A deeply ingrained assumption among scientists has been the need for detectable information carriers to travel through when we communicate—for instance, a stream of photons crossing an optical fiber, or through the air, allowing you to read this text. Or, indeed, the myriad neural signals bouncing around your brain when you do so. Over the past decade, however, a bunch of fellow physicists and I have overturned this assumption, with implications touching on the nature of reality itself.
My most recent work—From Counterportation to Local Wormholes—has been published on Thursday by the journal Quantum Science and Technology, of the Institute of Physics. It takes into new territory my previous joint work that was awarded a Top 10 Breakthrough 2017 by Physics World magazine. Among other things, it provides a 'smoking gun' for the existence of a physical reality underpinning the quantum description of the world.
It has a sister paper, a joint work with collaborators in Bristol, recently published in Nature Partner Journal Quantum Information, showing experimentally that The Laws of Physics Do Not Prohibit Counterfactual Communication. Here, counterfactual simply means that no particles are exchanged during communication, making it exchange-free. Information, remarkably, is gleaned from events that could have happened but did not in fact take place.
For the curious reader, the following diagram tries to succinctly explain the idea of exchange-free communication at its most basic. It does so with a quantum particle traveling through an interferometer—reminiscent of an early back-and-forth between Bohr and Einstein, and akin to a later thought experiment by Elitzur and Vaidman—but can otherwise be skipped.
Simplified scheme, showing—albeit inefficiently—that the laws of physics don't prohibit exchange-free communication. Let's call the communicating parties Alice and Bob. The mirror on the left (in black) is fully reflective. The pair of half-silvered mirrors (in grey) is such that a single photon entering the interferometer always ends up at detector DR on the right because the photon interferes with itself destructively at the other detector—provided Bob’s switchable mirror (in blue) is On, making it fully reflective. Bob sends bit 0 by switching his mirror On. He sends bit 1 by switching it Off, making it fully transparent, where the possibility of photon escape prevents self-interference, and consequently, detector DL has a chance of clicking. Alice starts by flipping a biassed coin. She sends in a photon if she gets heads, probability p, where p is high. If she gets tails, small probability 1-p, she doesn't send a photon, recording bit 0; the protocol succeeds. There are two scenarios when a photon is sent. In the first, Bob's mirror is On, causing detector DR to click: The protocol fails. In the second, Bob's mirror is Off, and no self-interference takes place. Alice’s photon now has a 1/4 chance (1/2 × 1/2) of triggering detector DL, in which case Alice records bit 1; the protocol succeeds. Bits corresponding to failed protocol runs are discarded. When Alice succeeds to record a bit, the error probability corresponding to Bob's mirror On is zero, while the error probability corresponding to Bob's mirror Off is (4-4p)/(4-3p). The latter is just (1-p) divided by the sum of (1-p) and p/4. Therefore, as p is increased, the error probability goes down, but the probability of a successful run, or efficiency, goes down too. Say we want to send a message, one bit at a time, such that if a single bit is discarded, the entire message is scrapped, and replaced with a new one. Eventually, a whole message would be sent accurately, during which no particles were exchanged. Ideally, by chaining enough interferometers together, both efficiency and accuracy can be made as close to perfect as desired. (Illustration by H.S.)
Imagine a cat at once alive and dead, and extrapolate that idea of incompatible states occurring together, down to the scale of tiny things such as photons, where we know it holds true. But how is counterportation different from ol' teleportation?
Star Trek aside, quantum teleportation transfers complete information about a quantum object, allowing it to be reconstituted elsewhere, such that it is indistinguishable in any meaningful way from the original, which duly disintegrates. The latter ensures that a fundamental limit preventing perfect copying is respected. While counterportation achieves the end goal of teleportation, namely disembodied transport, it does so without its two primary prerequisites—pre-shared entanglement and everyday-type (or classical) communication.
For teleportation, an entangled pair of quantum objects is generated ahead of time and distributed among the two communicating parties. The thing about entangled particles is that they can exhibit, for one thing, complete agreement when interrogated, no matter how far apart from each other: correlation beyond anything that can be accounted for using everyday intuition or classical physics.
Entanglement across space can in fact be described in terms of two distinct regions 'folded' together, as illustrated below—since, by the very nature of entanglement, a complete description of the quantum state of one region must include that of the other.
By means of a joint measurement by the sender (carried out over the quantum object to be teleported and one of the entangled particles), such correlation acts as a quantum backchannel so to speak. Depending on the measurement result, the sender finally tells the receiver, say by phone, what correction among a handful to apply. Once again, communication that entails detectable information carriers crossing.
