## Modern-day Alice trades looking glass for wormhole to explore quantum wonderland

## Black hole–entanglement link could be simulated in lab, new paper suggests

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If Lewis Carroll were alive today, he wouldn’t bother with a looking glass. His book would be called *Alice Through the Wormhole*.

Being the mathematician that he was, Carroll (aka Charles Dodgson) would have kept current with the latest developments in quantum physics. He would no doubt be intrigued by a new paper describing an idea for the creation (or at least the simulation) of a wormhole in the laboratory to test the latest ideas linking black holes with quantum weirdness.

Carroll would be particularly happy to see that little Alice had grown up to be a quantum physicist, collaborating with somebody named Bob (whose fictional precursor has yet to be identified). Alice and Bob are the (hypothetical) primary investigators of such mysteries as quantum cryptography and quantum entanglement. They are especially skilled at quantum teleportation, in which information needed to reconstruct a quantum particle can be transported from one lab (Alice’s) to another (Bob’s).

Teleporting a quantum particle (typically a photon, a particle of light) is a few centuries of science short of teleporting Captain Kirk from the *Enterprise* to the surface of some planet where danger is lurking. But the conceptual groundwork is now being put in place. The new paper, posted in the physics online archive, in fact, proposes a scheme allowing Alice to teleport a person (named Tom, for some reason) to Bob — through a wormhole.

Ordinarily, wormholes (if they exist) would connect distant regions of spacetime. They wouldn’t be useful for intergalactic Hyperloop travel, as anything entering a wormhole would cause it to collapse. But much work in recent years suggests that such spacetime tunnels might link two black holes, in which case travel through them becomes thinkable, even if not physically, emotionally or economically feasible.

Wormhole travel between black holes is thinkable because of quantum entanglement, one of Alice and Bob’s specialties. In a quantum universe (like the one you are living in), particles that interact can become “entangled” in such a way that they exist in a single “quantum state.” In such a state, a measurement performed on one of the particles can reveal information about the other particle, no matter how far away the second particle is. This spooky connection is hard to explain. Some theories seem to moderate the mystery by proposing that entangled particles are connected by wormholes.

In technical terms, this connection is designated by the “equation” ER=EPR. ER stands for Einstein and Rosen, the two physicists who wrote the seminal paper describing wormholes (otherwise known as Einstein-Rosen bridges). EPR stands for Einstein, Podolsky and Rosen (yes, the same Rosen — and the same Einstein, for that matter), the three physicists who wrote an early paper describing quantum entanglement (mainly in order to complain about it).

If the basic idea of ER=EPR is correct, then it might very well be possible for people to travel through wormholes, as Stanford physicist Leonard Susskind (among others) has discussed in a series of intriguing papers. In fact, Susskind contends, Alice and Bob could prove ER=EPR simply by jumping into two entangled black holes, linked by a wormhole. Alice and Bob would meet in the middle of the wormhole, thereby verifying the ER=EPR theory and winning themselves Nobel Prizes. Except for the slight snag that they could not get out of the wormhole (or even send a message), so nobody would ever know how things went once Alice and Bob finally met in person (or that they had met at all). They would be forever concealed behind the black holes’ event horizons, the surface through which no signal from the interior can escape.

In his latest paper, though, Susskind and Ying Zhao, also of Stanford, offer hope. It seems possible, Susskind and Zhao say, to mimic entangled black holes in the lab. Alice and Bob would not have to risk their futures — they could send Tom through the lab-created wormhole to see if he survived. “Combining quantum teleportation with the idea that entangled black holes are connected by Einstein-Rosen bridges implies that ER=EPR could in-principle be tested by observers who themselves never cross the horizon,” Susskind and Zhao assert.

OK, Tom is not really a person in this plan; he’s just a symbol for teleportee. A teleportee can simply be a photon, a particle containing quantum information that Alice would like to send to Bob. (Such a photon might, for instance, contain important information for a computation that Bob is performing.) Alice cannot simply measure the photon’s information, write it down and e-mail it to Bob. Looking at the photon reduces the multiple possible measurement outcomes to a single definite state (say, spin pointing up). Bob needs a particle that retains the multiple possible outcomes that make quantum information so rich.

All a particle’s quantum information can be teleported, though, if Bob and Alice share a pair of previously entangled photons. Alice allows her entangled photon to interact with Tom (the teleportee photon) and records the result. (This process DESTROYS the teleportee!) Alice then calls Bob up or texts him with the result. Bob then can perform an operation on his entangled photon, which has the effect of restoring Tom in his original state, bringing him BACK TO LIFE! (Metaphorically.)

If ER=EPR is right, Tom has in fact not died, but actually traveled through the wormhole connecting Bob and Alice’s entangled photons. In a thoroughly elaborate mathematical demonstration, Susskind and Zhao describe how this works. A key point is that the process of teleporting quantum information requires the communication of ordinary information through standard channels: To teleport one quantum bit (or qubit) of information, Alice must send Bob at least two ordinary bits of information by slower-than-light signaling of some sort. So there is no “instantaneous” spooky action at a distance going on, as some common misinterpretations suggest.

Susskind and Zhao admit that it is not very likely that Alice and Bob will ever venture into space to find two suitably connected black holes, let alone persuade somebody named Tom to come along. But it is possible to imagine a laboratory facsimile of such a paired black hole arrangement. Perhaps some clever condensed matter physicists could devise two “large shells of matter” that would mimic the properly weird gravitational spacetime geometry needed for the job. These shells would be connected by a wormhole, so Alice and Bob could jump in (they would have to “merge themselves with the matter forming the shells”) and meet “in some place outside ordinary spacetime.” But they still would not be able to inform anyone in the outer world of their success. Alice would have to induce Tom to merge with one of the shells so she could teleport him to Bob.

“When Tom emerges out of … Bob’s shell, he will recall everything he encountered, and can confirm that he really did traverse the wormhole,” Susskind and Zhao contend.

On the other hand (and this seems more promising), two quantum computers could be entangled to simulate wormhole travel. Simulating a real person would require quantum computers of unimaginably huge memory storage capacity. But with a 100-qubit quantum computer (much larger than anything available in labs today, but thinkable), a teleportee of 10 qubits could be sent through the wormhole. Small variations in the initial state of the teleportee would enable the computers to detect how it reacted to conditions in the wormhole, thereby providing the evidence needed to verify the wormhole’s existence, confirming that ER=EPR.

“There does not seem to be an in-principle obstruction to laboratory teleportation through the wormhole,” Susskind and Zhao say. “On the face of it this seems somewhat fantastical, but given that the lab is part of a quantum-gravitational world in which ER=EPR, the conclusion seems inevitable.”

As would be the subsequent book about the adventure. Forget Alice. It would be called *Tom Through the Wormhole*. Feed your head with that, White Rabbit.

*Follow me on Twitter: **@tom_siegfried*