Credit: Lucy Reading-Ikkanda/Simons Foundation

It from Qubit and the Gravity of a Quantum Universe

Now in its penultimate year, the Simons Collaboration on Quantum Fields, Gravity and Information has made a solid case for using quantum computing principles to understand spacetime, black holes and the structure of the universe as we know it.

Two It from Qubit Collaboration members at the 2019 Annual Meeting. Credit: MIchael Lisnet

Every human being has an intuitive understanding of gravity: It’s what keeps us planted on Earth, makes things fall when thrown in the air, and ensures that our sun rises each morning. Perhaps it’s ironic, then, that the exact nature of something so rooted in our everyday experience remains one of the grandest mysteries of our universe.

Ask a physicist, and they’ll explain that gravity is an inherent property of matter, one that warps the very fabric of space and time. This understanding of gravity, known as general relativity, provides the best description we have of the dynamics of planets and galaxies. But general relativity fails at the microscopic end of the spectrum, in the realm of subatomic particles whose behavior instead follows the strange laws of quantum mechanics. This theory, which says that objects can exist in multiple states simultaneously, fundamentally conflicts with the mathematics of general relativity. Resolving the clash between these two descriptions of our universe has been a daunting task.

That’s the ambitious goal of It from Qubit, the Simons Collaboration on Quantum Fields, Gravity and Information that formed in 2015. It’s an unlikely band of cosmologists, particle physicists and information scientists combining their expertise to develop a quantum mechanical description of gravity — and a fresh approach to the longtime problem. “The objective of the collaboration is to bring to bear new ideas and techniques being invented by people who are thinking about quantum computers,” says It from Qubit director Patrick Hayden, “to the task of understanding the origins of space and time.”

In most scenarios, general relativity and quantum mechanics never touch; gravity concerns objects with lots of mass, which are usually big — like planets — while quantum effects only dominate at tiny scales, when objects are tremendously light. But there are some exotic phenomena that involve both, like black holes and the Big Bang, the event that sprung our universe into existence.  When physicists try to produce a unified theory of quantum gravity, says It from Qubit collaborator Daniel Harlow of the Massachusetts Institute of Technology, “we get equations that don’t seem to make very much sense.”

Hayden, a quantum information theorist at Stanford University, acknowledges that the connection isn’t obvious. “One is something that happens mostly in quantum optics labs,” he says. “And the other is about the most esoteric branch of physics, asking the most fundamental questions about the universe.” However, as It from Qubit’s track record shows, the application of quantum information techniques has been remarkably useful.


Emerging ideas of spacetime

One of the most helpful of these techniques has been the error-correcting code, says Jason Pollack, an It from Qubit researcher at the University of Texas at Austin. First developed in 1947, error-correcting codes are algorithms that manage the corruption of data stored or sent through unreliable communication channels.

Consider the bit flip, for example — a common error occurring in computers. “Imagine you’ve got some binary string,” Pollack says, referring to the sequence of 0s and 1s that are the units, or bits, used by classical computers to encode information. Uncontrollable interactions with the environment, like a cosmic ray striking a computer chip, can cause some of these bits to flip from 0 to 1, or vice versa. “If it’s an image, maybe this isn’t a problem. That’s just going to change a pixel somewhere,” Pollack says. “But if you want to preserve some program that you’re going to run, it could be a total disaster.”

Bit Flip: Quantum error correction of for a single qubit ψ where at most one bit flip error can happen in the channel Ebit. Credit: Wikimedia Commons

Classical error-correcting codes offer a simple solution: Just duplicate the message. “Store a 0 as three 0s, and a 1 as three 1s,” Pollack says. That way, if a bit flip does occur, the original information can be recovered by analyzing the redundant copies and taking a majority vote. An analogous type of redundancy is used in the error-correcting codes of quantum computers, which store information across quantum bits, or qubits, that can exist as 0 or 1 or — under the rule of quantum mechanics — in a state that, when measured, can yield either value.

In 2014, Harlow and his colleagues discovered a profound link between these algorithms and the nature of spacetime. “We realized that by using the mathematics of quantum error correction, you could learn about how spacetime is emergent,” he says. For theoretical physicists, emergence is the idea that space and time are not fundamental entities; instead, they are properties that emerge from something more foundational.

“This idea, that some properties can be emergent, is common in many other areas of physics,” says Juan Maldacena, an It from Qubit collaborator at the Institute for Advanced Study. Trap a bunch of molecules in a box, for example, and they could form either a gas or a liquid — a characteristic not fundamental to the molecules themselves, but a result of their interacting behavior.

