Some phases of matter cannot be described using the conventional framework of symmetry breaking and exhibit a so-called quantum order. One type of quantum order, known as topological order, is characterized by long-range entanglement between particles across an entire system, a ground state degeneracy that depends on the global shape of the system, and a robustness against local disturbances.
Topological phases of matter primarily occur at zero temperature, as thermal fluctuations tend to destroy them and disrupt their underlying order. In a recent paper published in Physical Review Letters, however, researchers at Nanjing University, Yale University and other institutes reported a new 3D topological phase of matter characterized by an anomalous two-form symmetry that occurs at non-zero temperatures.
"In the last several years, we have made substantial progress in our ability to control quantum systems—over a range of different platforms: superconducting qubits, trapped ions, neutral atoms, photonics, and so on," Tyler D. Ellison, senior author of the paper, told Phys.org.
"This has opened up the potential for engineering interesting quantum states and quantum phases of matter in well-controlled experimental settings. However, unavoidably, the hardware is imperfect, and the system is not completely isolated from the environment. This means that the quantum system suffers from faulty operations (such as photon loss) and noise from the environment (e.g., from cosmic-ray particles)."
Due to this key limitation, quantum systems are not defined by 'pure' quantum states occurring in them, but rather by probability distributions of quantum states, arising from the probabilistic nature of noise-associated errors. Over the past decades, many physicists have been trying to better understand exotic physical phenomena and quantum states of matter that can be realized irrespective of background noise and a lack of control over them.
"Similarly, quantum systems at non-zero temperature are probability distributions of pure quantum states," said Ellison. "In this case, the distributions instead arise from thermal fluctuations. We realized that some of the same theoretical tools that have been developed recently in the context of noisy quantum systems can be used to characterize quantum systems at non-zero temperature."
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