By stark contrast, no particles are exchanged during counterportation, which begs the question of how the quantum object is transported across space. This is where local wormholes come into play—made of an often overlooked spatial entanglement of a single particle, depicted below. What's important to bear in mind about a single particle is that it can interfere with itself locally, reinforcing certain outcomes while eliminating others.
This, intriguingly, folds together two distinct regions of space via the dimension of time—the trick then is to reverse the process, as depicted below, unentangling the two objects such that the one to be counterported ends up across on the other side.
Why counterportation amounts to building a traversable wormhole is best seen using a new formulation of physics called constructor theory, which shifts the focus from laws of motion and initial conditions (Physics' bread and butter) to asking which physical tasks are possible and which are impossible and why. To be exact, a task is possible if a constructor, a physical system, can in principle carry out the task accurately, over and over again.
Wormholes—made popular by the motion picture 'Interstellar', which included physicist and Nobel laureate Kip Thorne among its crew—came to light about a century ago as quirky solutions to Einstein's gravity equation, shortcuts in the fabric of spacetime.
The defining task of a traversable wormhole, however, can be neatly abstracted from the viewpoint of constructor theory as making space traversable disjunctly; in other words, in the absence of any journey across observable space outside. The outside would consist simply in the universe we inhabit—in its mundane, unexotic shape. As it turns out, I have found just the constructor to carry such a task out: counterportation.
Traversable local wormhole. Space is represented horizontally. Time runs vertically, upwards. The two quantum objects, one on either side, start off at the bottom unentangled. The qubit to be counterported is the one on the right. As time elapses, the local wormhole gradually entangles the two objects across space, where the degree of entanglement is indicated by the saturation of red. Halfway up, entanglement is maximal. Information is no longer localised in space, which can be perceived as folded. The process is then reversed. The two objects are gradually unentangled—such that by the point when space has unfolded, once more via the time dimension, the qubit ends up across on the left. The orange and the greenish vertical lines, corresponding to two local journeys in observable spacetime, show that no detectable information carriers have traveled across. (Illustration by H.S.)
As implications go, the main one here hardly lacks reach. Let's first make the assumption that in an optical setting, any communication is explainable by at least one of the following. (1) Detectable photons traveling up and down the channel between the two communicating parties. (2) Measurements carried out in between initial preparation and final detection. (3) An underlying physical reality objectively existing prior to measurement.
By definitively ruling out the first two in my scheme, I provide what I believe is compelling evidence that an objective physical reality, underlying the quantum description of the world, is what has carried quantum information across space.
Three items appear in the paper that an aspiring physicist may like to tick off at some point. First, a simple equation that captures something new and hopefully interesting about the world. (If it's impactful enough it may end up inscribed on a tombstone down the line.) In this case, the equation for the exact probability of traversing a local wormhole, in terms of its physical parameters, which in the limit approaches one.
Second, a visual representation to go with it. In this case, the dashed lines in the quantum circuits depicted in the paper, which signify the new exchange-free paradigm of quantum computing. Third, an open problem for the scientific community to grapple with. In this case, finding a complete geometrical interpretation for counterportation, or equivalently for local wormholes.
P = cos2M(θM) cos2MN(θN)
Probability of traversing a local wormhole
It's a longish paper that could have been broken up into a handful of shorter ones, but I opted instead to tell as complete a story as possible. It was written in roughly two phases. The first preceded the pandemic. The second, at the height of the pandemic, while locking down with my parents, sister, and nieces, at home in Khartoum, Sudan (where a military coup also unfolded tragically, complete with the customary internet blackouts).
It was a perfect time to escape into physics in between civil rights protests, to contemplate what lies beyond, to explore the edge of reality. Then again, physicists have a long tradition of doing just that, coming up with some of their more interesting work during the 'plague'.
If my proposal for counterportation is to be realised, an entirely new type of quantum computer has to be built: an exchange-free one. But what scale are we talking? A two-qubit exchange-free quantum computer would allow any computation over two spatially separated qubits, without anything observable crossing.
In contrast to large-scale quantum computers that promise remarkable speed-ups (which no one yet knows how to build), the promise of exchange-free quantum computers of even the smallest scale (2 qubits) is to make seemingly impossible tasks—such as counterportation—possible, by incorporating space in a fundamental way alongside time.
The otherworldly-sounding mission of building a local wormhole is on. Collaborators in Bristol and Oxford and I are looking to demonstrate one in the lab, in the spirit of LIGO and CERN, the multi-billion ventures that exist to uncover new physical phenomena. But, at a fraction of the resources. My vision—and that of my co-founder at DotQuantum, Mahmoud Ashmawy, and our advisors at the Quantum Hub in York, and in Mountain View in Silicon Valley—is to ultimately provide wide access to local wormholes.
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