The text of various papers follows below. These items are froms from the
link:
https://www.quantamagazine.org/wormhole-experiment-called-into-question-20230323/?mc_cid=b32412d179&mc_eid=1c22739553
quantum gravity
Wormhole Experiment Called Into Question
By
Charlie Wood
March 23, 2023
Last fall, a team of physicists announced that they had teleported a qubit
through a holographic wormhole in a quantum computer. Now another group
suggests that’s not quite what happened.
5
Read Later
An illustration of a butterfly falling into a wormhole.
A holographic wormhole would scramble information in one place and reassemble
it in another. The process is not unlike watching a butterfly being torn apart
by a hurricane in Houston, only to see an identical butterfly pop out of a
typhoon in Tokyo.
Myriam Wares for Quanta Magazine
Introduction
In January 2022, a small team of physicists watched breathlessly as data
streamed out of Google’s quantum computer, Sycamore. A sharp peak indicated
that their experiment had succeeded. They had mixed one unit of quantum
information into what amounted to a wispy cloud of particles and watched it
emerge from a linked cloud. It was like seeing an egg scramble itself in one
bowl and unscramble itself in another.
In several key ways, the event closely resembled a familiar movie scenario: a
spacecraft enters one black hole — apparently going to its doom — only to pop
out of another black hole somewhere else entirely. Wormholes, as these
theoretical pathways are called, are a quintessentially gravitational
phenomenon. There were theoretical reasons to believe that the qubit had
traveled through a quantum system behaving exactly like a wormhole — a
so-called holographic wormhole — and that’s what the researchers concluded.
When it was published in November, the experiment graced the cover of Nature
and was widely covered in the media, including in this magazine.
Now another group of physicists has analyzed the result and determined that,
while the experiment may have produced something vaguely wormhole-like, it
wasn’t really a holographic wormhole in any meaningful sense. In light of the
new analysis, independent researchers are coming to doubt that the
teleportation experiment has anything to do with gravity after all.
“I feel that the evidence for a gravitational interpretation is weakening,”
said John Preskill, a theoretical physicist at the California Institute of
Technology who was not involved with either study.
The group did teleport something on the Sycamore chip, however, and they did it
in a way that — at least on the surface — looked more wormhole-like than
anything produced by earlier experiments. The dispute over how to interpret the
experiment springs from rapid developments involving holography, which
functions as a sort of mathematical pair of 3D glasses that lets physicists
view a quantum system as a gravitational one. Studying wormholes through the
gravitational lens has uncovered new ways to teleport quantum information,
raising hopes that such quantum experiments might someday go in the other
direction and probe quantum gravity in the lab. But the wormhole brouhaha
highlights the fact that determining when the holographic lens works — and
therefore whether certain aspects of quantum gravity might be accessible on
quantum computers — may require greater subtlety than physicists imagined.
When he read the new response, Vincent Su, a physicist at the University of
California, Berkeley who studies wormhole-like teleportation and is not
involved with either group, wondered, “Is quantum gravity in the lab dead?”
Scrambling Wormholes
Wormholes have long been a fixture of science fiction writers in need of a
mechanism for quickly moving their characters across the vastness of space, but
the wormholes that appeared in Einstein’s theory of gravity initially seemed
extremely improbable, requiring tricky manipulations of space-time that
inevitably led to time-travel paradoxes. That changed in 2016, when three
physicists — Ping Gao and Daniel Jafferis at Harvard University and Aron Wall,
then at the Institute for Advanced Study — found an unexpectedly simple and
paradox-free way to prop open a wormhole with a shock wave of negative energy.
How does gravity work in the quantum regime? A holographic duality from string
theory offers a powerful tool for unraveling the mystery.
Video: How does gravity work in the quantum regime? A holographic duality from
string theory offers a powerful tool for unraveling the mystery.
Directed by Emily Driscoll and animated by Jonathan Trueblood for Quanta
Magazine
Introduction
“It’s quite beautiful. It started the whole thinking in this direction,” said
Hrant Gharibyan, a quantum physicist at Caltech. “There’s a narrow window that
you can throw stuff from the left universe to the right.”
The foundation of the work was one of the hotter trends in modern physics,
holography.
Holography involves the study of profound relationships known as dualities. On
their face, dual systems look completely different. They have different parts
and play by different rules. But if two systems are dual, every aspect of one
system can be precisely related to an element of the other system. Electric
fields are dual to magnetic fields, for instance. A major finding in modern
physics is that dualities also seem to link certain gravitational systems to
quantum systems.
