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

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?

• What do you want to entangle, and what do you want to do with it? Entanglement is mostly about information and time. Technically you can entangle a whole lab from a certain point of view with todays technology, it just do you any good. – Whitecold Jun 12 '20 at 18:03

## 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.

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.

• But there's still the problem of the speed of the entangling rate, as L. Dutch mentioned above. "Even if you increase the entangling rate of a factor 10 million*, you would still need an entire Universe age to complete the task." Wouldn't this mean that even if the decoherence rate was shortened, it would still take a super long time to entangle the particles in the first place? Ideally I want my character to be able to entangle them instantaneously on demand. – Strivs Apr 20 '20 at 18:00
• @Strivs No process can happen instantaneously; everything happens at a finite rate. You're very much dependent on the equipment you're using, and that presents a whole bunch of limitations. To solve those . . . well, you'd need a lot of magic. – HDE 226868 Apr 20 '20 at 18:02
• @Strivs The other thing is, why would you need that many particles? You only need a small amount of qubits, for instance, for a functioning quantum computer. A gram of hydrogen contains many orders of magnitude more particles to be entangled than you'd ever reasonably need. – HDE 226868 Apr 20 '20 at 18:24

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

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

• Hmmmm... the hydrogen in those molecules are more that entangled already, they are bound. Just make sure there enough cool (i.e. no plasma) hydrogen around. See? job done by the magic of chemistry, no sweat and at subsecond time scales :grin: – Adrian Colomitchi Apr 20 '20 at 17:13
• @L.Dutch - Reinstate Monica I see. Well, what if there was some sci-fi plot device maybe that I could add to make the entanglement for just about anything instantaneous? I'm not exactly sure how it would work, what it'd be or where it'd come from, but what would you suggest? – Strivs Apr 20 '20 at 17:44
• My one object here is that it seems unlikely you'd need that many entangled particles. I'm not sure what the use of $\sim10^{24}$ entangled atoms would be. I suspect the entanglement rate isn't that much of an issue. – HDE 226868 Apr 20 '20 at 18:29
• @HDE226868, to mass produce something I assume large quantities are in order. 1 g of hydrogen is just a pinch for a large quantity – L.Dutch - Reinstate Monica Apr 20 '20 at 18:46
• @L.Dutch-ReinstateMonica A "large quantity" of entangled particles, though, is not very high. $10^{24}$ of anything is quite a lot - $10^{24}$ dollars, for instance, is absurdly large. A single quantum computer would need $\sim1000$ qubits to do some powerful work - why make so many entangled pairs? – HDE 226868 Apr 20 '20 at 18:50

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.

• I'm not sure this addresses the OP's primary question of mass entanglement. – HDE 226868 Apr 20 '20 at 17:35
• Good point. I've made it more obvious that pretty much every form of "action at a distance" counts as communication. – Daron Apr 20 '20 at 17:51