Maldacena explains that many theorists liken space to the volume inside a cylinder: a three-dimensional bulk within a two-dimensional surface that holds all the information needed to create a universe, not unlike our own, inside. Particles that live on the surface obey quantum mechanics, and the interactions between them somehow construct an inner world subject to the laws of gravity that we experience every day.

“So although we think we’re living in three space dimensions and one time dimension, we’re really not,” Harlow says. “But that sounds nuts, right? How can you mathematically have a theory that has a lower number of dimensions, but pretends to have a higher number of dimensions?”

A primary goal of It from Qubit is to find out, and the collaboration believes that the mathematics of quantum error correction is essential to doing so. “A big part of It from Qubit has been understanding more about how this works,” Harlow says. “What are the properties of this code? What are the limits? If you push too hard, what kinds of things break down?”

It from Qubit alumna Ying Zhao (left) and Leonard Susskind, a principal investigator with the Collaboration, during the 2017 Annual Meeting. Credit: MIchael Lisnet

Much of the progress has come from the exchange of ideas at workshops hosted by It from Qubit. According to Hayden, their first major event in 2016 was just focused on getting everyone in the same room. “We all taught each other, because we didn’t know how to speak each other’s languages,” he says, since members were coming from distinct disciplines that lacked communication with each other. These meetings are also open to researchers who are not collaborators. “We always end up having participation from other people who are working in this sphere,” Hayden says. “The collaboration has been really crucial for building momentum.”

While not a formal member of It from Qubit, Monica Jinwoo Kang, a theoretical physicist at the California Institute of Technology, has attended collaboration meetings to discuss new ideas and share her own research. “It’s been fun,” she says, and she looks forward to future gatherings. “It’s given us vivid new tools that have created real results.”

Kang explores the relationship between a bulk theory of gravity and the corresponding quantum theory on its boundary. “Gravity is like a hologram,” she says, because although these objects are two-dimensional, they appear to store three-dimensional information. If spacetime is emergent, there should be interplay between the two theories: Something happens on the surface, and it creates a predictable effect inside. Researchers in the field are still trying to fully understand this connection, but they have already realized that the boundary particles can interact in a variety of ways to reproduce the same physics in the bulk.

“This redundant reconstruction of information is exactly the sort of feature you want in an error-correcting code,” says Pollack, who thinks of gravity as a coarse approximation of a deeper underlying theory of the universe, one that we cannot observe without nearly impossible measurements. Pollack currently studies which features in a quantum theory are necessary to create something that looks like gravity as a consequence. “What I find most compelling is to think about why the world around us looks classical in the first place,” he says. “What creates reality?” Quantum information algorithms have been important for this work, and he embraces the cross-collaboration that is at the heart of It from Qubit — he even included a section in his most recent publication for information scientists to get up to speed on the language of gravitational physics.


The depths of black holes

The mathematics of quantum error correction has proved paramount in shedding light on another long-standing conundrum in theoretical physics. “Maybe the biggest result that came out of the collaboration was a detailed calculation of the way that information gets out of a black hole,” Hayden says.

Often created from the explosive deaths of stars, black holes arise when enough mass is packed into a small enough region, creating a gravitational pull so great that, according to general relativity, nothing can escape.

In the 1970s, however, the acclaimed cosmologist Stephen Hawking announced that thermal energy can break free from a black hole’s clutches. This energy, known today as Hawking radiation, is emitted over long periods of time, causing black holes to slowly evaporate and eventually disappear altogether. Hawking computed the radiation’s entropy (a quantity that measures randomness) and determined it was high. This meant that the state of this radiation is completely random, not at all influenced by anything entering the black hole in its past.

It may be, then, that a black hole is the perfect place to get rid of something: Toss it inside, and the information it contains can never be recovered, an alarming outcome for quantum gravity researchers. “If that were true,” says It from Qubit collaborator Geoff Penington, a physicist at the University of California, Berkeley, “it’d be overturning the basic principles of physics as we know it.”

The principle in question lies at the heart of physics: The state of a system at one point in time should influence what it looks like in the future. “The fact that you send something in should change something,” Maldacena says. But from Hawking’s result, “it looks as if the black hole returns to its initial state.” This contradiction, dubbed the black hole information paradox, sent shock waves through the physics community at the time. Was quantum mechanics wrong?