We might consider a collection of interacting particles, for instance, entirely
within the framework of quantum theory. Or, as if by popping on a pair of 3D
glasses, we might see the collection of particles as a black hole governed by
the rules of gravity. Physicists have spent decades developing mathematical
“dictionaries” that let them translate quantum elements into gravitational
elements and vice versa, effectively putting on and taking off the glasses.
They watch how particles, black holes and wormholes transform as one switches
between the two perspectives. Calculations that are hard to do from one
perspective are often easier from the other. A major hope of the field is to
develop the ability to access the still mysterious rules of quantum gravity by
studying better-understood quantum theories.
But questions abound as to how far the glasses trick will hold. Does every
conceivable quantum theory pop into a gravity theory when viewed
holographically? Can physicists understand gravity in our universe by finding
its better-behaved quantum twin? No one knows. But many theorists have
dedicated their careers to exploring a few well-understood holographic pairs of
theories and are constantly searching for new examples.
Gao, Jafferis and Wall had already suggested in 2016 that passing through a
wormhole (a gravitational enterprise) might have a quantum interpretation
without the 3D glasses: the teleportation of quantum information. A couple of
years later, another team made their speculation concrete.
A smiling man in front of a chalk board.
Daniel Jafferis, a theoretical physicist at Harvard University, helped develop
the wormhole teleportation protocol. He was also one of the leaders of last
year’s wormhole team.
Paul Horowitz
In 2019, Gharibyan and his collaborators translated traversable wormholes into
quantum language, publishing a step-by-step recipe for a peculiar quantum
experiment that showcases the essence of holography. With the 3D glasses on,
you see a wormhole. An object enters one black hole, traverses a sort of
space-time bridge, and exits the other black hole. Take the glasses off,
however, and you see the dual quantum system. Two black holes become two
gigantic clouds of particles. The space-time bridge becomes a quantum
mechanical link known as entanglement. And the act of traveling through the
wormhole becomes an event that appears quite surprising from the quantum
perspective: A particle carrying a qubit, a unit of quantum information, enters
one cloud and becomes scrambled beyond all recognition. The qubit unscrambles
and exits the entangled cloud as another particle — a development as unexpected
as watching a butterfly being torn apart by a hurricane in Houston, only to see
an identical butterfly pop out of a typhoon in Tokyo.
“Naïvely you’d never guess,” Gharibyan said, “that you could scramble and
unscramble very chaotically, and the information comes out.”
But viewed through a holographic lens, the proceedings make perfect sense. The
entangled clouds of particles are not a literal wormhole in our universe. But
they are dual to a wormhole, meaning that they have a matching behavior for
anything a traversable wormhole can do — including transporting a qubit.
This is what the team announced in the November Nature paper. They simulated
the behavior of two clouds of entangled particles in a quantum computer and
performed a teleportation that captured the essential aspects of traversing a
wormhole from the holographic perspective.
But that wasn’t the only way to interpret their experiment.
Not All That Teleports Is Gravity
Over the past few years, researchers made another surprising discovery.
Although they had spotted the scrambling teleportation recipe while using the
gravitational lens, gravity wasn’t always essential.
Gravity scrambles information in a very particular way. In fact, theorists have
argued that black holes must be the most efficient scramblers in nature. But
when Gharibyan and his colleagues used clouds of particles that scrambled by
different quantum rules than gravity, they realized that the clouds could still
teleport by scrambling, albeit less efficiently. And when they looked at the
alternative clouds through a holographic lens, they saw nothing — no wormholes.
Gharibyan’s group and another team led by Norman Yao at Berkeley put everything
together in a pair of simultaneous papers in 2021. (Yao has since moved to
Harvard.)
A black and white photo of a man smiling.
Norman Yao, a physicist at Harvard University, led the team that poked holes in
last year’s wormhole paper.
Noah Berger for UC Berkeley
Introduction
These papers laid out some of the characteristics that seemed to distinguish
gravitational teleportation from teleportation by more vanilla sorts of
scrambling. In particular, they identified a feature of all quantum systems
known as size winding, which can be linked holographically to the speed of a
particle falling through the wormhole. When gravity was responsible for the
scrambling, size winding had a particular mathematical property and was said to
be “perfect” in the systems they studied. That gave the Nature team a specific
signal to hunt for.
“What was predicted in these earlier papers was that size winding is a
holographic signature, almost like a smoking gun,” Su said.
