}}=39\text{ Hz}-4\text{ Hz}=35\text{ Hz}$$ If we want to produce $N\sim10^3$ particles for our thousand-qubit quantum, computer this would take a time $$\tau=\frac{N}{\Delta}\approx30\text{ seconds}$$ Even if that's off by an order of magnitude, that wouldn't be bad. Mass entanglement of the sort needed for reasonable numbers of entangled particles is feasible.

Answer 2

Forget about entangling particles on mass scale. It's an overwhelming task. Why?

Let's start with your statement

a method that can generate 40 entanglements on demand in a single second.

How many atoms there in a substantial amount of mass? Let's stay simple and consider hydrogen. 1 gram of hydrogen contains 2 moles of atomic hydrogen, which means $2 \cdot 6.022 \cdot 10^{23} = 1.2 \cdot 10^{24}$ atoms.

How long will it take to entagle all those atoms at the pace you specified?

$1.2 \cdot 10^{24}/40 = 3 \cdot 10^{22}$ seconds.

How much time is it?

According to this Wikipedia page, the age of the Universe is about $10^{15}$ seconds, so it will take 10 million times the age of the universe to finish entangling 1 miserable gram of hydrogen at that pace!

Even if you increase the entangling rate of a factor 10 million*, you would still need an entire Universe age to complete the task.

*no human technology has come close to this improvement rate, aside from IC manufacturing

Answer 3

No Action at a Distance.

THE fundamental limitation of quantum entanglement is that it cannot be used to transmit information. Ideally you would like to entangle particle A with particle B so that when particle A vibrates upwards then particle B vibrates upwards, and then particle A vibrates downwards then particle B vibrates downwards. Put each particle in spaceships and send them off in opposite directions. When you want to send an information between the spaceships you just wiggle your particle in Morse code and the other particle spells out the same message on the other side.

Easy right? Wrong! Can't be done! I'm not actually sure why. I think it's because the first time one of them opens the box and looks at their particle the entanglement breaks and the particles no longer respond to each other.

So Alice opens the box and says "huh, it's vibrating upwards. I guess Bob's is vibrating upwards as well." And that's the end of the story. She cannot tell whether it's vibrating upwards because Bob told it to, or maybe it was always vibrating upwards and Bob never touched it.

Since we cannot communicate, we also cannot do anything that could be indirectly to communicate. That includes pretty much any form of action at a distance. So you cannot entangle two things that are far away. You have to bring them together, entangle, and then send them apart.