Quantum 'magic' could help explain the origin of spacetime

 

A quantum property dubbed "magic" could be the key to explaining how space and time emerged, a new mathematical analysis by three RIKEN physicists suggests. The research is published in the journal Physical Review D.

It's hard to conceive of anything more basic than the fabric of spacetime that underpins the universe, but theoretical physicists have been questioning this assumption. "Physicists have long been fascinated about the possibility that space and time are not fundamental, but rather are derived from something deeper," says Kanato Goto of the RIKEN Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS).
This notion received a boost in the 1990s, when theoretical physicist Juan Maldacena related the gravitational theory that governs spacetime to a theory involving quantum particles. In particular, he imagined a hypothetical space—which can be pictured as being enclosed in something like an infinite soup can, or "bulk"—holding objects like black holes that are acted on by gravity. Maldacena also imagined particles moving on the surface of the can, controlled by quantum mechanics. He realized that mathematically a quantum theory used to describe the particles on the boundary is equivalent to a gravitational theory describing the black holes and spacetime inside the bulk.

"This relationship indicates that spacetime itself does not exist fundamentally, but emerges from some quantum nature," says Goto. "Physicists are trying to understand the quantum property that is key."

The original thought was that quantum entanglement—which links particles no matter how far they are separated—was the most important factor: the more entangled particles on the boundary are, the smoother the spacetime within the bulk.

"But just considering the degree of entanglement on the boundary cannot explain all the properties of black holes, for instance, how their interiors can grow," says Goto.

So Goto and iTHEMS colleagues Tomoki Nosaka and Masahiro Nozaki searched for another quantum quantity that could apply to the boundary system and could also be mapped to the bulk to describe black holes more fully. In particular, they noted that black holes have a chaotic characteristic that needs to be described.

"When you throw something into a black hole, information about it gets scrambled and cannot be recovered," says Goto. "This scrambling is a manifestation of chaos."

The team came across "magic," which is a mathematical measure of how difficult a quantum state is to simulate using an ordinary classical (non-quantum) computer. Their calculations showed that in a chaotic system almost any state will evolve into one that is "maximally magical"—the most difficult to simulate.

This provides the first direct link between the quantum property of magic and the chaotic nature of black holes. "This finding suggests that magic is strongly involved in the emergence of spacetime," says Goto.

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Physicists take step toward fault-tolerant quantum computing

 

Some classical computers have error correction built into their memories based on bits; quantum computers, to be workable in the future, will need error correction mechanisms, too, based on the vastly more sensitive qubits.




Cornell researchers have recently taken a step toward fault-tolerant quantum computing: they constructed a simple model containing exotic particles called non-Abelian anyons, compact and practical enough to run on modern quantum hardware. Realizing these particles, which can only exist in two dimensions, is a move towards implementing it in the real world.


Thanks to some creative thinking, Yuri Lensky, a former Bethe/Wilkins/Kavli Institute at Cornell (KIC) postdoctoral fellow in physics in the College of Arts and Sciences (A&S), collaborating with Eun-Ah Kim, professor of physics (A&S), came up with a simple "recipe" that could be used for robustly computing with non-Abelian anyons, including specific instructions for executing the effect experimentally on devices available today.

Their paper, "Graph Gauge Theory of Mobile Non-Abelian Anyons in a Qubit Stabilizer Code," written in collaboration with theorists at Google Quantum AI, published March 24 in Annals of Physics. Google Quantum AI researchers, together with Lensky and Kim, have proved the theory with a successful experiment as reported in a preprint publication, "Observation of Non-Abelian Exchange Statistics on a Superconducting Processor," on the research-sharing platform arXiv.

"This two-dimensional state is interesting both from a quantum condensed matter physics perspective—it has some novel properties that are very special to 2D physics—and from a quantum information perspective," Lensky said. "It's something truly quantum, but it's also potentially useful for quantum computation. It protects bits of quantum information by storing them non-locally, and our protocol allows us to compute with these bits."

Kim explained the principle that animates non-Abelian anyons by holding out two identical one-pound barbells. When she crosses her arms, the identical barbells change positions, but as objects defined by classical physics, their state remains the same. They are interchangeable.

If those barbells represent two identical quantum particles, remarkably in certain 2D systems their trails through space-time can produce a measurable record of the change (picture the crossed arms.) This process of exchange is called a braid, after the shapes of the particle trails.

"Quantum mechanically, when you move one particle around the other," Kim said, holding one weight still and moving the other in a circle around it, "the wave function, which is a solution to the Schrödinger equation describing quantum mechanical motion, can be multiplied by a phase factor or it can become something that's very different."

When the wave function gains a global sign that can only be observed through interferometry, a measurement of the interference of waves, that's called an Abelian anyon. When the wave function becomes measurably different, it's a non-Abelian anyon, she said.

Non-Abelian anyons could be harnessed to create qubits defined not on a single particle, but on a pair of identical quantum particles: nonlocally encoded.

"If I put the qubit shared between these particles in a zero state and move them apart, then whatever happens locally to one of these anyons, the zero state will remain. The qubit set to zero is safe from corruption," Kim said. "Non-Abelian anyons could be used in a platform for protected qubits."

But while physicists have theorized about these exotic particles for years—Alexei Kitaev proposed operating on protected bits of quantum memory by braiding non-Abelian anyons back around 2001, Lensky said—they have never been observed in a physical system before now.