More Particles, More Problems
Last spring, while the Nature paper was going through the peer-review process,
Su and his collaborators carried out a teleportation-by-scrambling experiment
on two quantum computers, one operated by IBM and another by Quantinuum. They
called their teleportation demo “wormhole-inspired,” since they knew their
quantum model used one of the nongravitational scrambling recipes. At the time,
they suspected that an experimental demonstration of true gravitational
teleportation would take a decade or longer.
To understand why gravitational teleportation is so tough to pull off, it helps
to keep in mind that these quantum computers don’t literally contain clouds of
particles that scramble and unscramble information of their own accord.
Instead, they contain qubits, which are objects that act like particles (qubits
can be made from either literal atoms or artificial ones). When scientists
program the computer, they tell it to make quantum changes to the qubits
according to an energy equation called a Hamiltonian. The Hamiltonian describes
how the qubits change from one moment to the next. Effectively, this equation
lets them customize the laws of quantum physics for the qubits. As the computer
runs, it carries out a sort of simulation of how real clouds of particles
governed by those laws would act.
Here’s the rub: For a definitive showcase of gravitational teleportation, you
need big clouds of particles. How big? The bigger the better. The theorists had
done all the math in the context of essentially infinitely large clouds. For an
experiment, researchers generally agree that 100 particles per cloud would
suffice for indisputable wormhole-behavior to emerge.
A gloved hand holding a square wafer.
Last year’s experiment was run on seven qubits of Google’s Sycamore quantum
computing chip.
Peter Kneffel/dpa/Alamy Live News
Introduction
Yet as the number of particles goes up, the size of the Hamiltonian explodes.
If you’re modeling the particles using one of the more tractable models of
gravity, called the SYK model, your Hamiltonian must reflect the fact that
every member of a group of particles can directly influence every other member.
The Hamiltonian for 100 densely linked particles is an equation with a
staggering 3,921,225 terms. This is far beyond what today’s quantum computers
can simulate with a few dozen qubits. Even if one were willing to settle for a
fuzzy wormhole dual to clouds of just 20 particles, the Hamiltonian would go on
for an overwhelming 4,845 terms. This hurdle was a key reason why Su’s group
thought that a true wormhole simulation was a decade away.
Then last November, a team of researchers led by Jafferis, Joseph Lykken of the
Fermi National Accelerator Laboratory and Maria Spiropulu of Caltech surprised
the community by announcing that they had run a quantum experiment displaying
perfect size winding — the key signature thought to establish the existence of
a gravitational dual, and thus a wormhole — using just seven particles. Even
more surprising, they were able to stuff the behavior of this seven-particle
system into a Hamiltonian with only five terms.
A Holographic Wormhole on a Chip
The core of the group’s work was a novel way of pruning many of those
particle-to-particle connections described by the unwieldy SYK Hamiltonian.
Numerous physicists have “sparsified” the SYK model for a given cloud size by
dropping random terms, finding that simpler versions can keep the holographic
properties of the original Hamiltonian.
Instead of deleting connections at random, Jafferis and his collaborators
thought to use machine learning to intelligently prune only the connections
that don’t affect the cloud’s ability to teleport, a simplification strategy
praised by other researchers.
“I thought it was actually very clever,” Gharibyan said. “The sparsification I
thought was a very great insight.”
“It was a good idea,” Preskill said.
The researchers took aim at the 10-particle SYK model, which has a Hamiltonian
of 210 terms. They simulated teleportation between clouds of 10 particles on a
standard computer and designed a machine learning algorithm to simplify the
Hamiltonian as much as possible without breaking its capacity to teleport. The
algorithm returned an extremely sparse Hamiltonian measuring just five terms
that captured teleportation between two seven-particle clouds. (The machine
learning algorithm apparently decided that three of the particles weren’t
meaningfully contributing to the process.) The equation was simple enough to
run on Google’s Sycamore quantum processor, a notable achievement.
A cryostat with lots of metal tubes.
Google’s Sycamore quantum processor must be kept just above absolute zero in a
cryostat such as this one.
Google
“It’s cool that they were able to run something on quantum hardware,” Su said.
The Sycamore experiment confirmed that the Hamiltonian could carry out the
teleportation, just as it had been trained to. But what really excited
researchers was the fact that this gang of qubits also displayed perfect size
winding — the supposed signature of a gravitational dual. Somehow a toy model
of a toy model of a toy model of gravity had managed to maintain the
holographic essence of its grandparent model. The researchers appeared to have
done the equivalent of boiling down a tornado to a handful of molecules, which,
despite being largely unable to interact with each other, still manage to keep
the characteristic funnel shape.