A breakthrough came in 2019, thanks to a landmark paper published by Penington as a graduate student. “People from It from Qubit had developed all these tools, and I just had to think to apply them in the right context,” he says. Penington redid Hawking’s famous calculation with a more modern formulation of entropy that had been developed and expanded upon by many of his collaborators. He found that the entropy is small; Hawking radiation isn’t random at all.

But he didn’t stop there. Just because this radiation exists in a unique state doesn’t mean it’s influenced by information from the black hole’s past. Penington also needed to show that by measuring the Hawking radiation, someone could, in theory, work backward to retrieve the original information. “And that’s where quantum computing came in,” he says, because information recovery is the purpose of error-correcting codes. “It turns out, you can apply those standard techniques to the black hole case,” Penington says. “And lo and behold, you see that the information is there. You can get it back.”

Schematic behavior of the entropy of the outgoing radiation. The precise shape of the lines depends on the black hole and the details of the matter fields being radiated. Credit: Ahmed Almheiri, Thomas Hartman, Juan Maldacena, Edgar Shaghoulian and Amirhossein Tajdini Rev. Mod. Phys. 93, 035002

So Hawking was wrong, and quantum mechanics is safe. “The mistake Hawking made was that he was treating spacetime as fundamental in his calculation,” Harlow says. “Whereas really, spacetime should be emergent.” The discovery that spacetime behaves like an error-correcting code, and the development of this idea by It from Qubit collaborators, paved a path for Penington to update calculations that had been performed incorrectly for nearly half a century.

With key progress made, It from Qubit still has more to discover about the nature of black holes. While Penington proved that the calculation can be done, the actual steps needed to recover the original information remain out of reach. “We don’t have a clue how to do it,” he says. “It’s not even that it’s too hard mathematically. It’s that we don’t even know the rules.” Still, this is a groundbreaking win for It from Qubit and the broader theoretical physics community. “Most stuff is way too hard to tackle,” Penington says. “But occasionally, you have enough scaffolding that other people have built up that you can fit a piece in. And it all fits perfectly.”


Bridging scholarly borders

Penington wasn’t the only physicist to elucidate the black hole information paradox. On the same day that his result was posted on the arXiv preprint server, four researchers in the field uploaded similar, but independent, calculations that led to the same conclusion. Two of the co-authors, Donald Marolf and Henry Maxfield, are It from Qubit members or alumni; the others, Netta Engelhardt and Ahmed Almheiri, are close friends of the collaboration. Almheiri, a quantum field theorist at the Institute for Advanced Study, also collaborated with Harlow in 2014 on the connection between quantum error correction and emergent spacetime.

Almheiri remembers meeting Penington at a quantum gravity workshop. “That’s when we realized we were doing the same thing,” Almheiri says, and from that point on it was a race to the finish line. “It was friendly competition!” That both teams reached the same conclusions about the paradox makes their results more likely to be valid.

It’s also a testament to how the collaboration has knitted the community together. “Most of the leaders in the field have some connection to It from Qubit,” Almheiri says. It provides a central hub for those pondering quantum gravity, and ample support for students and postdoctoral researchers. That’s important, Almheiri says: “It’s the young generation that really fuels the progress.”

With meetings held all over the world (including Japan, Argentina and Canada), It from Qubit has sparked discussion across national and academic borders. Its postdoctoral fellows are encouraged to spend time at other research institutions to collaborate further. And the collaboration has a pedagogical focus too: It has sponsored a number of schools to equip up-and-coming scholars with the tools needed to bring new insight to ongoing research questions.

Collaborators are buzzing with ideas about where the field is headed next. Both Penington and Maldacena are optimistic about potential input from quantum computers, which will be able to compute more complex problems, since only the most basic of calculations can be solved using pen and paper. This experimental direction will help guide theoretical work in the community.

Regardless of what the future holds, the success of It from Qubit is clear: Tackling quantum gravity with an injection of ideas from information science was a fringe perspective that has now been thrust into the mainstream. “Almost everybody working in the field at this point would agree that this is the right way to think about a lot of these issues,” Hayden says.

It from Qubit will run for another year, and its ending will be bittersweet. But its impact will outlive the collaboration itself — the field is left with new tools, new connections and new opportunities for discovery. “This community is larger than It from Qubit,” Hayden says, adding that the effort has given institutional legitimacy to research that might otherwise have fallen through the cracks. “And so the science is going to continue.”

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