When Google Quantum AI developed the quantum processor platform capabilities to realize the surface code and braiding of Abelian anyons in a physical system, Lensky said, "This was [our] inspiration to look for a way to realize the physics of non-Abelian anyons as soon as possible."

"We knew they had the working ingredients, but they didn't have a recipe," Kim said. "We figured out how to move these non-Abelian anyons, then we told the experimentalists what to do. It was possible because Yuri and I were thinking in a flexible, creative and open-minded way."

Past theoretical research identified non-Abelian properties, but came up short on how to move them, a necessary step. A key insight from Lensky and Kim was to give up the regularity of a grid and arrange qubits in an almost hand-drawn manner but backed up by robust mathematics.

"After this simple geometric insight, using gauge theory, we were able to come up with the protocol of taking this picture and implementing it on a chip in a robust and efficient way," Kim said. "With this 10-qubit system, we were able to encode multiple non-Abelian anyons, and therefore multiple logical information-carrying qubits, and a precise recipe for what the experimentalists need to do every step of the way."

"Although the focus of the theory and experiment is simply to realize non-Abelian anyons in the real world, this can also be viewed as a first small step towards implementing computation by braiding," Lensky said

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Researchers make an important step towards the quantum internet using diamond nanostructures

 

Diamond material is of great importance for future technologies such as the quantum internet. Special defect centers can be used as quantum bits (qubits) and emit single light particles that are referred to as single photons.

To enable data transmission with feasible communication rates over long distances in a quantum network, all photons must be collected in optical fibers and transmitted without being lost. It must also be ensured that these photons all have the same color, i.e., the same frequency. Fulfilling these requirements has been impossible until now.

Researchers in the "Integrated Quantum Photonics" group led by Prof. Dr. Tim Schröder at Humboldt-Universität zu Berlin have succeeded for the first time worldwide in generating and detecting photons with stable photon frequencies emitted from quantum light sources, or, more precisely, from nitrogen-vacancy defect centers in diamond nanostructures.

This was enabled by carefully choosing the diamond material; sophisticated nanofabrication methods carried out at the Joint Lab Diamond Nanophotonics of the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik; and specific experimental control protocols. By combining these methods, the noise of the electrons, which previously disturbed data transmission, can be significantly reduced, and the photons are emitted at a stable (communication) frequency.




In addition, the Berlin researchers show that the current communication rates between spatially separated quantum systems can prospectively be increased more than 1,000-fold with the help of the developed methods—an important step closer to a future quantum internet.

The scientists have integrated individual qubits into optimized diamond nanostructures. These structures are 1,000 times thinner than a human hair and make it possible to transfer emitted photons in a directed manner into glass fibers.

However, during the fabrication of the nanostructures, the material surface is damaged at the atomic level, and free electrons create uncontrollable noise for the generated light particles. Noise, comparable to an unstable radio frequency, causes fluctuations in the photon frequency, preventing successful quantum operations such as entanglement.

A special feature of the diamond material used is its relatively high density of nitrogen impurity atoms in the crystal lattice. These possibly shield the quantum light source from electron noise at the surface of the nanostructure. "However, the exact physical processes need to be studied in more detail in the future," explains Laura Orphal-Kobin, who investigates quantum systems together with Prof. Dr. Tim Schröder.

The conclusions drawn from the experimental observations are supported by statistical models and simulations, which Dr. Gregor Pieplow from the same research group is developing and implementing together with the experimental physicists.

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Seeing is more than believing: Exploring 'de Sitter space' to explain gravity in the expanding early universe

 

Having more tools helps; having the right tools is better. Utilizing multiple dimensions may simplify difficult problems—not only in science fiction but also in physics—and tie together conflicting theories.





For example, Einstein's theory of general relativity—which resides in the fabric of space-time warped by planetary or other massive objects—explains how gravity works in most cases. However, the theory breaks down under extreme conditions such as those existing in black holes and cosmic primordial soups.

An approach known as superstring theory could use another dimension to help bridge Einstein's theory with quantum mechanics, solving many of these problems. But the necessary evidence to support this proposal has been lacking.

Now, a team of researchers led by Kyoto University is exploring 'de Sitter space' to invoke a higher dimension to explain gravity in the expanding early universe. They have developed a concrete method to compute correlation functions among fluctuations on expanding universe by making use of holography.

"We came to realize that our method can be applied more generically than we expected while dealing with quantum gravity," says Yasuaki Hikida, from the Yukawa Institute for Theoretical Physics.

Dutch astronomer Willem de Sitter's theoretical models describe space in a way that fits with Einstein's general theory of relativity, in that the positive cosmological constant accounts for the expansion of the universe.

Starting with existing methods for handling gravity in anti-de Sitter space, Hikida's team reshaped them to work in expanding de Sitter space to more precisely account for what is already known about the universe.

"We are now extending our analysis to investigate cosmological entropy and quantum gravity effects," adds Hikida.

Although the team's calculations only considered a three-dimensional universe as a test case, the analysis may easily be extended to a four-dimensional universe, allowing for the extraction of information from our real world.

"Our approach possibly contributes to validating superstring theory and allows for practical calculations about the subtle changes that rippled across the fabric of our early universe."

The study is published in the journal Physical Review Letters.

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