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1. Formation of robust bound states of interacting microwave photons

   https://doi.org/10.1038/s41586-022-05348-y  

   Morvan, A. ; Andersen, T. I. ; Mi, X. ; Neill, C. ; Petukhov, A. ; Kechedzhi, K. ; Abanin, D. A. ; Michailidis, A. ; Acharya, R. ; Arute, F. ; et al ( December 2022 , Nature)

   Systems of correlated particles appear in many fields of modern science and represent some of the most intractable computational problems in nature. The computational challenge in these systems arises when interactions become comparable to other energy scales, which makes the state of each particle depend on all other particles¹. The lack of general solutions for the three-body problem and acceptable theory for strongly correlated electrons shows that our understanding of correlated systems fades when the particle number or the interaction strength increases. One of the hallmarks of interacting systems is the formation of multiparticle bound states^2–9. Here we develop a high-fidelity parameterizable fSim gate and implement the periodic quantum circuit of the spin-½ XXZ model in a ring of 24 superconducting qubits. We study the propagation of these excitations and observe their bound nature for up to five photons. We devise a phase-sensitive method for constructing the few-body spectrum of the bound states and extract their pseudo-charge by introducing a synthetic flux. By introducing interactions between the ring and additional qubits, we observe an unexpected resilience of the bound states to integrability breaking. This finding goes against the idea that bound states in non-integrable systems are unstable when their energies overlap with the continuum spectrum. Our work provides experimental evidence for bound states of interacting photons and discovers their stability beyond the integrability limit.

    

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2. Subdiffusive hydrodynamics of nearly integrable anisotropic spin chains

   https://doi.org/10.1073/pnas.2202823119  

   De Nardis, Jacopo ; Gopalakrishnan, Sarang ; Vasseur, Romain ; Ware, Brayden ( August 2022 , Proceedings of the National Academy of Sciences)

   We address spin transport in the easy-axis Heisenberg spin chain subject to different integrability-breaking perturbations. We find subdiffusive spin transport characterized by dynamical exponent z = 4 up to a timescale parametrically long in the anisotropy. In the limit of infinite anisotropy, transport is subdiffusive at all times; for finite anisotropy, one eventually recovers diffusion at late times but with a diffusion constant independent of the strength of the perturbation and solely fixed by the value of the anisotropy. We provide numerical evidence for these findings, and we show how they can be understood in terms of the dynamical screening of the relevant quasiparticle excitations and effective dynamical constraints. Our results show that the diffusion constant of near-integrable diffusive spin chains is generically not perturbative in the integrability-breaking strength. 

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3. Quantum-critical scale invariance in a transition metal alloy

   https://doi.org/10.1038/s42005-020-00448-5  

   Nakajima, Yasuyuki ; Metz, Tristin ; Eckberg, Christopher ; Kirshenbaum, Kevin ; Hughes, Alex ; Wang, Renxiong ; Wang, Limin ; Saha, Shanta R. ; Liu, I-Lin ; Butch, Nicholas P. ; et al ( October 2020 , Communications Physics)

   Quantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe_1/3Co_1/3Ni_1/3)₂As₂, indicative of quantum criticality at zero temperature and applied magnetic field. Beyond a linear-in-temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system unhindered by a pairing instability.

    

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4. Transport in helical Luttinger liquids in the fractional quantum Hall regime

   https://doi.org/10.1038/s41467-021-25631-2  

   Wang, Ying ; Ponomarenko, Vadim ; Wan, Zhong ; West, Kenneth W. ; Baldwin, Kirk W. ; Pfeiffer, Loren N. ; Lyanda-Geller, Yuli ; Rokhinson, Leonid P. ( September 2021 , Nature Communications)

   Domain walls in fractional quantum Hall ferromagnets are gapless helical one-dimensional channels formed at the boundaries of topologically distinct quantum Hall (QH) liquids. Naïvely, these helical domain walls (hDWs) constitute two counter-propagating chiral states with opposite spins. Coupled to an s-wave superconductor, helical channels are expected to lead to topological superconductivity with high order non-Abelian excitations^1–3. Here we investigate transport properties of hDWs in theν = 2/3 fractional QH regime. Experimentally we found that current carried by hDWs is substantially smaller than the prediction of the naïve model. Luttinger liquid theory of the system reveals redistribution of currents between quasiparticle charge, spin and neutral modes, and predicts the reduction of the hDW current. Inclusion of spin-non-conserving tunneling processes reconciles theory with experiment. The theory confirms emergence of spin modes required for the formation of fractional topological superconductivity.

    

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5. Hydrodynamic nonlinear response of interacting integrable systems

   https://doi.org/10.1073/pnas.2106945118  

   Fava, Michele ; Biswas, Sounak ; Gopalakrishnan, Sarang ; Vasseur, Romain ; Parameswaran, S. A. ( September 2021 , Proceedings of the National Academy of Sciences)

   We develop a formalism for computing the nonlinear response of interacting integrable systems. Our results are asymptotically exact in the hydrodynamic limit where perturbing fields vary sufficiently slowly in space and time. We show that spatially resolved nonlinear response distinguishes interacting integrable systems from noninteracting ones, exemplifying this for the Lieb–Liniger gas. We give a prescription for computing finite-temperature Drude weights of arbitrary order, which is in excellent agreement with numerical evaluation of the third-order response of the XXZ spin chain. We identify intrinsically nonperturbative regimes of the nonlinear response of integrable systems. 

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