“They had actually a pretty nice way to measure the size winding as well,”
Gharibyan said. “It was pretty exciting.”
Many in the field were struck by just how simple the toy model was. One group
in particular —Yao and his Berkeley colleagues Bryce Kobrin and Thomas Schuster
— started to dig into how such a simple model could possibly capture the
unspeakable chaos of gravity.
Too Small to Scramble
On February 15, the trio posted the results of their investigation, which
involved analyzing the mathematical properties and behavior of the Nature
team’s simple Hamiltonian. It has not been peer-reviewed. Their main finding is
that the simple model departs from its parent model of gravity in crucial ways.
These differences, the group argues, imply that the signals the researchers
considered hallmarks of gravity no longer apply, and because of this, the best
description of what the Nature team saw is not gravitational teleportation.
The least gravitational thing about the simplified Hamiltonian is that, unlike
in the original SYK model, the five terms are “fully commuting,” which means
that they don’t have a certain kind of interdependence. Commutativity makes it
much easier to simulate the clouds of particles, but it implies that the clouds
can’t scramble chaotically. Since chaotic scrambling is considered a defining
property of black holes and is an essential ingredient in gravitational
teleportation, experts doubt that such a simple Hamiltonian could possibly
capture complicated wormhole-like behavior. Put loosely, the system more
closely resembles the gentle spiral of draining bathwater than it does the
churning turbulence of Class V river rapids.
A blonde woman with glasses in front of a laptop.
Maria Spiropulu, a physicist at the California Institute of Technology, was one
of the leaders of last year’s wormhole experiment.
Bongani Mlambo for Quanta Magazine
Introduction
The researchers also proposed a nongravitational explanation for the supposed
signature of holography, perfect size winding. The five-term Hamiltonian does
have it, but so do other random five-term, commuting Hamiltonians that they
tested. Moreover, when they tried to bump up the number of particles while
keeping the commuting property, the size winding signal should have
strengthened. Instead, it disappeared. The physicists reached a conclusion that
researchers had not previously grasped because no one had studied such simple
models holographically: Many fully commuting, small Hamiltonians seem to have
perfect size winding, even though these models don’t have gravitational duals.
This finding implies that, in small systems, perfect size winding isn’t a sign
of gravity. It’s just a side effect of the system being small.
Both groups declined to comment while they work out their differences through
peer-reviewed publications. The Yao group has submitted their analysis to
Nature, and the Jafferis, Lykken and Spiropulu group will likely have a chance
to respond. But five independent experts familiar with holography consulted for
this article agreed that the new analysis seriously challenges the experiment’s
gravitational interpretation.
Holographic Dreams
The holographic future may not be here yet. But physicists in the field still
believe it’s coming, and they say that they’re learning important lessons from
the Sycamore experiment and the ensuing discussion.
Related:
The Most Famous Paradox in Physics Nears Its End
Wormholes Reveal a Way to Manipulate Black Hole Information in the Lab
A Deepening Crisis Forces Physicists to Rethink Structure of Nature’s Laws
First, they expect that showing successful gravitational teleportation won’t be
as cut and dry as checking the box of perfect size winding. At the very least,
future experiments will also need to prove that their models preserve the
chaotic scrambling of gravity and pass other tests, as physicists will want to
make sure they’re working with a real Category 5 qubit hurricane and not just a
leaf blower. And getting closer to the ideal benchmark of triple-digit numbers
of particles on each side will make a more convincing case that the experiment
is working with billowing clouds and not questionably thin vapors.
No one expects today’s rudimentary quantum computers to be up to the challenge
of the punishingly long Hamiltonians required to simulate the real deal. But
now is the time to start chiseling away at them bit by bit, Gharibyan believes,
in preparation for the arrival of more capable machines. He expects that some
might try machine learning again, this time perhaps rewarding the algorithm
when it returns chaotically scrambling, non-commuting Hamiltonians and
penalizing it when it doesn’t. Of the resulting models, any that still have
perfect size winding and pass other checks will become the benchmark models to
drive the development of new quantum hardware.
If quantum computers grow while holographic Hamiltonians shrink, perhaps they
will someday meet in the middle. Then physicists will be able to run
experiments in the lab that reveal the incalculable behavior of their favorite
models of quantum gravity.
“I’m optimistic about where this is going,” Gharibyan said----
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My recent comments on vortex-l give with the theories presented above; however
, the link fails to connect gravity and magnetic dipole fields with entangled
wuth primary particle spin—energy and angular momentum – identified by a
Hamiltonians equation which described the balance of potential energy and
kinetic energy …
Bob Cook
