Macroscopic Quantum Entanglement

The discovery of quantum mechanics in 1925 was followed by a fierce debate about its meaning and implications. And this debate still rages on, but there have been many twists and turns since the early days. This blog is about the twists that I think are about to happen.

The fact that quantum mechanics implies that measurements “create reality” was very uncomfortable to people like Einstein, who famously said: “Out there is this huge world, which exists independently of us human beings and which stands before us like a great, eternal riddle, at least partially accessible to our inspection.” In contrast to this stood Bohr who thought that it was not meaningful to take the “realistic“ position of Einstein. Instead, he said, and here I quote Bohr: “There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.” According to Bohr, therefore, microscopic objects behave strangely, but we should not lose any sleep over it since science is only concerned with what we can say about nature and not what nature really is like (a philosophical position that might be labeled as “instrumentalism”).

All this was profoundly changed by Schrödinger in the mid-thirties. He effectively amplified the split between a realist, like Einstein, and a pragmatist, like Bohr.

Schrödinger said something like this. If you think that small objects are weird (and Bohr and Einstein both agree on this, and so does everyone else for that matter) then large objects must be weird too (despite appearing to the contrary). He illustrated this with an example that has now entered the realms of scientific classics, a thought experiment involving a decaying particle (small object), a fragile bottle with poison (a large object), and a cat (another large object), all enclosed in one room.

Photo by Giorgio Trovato on Unsplash

Quantum mechanics suggests that the decaying particle is typically in the state where it has decayed and not decayed at the same time (it’s a quantum superposition of these two possibilities). If it has decayed then the bottle is broken, the poison released and the cat dead; if the particle has not decayed, the bottle stays intact and the cat is alive. But quantum mechanics suggests that these two possibilities, these two very different worlds, exist at the same time. So the microscopic quantum weirdness implies the simultaneous existence of dead and alive cats, and these are two very different macroscopic possibilities. Therefore it is difficult to maintain Einstein’s naive realism, and it is also surely difficult to maintain Bohr’s ultra pragmatism.

In fact, the state involving the particle, poison, and cat is a (macroscopically) entangled state, and Schrödinger believed that entanglement is the characteristic feature of quantum physics. It’s the entanglement that makes quantum physics different from classical.

But wait a minute. Are there any experiments vindicating Schrödinger? Well, yes and no. There are, of course, no observations of dead and alive cats (yet!), but there are observations of entanglement in solids containing the number of atoms comparable to that making up a cat. So macroscopic entanglement has now become an experimental reality and it’s probably only just a matter of time before we can experiment with entangled living creatures (The Gordon and Betty Moore Foundation is presently funding me to do just that). And this, I think, would be the biggest twist in the debate started by Borh and Einstein.

What physicists are typically able to do is as follows. They take a photon (or a bunch of them) – which plays the role of the decaying particle in Schrödinger’s thought experiment – and bounce it off a large mirror typically containing one thousand billion atoms, and this mirror plays the role of a cat (and bottle with poison if you like). If the photon is reflected, the mirror has to recoil (to conserve the total momentum), while, if the photon is transmitted, the mirror remains stationary (again, because the total momentum is conserved). The two states of the mirror are the analogs of the dead and alive cat. Experiments of this type are currently being conducted by various groups around the world and there is little doubt in anyone’s mind that they will corroborate Schrödinger. Still, it’s nowhere near the complexity required to entangle living systems.

All these experiments with macroscopic entanglement are generically difficult, but – and this is good news – we don’t have to work so hard to detect macroscopic entanglement. We can, for example, measure some macroscopic properties of solids, such as their magnetic response to the external magnetic field, and this response can tell us if the atoms in the solid are entangled (roughly speaking, the faster they magnetize the higher the entanglement). We‘ve had experiments at room temperature and higher, done in many places, using for example Cooper Carboxylate to confirm entanglement between its atoms (the number of entangled atoms being on the order of Avogadro’s number). These kinds of experiments use methods of detecting entanglement that I have been developing over the last 20 years or so and are not terribly difficult to perform (piece of cake compared to Schrödinger).

All this tells us, as far as I am concerned, that macroscopic entanglement is beyond any reasonable doubt. To move closer to Schrödinger’s thinking, however, we might need to ask if macroscopic entanglement occurs in living systems and if it plays any useful role in biology. The jury is certainly still out on this one, but, if I were you, I’d watch this space closely from now on.