What makes physics beautiful? We asked some top researchers
Quantum field theory forms the base of all physics, and before students can become fluent in the language they must identify gaps in their knowledge. That’s the concept behind a non-credit minicourse at Perimeter about ‘beautiful’ papers in physics, taught by Pedro Vieira, Research Faculty member and Clay Riddell Dirac Chair in Theoretical Physics at Perimeter. The course covers a selection of papers, from Steven Weinberg’s 1965 article on infrared photons and gravitons to Pasquale Calabrese and John Cardy’s 2004 work on entanglement entropy, with the repeating theme of a winding path to discovery and exploration. The course alternates between lectures on nine influential papers and student-led presentations about how these monumental papers influenced physics.
From its mountaintop location in Arizona, DESI maps the universe. Credit: Marilyn Sargent/Berkeley Lab
Evidence mounting that dark energy evolves over time
According to the Standard Model of Cosmology, the expansion of our universe is driven by the simplest possible version of dark energy: an unchanging ‘cosmological constant’ called lambda. That longstanding model may be about to change. Will Percival, Research Associate Faculty member at Perimeter, is co-spokesperson for the Dark Energy Spectroscopic Instrument (DESI) collaboration, which has built the largest 3D map of our universe to date by observing millions of galaxies and quasars. The latest evidence from DESI suggests that dark energy may in fact be evolving over time.
DESI is an international experiment with more than 900 researchers from over 70 institutions around the world and is managed by the US Department of Energy’s Lawrence Berkeley National Laboratory. DESI measures dark energy by tracking the size of bubble-like formations called Baryonic Acoustic Oscillations, caused by acoustic waves in the hot plasma of the early universe. These primordial bubbles can now be seen imprinted on the large-scale distribution of galaxies and matter in the universe.
Read further: New DESI Results Weigh In On Gravity, Hints that dark energy could vary over time excite cosmologists
European Southern Observatory's Very Large Telescope (VLT) is a flagship facility for European ground-based astronomy. Credit: ESO/G.Hüdepohl
A new (old) lens on the universe
An old optical telescope technique known as “intensity interferometry” – first established for early radio telescopes in the 1950s – holds promise for today’s researchers. The technique, based on the idea of combining information from two or more smaller telescopes to act as one larger instrument with greater precision, was used with some success to measure the diameters of many stars, but was eventually abandoned because the instruments were too slow.
Now, with better and faster technology, scientists see new possibilities for intensity interferometry. Current and former Perimeter researchers Neal Dalal, Marios Galanis, Junwu Huang, and Masha Baryakhtar hosted a conference in 2024 to discuss technology that will make the technique viable. Dalal and Galanis co-authored a paper that discusses the possibility of using intensity interferometry to get better images of active galactic nuclei (AGN): supermassive black holes that emit bright jets and winds that shape their galaxies.
Reference:
N. Dalal (Perimeter), M. Galanis (Perimeter), C. Gammie (Illinois U., Urbana), S. E. Gralla (Arizona U.), N. Murray (CITA), “Probing 𝐻0 and resolving AGN disks with ultrafast photon counters,” Phys. Rev. D. (2024), DOI: 10.1103/PhysRevD.109.123029. [link]
The luminous Milky Way and hundreds of stars stretch across the sky behind the silhouette of Vera C. Rubin Observatory. Credit: Bruno C. Quint
Preparing for an unprecedented survey of the southern hemisphere night sky
High atop the El Peñón peak of Cerro Pachón in northern Chile, the Vera C. Rubin Observatory is about to perform a scientific first: survey the entire southern hemisphere night sky for 10 consecutive years. The Legacy Survey of Space and Time, or LSST, is expected to start at the Rubin Observatory in 2025 and will use the world’s largest digital camera to capture a high-definition time-lapse of the visible sky. Ana Ennis, a joint postdoctoral researcher at Perimeter Institute and the University of Waterloo Centre for Astrophysics, is the co-chair for the Star Clusters Working Group within the LSST’s science collaborations. As part of the leadup to the telescope’s grand unveiling, Ennis is working alongside a team of international astronomers to prepare for the incoming flood of data.
The LSST will be accomplishing two goals at the same time. First, cover as much of the southern hemisphere as quickly as possible to monitor for events like objects moving in space and transient gamma-ray bursts. Once detected, the LSST will send out an alert so that other telescopes can focus in on these transitory phenomena. The second goal is taking very deep images of the sky. Deep images require long exposure times to collect all the photons produced by faraway stars and galaxies.
Is ‘time’ fundamental in quantum mechanics?
No matter how we measure it – by the tick of a clock or by the number of grey hairs on our heads – time moves in a relentless one-way direction from past to present to future. Some theoretical physicists think time, at its most basic quantum level, is not fundamentally real, while others think time is basic to everything. What if time is something that “emerges” from relationships between quantum particles? Perimeter Postdoctoral Researcher Luca Ciambelli and Research Faculty member Laurent Freidel, alongside University of Illinois Urbana-Champaign Professor and Perimeter Visiting Fellow Robert Leigh, published a paper in the Journal of High Energy Physics about a possible solution to the problem of time using a mathematical construct called a “null hypersurface.” By working with this null hypersurface, the team offered a way forward to quantum time.
