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1. Exponential acceleration of macroscopic quantum tunneling in a Floquet Ising model

   George Grattan, Brandon A. (November 2023, arXivorg)

   The exponential suppression of macroscopic quantum tunneling (MQT) in the number of elements to be reconfigured is an essential element of broken symmetry phases. This suppression is also a core bottleneck in quantum algorithms, such as traversing an energy landscape in optimization, and adiabatic state preparation more generally. In this work, we demonstrate exponential acceleration of MQT through Floquet engineering with the application of a uniform, high frequency transverse drive field. Using the ferromagnetic phase of the transverse field Ising model in one and two dimensions as a prototypical example, we identify three phenomenological regimes as a function of drive strength. For weak drives, the system exhibits exponentially decaying tunneling rates but robust magnetic order; in the crossover regime at intermediate drive strength, we find polynomial decay of tunnelling alongside vanishing magnetic order; and at very strong drive strengths both the Rabi frequency and time-averaged magnetic order are approximately constant with increasing system size. We support these claims with extensive full wavefunction and tensor network numerical simulations, and theoretical analysis. An experimental test of these results presents a technologically important and novel scientific question accessible on NISQ-era quantum computers. 

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2. Nuclear magnetic resonance studies in a model transverse field Ising system

   https://doi.org/10.3389/fphy.2024.1393229  

   Nian, Y-H; Vinograd, I; Chaffey, C; Li, Y; Zic, M P; Massat, P; Singh, R_R P; Fisher, I R; Curro, N J (June 2024, Frontiers in Physics)

   The suppression of ferroquadrupolar order in TmVO₄in a magnetic field is well-described by the transverse field Ising model, enabling detailed studies of critical dynamics near the quantum phase transition. We describe nuclear magnetic resonance measurements in pure and Y-doped single crystals. The non-Kramers nature of the ground state doublet leads to a unique form of the hyperfine coupling that exclusively probes the transverse field susceptibility. Our results show that this quantity diverges at the critical field, in contrast to the mean-field prediction. Furthermore, we find evidence for quantum critical fluctuations present near Tm-rich regions in Y-doped crystals at levels beyond which long-range order is suppressed, suggesting the presence of quantum Griffiths phases. 

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3. Second order Zeeman interaction and ferroquadrupolar order in TmVO4

   https://doi.org/10.1038/s41535-022-00475-1  

   Vinograd, I.; Shirer, K. R.; Massat, P.; Wang, Z.; Kissikov, T.; Garcia, D.; Bachmann, M. D.; Horvatić, M.; Fisher, I. R.; Curro, N. J. (June 2022, npj Quantum Materials)

   Abstract TmVO₄exhibits ferroquadrupolar order of the Tm 4f electronic orbitals at low temperatures, and is a model system for Ising nematicity. A magnetic field oriented along thec-axis constitutes a transverse effective field for the quadrupolar order parameter, continuously tuning the system to a quantum phase transition as the field is increased from zero. In contrast, in-plane magnetic fields couple to the order parameter only at second order, such that orienting along the primary axes of the quadrupole order results in an effective longitudinal field, whereas orienting at 45 degrees results in a second effective transverse field. Not only do in-plane fields engender a marked in-plane anisotropy of the critical magnetic and quadrupole fluctuations above the ferroquadrupolar ordering temperature, but in-plane transverse fields initially enhance the ferroquadrupolar order, before eventually suppressing it, an effect that we attribute to admixing of the higher crystalline electric field levels. 

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4. Effect of electron–electron interaction on magnitude of quantum oscillations of dissipative resistance in magnetic fields

   https://doi.org/10.1063/5.0127286  

   Abedi, Sara; Vitkalov, Sergey; Bykov, A. A.; Bakarov, A. K. (December 2022, Journal of Applied Physics)

   Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields [Formula: see text] tilted from the normal to the 2D electron layer at different temperatures [Formula: see text]. The in-plane component ([Formula: see text]) of the field [Formula: see text] induces magnetic entanglement between subbands, leading to beating in oscillating density of states (DOS) and to MISO suppression. Model of the MISO suppression is proposed. Within the model, a comparison of MISO amplitude in the entangled and disentangled ([Formula: see text]) 2D systems yields both difference frequency of DOS oscillations, [Formula: see text], and strength of the electron–electron interaction, described by parameter [Formula: see text], in the 2D system. These properties are analyzed using two methods, yielding consistent but not identical results for both [Formula: see text] and [Formula: see text]. The analysis reveals an additional angular dependent factor of MISO suppression. The factor is related to spin splitting of quantum levels in magnetic fields. 

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5. Emergent helical edge states in a hybridized three-dimensional topological insulator

   https://doi.org/10.1038/s41467-022-33643-9  

   Chong, Su Kong; Liu, Lizhe; Watanabe, Kenji; Taniguchi, Takashi; Sparks, Taylor D.; Liu, Feng; Deshpande, Vikram V. (October 2022, Nature Communications)

   Abstract As the thickness of a three-dimensional (3D) topological insulator (TI) becomes comparable to the penetration depth of surface states, quantum tunneling between surfaces turns their gapless Dirac electronic structure into a gapped spectrum. Whether the surface hybridization gap can host topological edge states is still an open question. Herein, we provide transport evidence of 2D topological states in the quantum tunneling regime of a bulk insulating 3D TI BiSbTeSe₂. Different from its trivial insulating phase, this 2D topological state exhibits a finite longitudinal conductance at ~2e²/h when the Fermi level is aligned within the surface gap, indicating an emergent quantum spin Hall (QSH) state. The transition from the QSH to quantum Hall (QH) state in a transverse magnetic field further supports the existence of this distinguished 2D topological phase. In addition, we demonstrate a second route to realize the 2D topological state via surface gap-closing and topological phase transition mechanism mediated by a transverse electric field. The experimental realization of the 2D topological phase in a 3D TI enriches its phase diagram and marks an important step toward functionalized topological quantum devices. 

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