For decades, particle physicists have been searching to no avail for hints of supersymmetry. But could it lie closer to home than we think? New findings from researchers at PSI reveal supersymmetric behaviour in a quantum material, demonstrating that it can emerge naturally in condensed matter. The findings, which have been published in Nature Communications have promising practical implications for making stable qubits for quantum computing.
The theory of supersymmetry tells us that every matter (fermion) particle should have a supersymmetric force-carrying (boson) partner. If proven relevant to the physics of our universe, this would provide crucial evidence of physics beyond the Standard Model, with implications for unresolved mysteries in physics such as dark matter. Yet, despite decades of experiments at the large hadron collider (LHC) at CERN, no direct evidence of its existence has been found.
Although supersymmetry has yet to – and may never – be discovered at the high energies where new particles are created and destroyed, its mathematical structure can still emerge in materials at lower energies, governing the behaviour of quantum states, as a new study led by PSI researchers shows.
Supersymmetry in a quantum spin ladder
The researchers studied a class of materials known as quantum spin ladders, a type of insulating magnetic material where atoms are arranged in pairs of coupled chains. In the ladder, charge appears as a hole in the magnetic pattern, with its properties intrinsically tied to the spins of the electrons. These spins and these charge-like excitations influence their neighbours along the chains and across the rungs of the ladder, following quantum rules – much like how filling in one number in a Sudoku puzzle constrains the possibilities for surrounding numbers. This interconnectedness leads to unusual, highly correlated behaviour.
Using inelastic neutron scattering experiments on the ThALES instrument at the Institut Laue Langevin (ILL) and the LET instrument at the ISIS Neutron and Muon Source, the researchers investigated two spin ladder compounds, (C₅D₁₂N)₂CuBr₄ and (C₅D₁₂N)₂CuCl₄, where magnetic copper ions form the chains of the ladder. By bombarding the materials with neutrons and measuring how they scattered, they could infer how the neutrons had interacted with the material and hence gain insight into the spin and charge excitations in the spin ladder. To model these ladder excitations with high accuracy, theorists from PSI together with the University of Geneva and University of Bonn used state-of-the-art numerical methods known as matrix-product-state (MPS) simulations.
The key evidence for supersymmetry in the material came by performing the neutron scattering experiments across different temperatures. Here, the researchers observed a distinct spectral signature: a peak that remained sharp whatever the temperature. Normally, quantum states are disrupted by thermal fluctuations, causing spectral features to broaden and sometimes shift.
Guided by their theoretical models, the researchers could explain the persistence of this peak with the mathematics of supersymmetry. In the same way that in particle physics supersymmetry pairs fermions with bosons, in the quantum spin ladder, supersymmetry emerges not as new particles but as a deep mathematical relationship between spin and charge excitations. This stable link between excitations protects them from thermal fluctuations.
Implications for quantum computing
Scientists have theorised for some time that condensed matter systems could show supersymmetric behaviour, but this is the first time that it has been directly observed in a real material.
“What we’ve observed is not fundamental supersymmetry as would be discovered at CERN with the observation of a supersymmetric partner particle; it’s what we would describe as an emergent supersymmetry,” explains Bruce Normand, senior scientist at PSI, who worked on the numerical modelling. “Nonetheless being able to see a new symmetry for the first time is intrinsically very exciting; and, importantly, seeing it enables us to do experiments with it.”
Beyond the implications for our fundamental understanding of materials, there is a practical consequence of their discovery. “The manifestation of supersymmetry that we see is this peak - this quantum state - that is protected. This is a quality that is extremely sought after in quantum information processing,” says Normand.
In quantum computing, decoherence – whereby qubits rapidly lose quantum information – is a major obstacle to scaling-up toward fully functional quantum processors. Supersymmetry may not yet give us the answers to the meaning of life, the universe and dark matter. But it may, in the future, be a means to stable qubits.
Article courtesy of Miriam Arrell, Paul Scherrer Institute