Reference:
L. Ciambelli (Perimeter), L. Freidel (Perimeter), R. G. Leigh (UIUC/Perimeter), “Quantum null geometry and gravity,” J. High Energ. Phys. (2024), DOI: 10.1007/JHEP12(2024)028 [link]
A new link between generalized quantum error-correcting codes, complexity, and physics
One of the key barriers to practical quantum computing is the fragility of quantum systems. Quantum computers rely on entangled particles to work, but entanglement is easily disrupted by ‘noise’: interactions with the outside world. As a result, scientists are pouring their energy into devising error-correcting codes that can keep quantum computers from making mistakes. Quantum information expert Daniel Gottesman, a Distinguished Visiting Research Chair at Perimeter and former Research Faculty member, teamed up with current and former Perimeter scientists including Zi-Wen Liu, Jinmin Yi, and Weicheng Ye for a paper in Nature Physics that establishes a new framework for understanding quantum error-correction codes.
The team’s work tackles approximate quantum error correction (AQEC). The paper establishes that when AQEC codes have an error – a subsystem variance – that is sufficiently small, then that subjects the code to a lower bound on the time needed for all the qubits to connect up. This puts a constraint on the circuit complexity. The insight has given researchers a way to fundamentally characterize the ‘components’ of a nontrivial AQEC code for the first time. It's an important step forward for quantum information theory.
References:
J. Yi (Perimeter/Waterloo U.), W. Ye (Perimeter), D. Gottesman (Maryland U., College Park), Z.-W. Liu (Perimeter/Tsinghua U.), “Complexity and order in approximate quantum error-correcting codes,” Nat. Phys. (2024), DOI: 10.1038/s41567-024-02621-x. [link]
Summary paper: “A general theory of quantum codes connecting quantum computation, complexity and physics,” Nat. Phys. (2024), DOI: 10.1038/s41567-024-02622-w. [link]
Beyond Qubits
Christine Muschik, quantum theorist and associate faculty member at Perimeter Institute, is helping pioneer a new era of quantum computing by exploring multi-level systems known as qudits.
Quantum computers could be much more powerful than classical computers, thanks to two concepts: superposition and entanglement. Superposition means a particle can be in two states at the same time. Entanglement is the strong correlation between particles at great distances. Christine Muschik, a research associate faculty member at Perimeter and a professor at the University of Waterloo’s Institute for Quantum Computing (IQC), is one of two co-leads on a paper in Nature Physics. Her research aims to work out the complex interactions of particles and forces that are integral to particle physics.
A regular computer has logic gates, which are like switches that turn off or on, forming the building blocks for digital circuits. But quantum computers have quantum gates, which are quantum circuits using small numbers of qubits – a quantum bit that is a basic unit of information in quantum computing. Qubits are represented as superpositions of zeros and ones at the same time, allowing them to contain much more information. But Muschik is also exploring qudits, multi-level systems that can exist in multiple states, that could represent a new era of quantum computing.
Reference:
M. Meth (Innsbruck U.), J. Zhang (Waterloo U.), J. F. Haase (Waterloo U./Ulm U.), C. Edmunds (Innsbruck U.), L. Postler (Innsbruck U.), A. J. Jena (Waterloo U.), A. Steiner (Innsbruck U.), L. Dellantonio (Waterloo U./Exeter U.), Rainer Blatt (Innsbruck U. / Innsbruck U., Quant. Opt. and Info.), P. Zoller (Innsbruck U., Quant. Opt. and Info. and Innsbruck U., Inst. Astrophys.), T. Monz (Innsbruck U.), P. Schindler (Innsbruck U.), C. Muschik (Waterloo U./Perimeter Inst. Theor. Phys.), M. Ringbauer (Innsbruck U.), “Simulating two-dimensional lattice gauge theories on a qudit quantum computer,” Nat. Phys. (2025), DOI: 10.1038/s41567-025-02797-w. [link]
“[Quantum Computing using Qudits] is still a young field. My team and I feel like pioneers.”
—Christine Muschik, Research Associate Faculty
Marcela Carena, Executive Director of Perimeter Institute, will serve on a newly formed expert panel advising the Canadian government on optimizing research infrastructure.
Perimeter Executive Director Marcela Carena to serve on newly formed CCA advisory panel
The Canadian Council of Academies (CCA) has established a new expert panel designed to support Canada’s federal government on issues related to enhancing research infrastructure. Perimeter’s Executive Director Marcela Carena is one of 12 experts appointed to the panel as part of an assessment supported by the Government of Canada through the Strategic Science Fund. The panel was formed at the request of Innovation, Science, and Economic Development Canada.