In many-body localized (MBL) systems, entanglement propagates throughout the system despite the absence of transport. Early experiments have relied on population measurements to indirectly probe these entanglement dynamics. However, because the entanglement results from phase relationships between localized orbitals, it is more naturally probed with phase sensitive algorithms and measurement. In this thesis, we use an array of nearest neighbor coupled superconducting qubits to introduce phase sensitive protocols to the experimental study of MBL systems. We establish that system is MBL by demonstrating disorder induced ergodicity breaking and the presence of effective nonlocal interactions. We then use density matrix reconstructions to observe the hallmark slow growth of entanglement and provide a site-resolved spatial and temporal map of the developing entanglement. We also inspect the capacity of the MBL phase to preserve quantum correlations by observing the decay of distillable entanglement when Bell pair embedded in an MBL environment and dephased by remote excitation. In superconducting quantum processors, such as that used in the MBL study above, dissipation leads to computational errors and must be minimized. To that end, we also describe coherence engineering experiments in terms of the low power internal quality factor Qi of coplanar waveguide (CPW) resonators, a figure of merit characterizing dissipation in the quantum computing regime. We investigate titanium nitride as a superconducting base metal for quantum circuits. By optimizing the deposition conditions, we achieve a record low-power Qi in CPW resonators. We also characterize the dielectric loss due to flux trapping hole arrays. Since flux traps are commonly used to prevent magnetic vortex formation and dielectric loss is a limiting dissipation mechanism, it is important to estimate the contribution of flux traps to the dielectric dissipation budget. We find that for reasonable hole patterns the dielectric loss can be small while preventing vortex formation.
In many-body localized (MBL) systems, entanglement propagates throughout the system despite the absence of transport. Early experiments have relied on population measurements to indirectly probe these entanglement dynamics. However, because the entanglement results from phase relationships between localized orbitals, it is more naturally probed with phase sensitive algorithms and measurement. In this thesis, we use an array of nearest neighbor coupled superconducting qubits to introduce phase sensitive protocols to the experimental study of MBL systems. We establish that system is MBL by demonstrating disorder induced ergodicity breaking and the presence of effective nonlocal interactions. We then use density matrix reconstructions to observe the hallmark slow growth of entanglement and provide a site-resolved spatial and temporal map of the developing entanglement. We also inspect the capacity of the MBL phase to preserve quantum correlations by observing the decay of distillable entanglement when Bell pair embedded in an MBL environment and dephased by remote excitation. In superconducting quantum processors, such as that used in the MBL study above, dissipation leads to computational errors and must be minimized. To that end, we also describe coherence engineering experiments in terms of the low power internal quality factor Qi of coplanar waveguide (CPW) resonators, a figure of merit characterizing dissipation in the quantum computing regime. We investigate titanium nitride as a superconducting base metal for quantum circuits. By optimizing the deposition conditions, we achieve a record low-power Qi in CPW resonators. We also characterize the dielectric loss due to flux trapping hole arrays. Since flux traps are commonly used to prevent magnetic vortex formation and dielectric loss is a limiting dissipation mechanism, it is important to estimate the contribution of flux traps to the dielectric dissipation budget. We find that for reasonable hole patterns the dielectric loss can be small while preventing vortex formation.
The interplay of interactions and strong disorder can lead to an exotic quantum many-body localized (MBL) phase of matter. Beyond the absence of transport, the MBL phase has distinctive signatures, such as slow dephasing and logarithmic entanglement growth; they commonly result in slow and subtle modifications of the dynamics, rendering their measurement challenging. Here, we experimentally characterize these properties of the MBL phase in a system of coupled superconducting qubits. By implementing phase sensitive techniques, we map out the structure of local integrals of motion in the MBL phase. Tomographic reconstruction of single and two-qubit density matrices allows us to determine the spatial and temporal entanglement growth between the localized sites. In addition, we study the preservation of entanglement in the MBL phase. The interferometric protocols implemented here detect affirmative quantum correlations and exclude artifacts due to the imperfect isolation of the system. By measuring elusive MBL quantities, our work highlights the advantages of phase sensitive measurements in studying novel phases of matter.
The law of statistical physics dictates that generic closed quantum many-body systems initialized in nonequilibrium will thermalize under their own dynamics. However, the emergence of many-body localization (MBL) owing to the interplay between interaction and disorder, which is in stark contrast to Anderson localization, which only addresses noninteracting particles in the presence of disorder, greatly challenges this concept, because it prevents the systems from evolving to the ergodic thermalized state. One critical evidence of MBL is the long-time logarithmic growth of entanglement entropy, and a direct observation of it is still elusive due to the experimental challenges in multiqubit single-shot measurement and quantum state tomography. Here we present an experiment fully emulating the MBL dynamics with a 10-qubit superconducting quantum processor, which represents a spin-1/2 XY model featuring programmable disorder and long-range spin-spin interactions. We provide essential signatures of MBL, such as the imbalance due to the initial nonequilibrium, the violation of eigenstate thermalization hypothesis, and, more importantly, the direct evidence of the long-time logarithmic growth of entanglement entropy. Our results lay solid foundations for precisely simulating the intriguing physics of quantum many-body systems on the platform of large-scale multiqubit superconducting quantum processors.
Many-body localization (MBL) provides a mechanism by which interacting quantum systems evade thermalization, leading to persistent memory of initial conditions and slow entanglement growth. Probing these dynamical signatures in large systems and at long evolution times remains challenging for both classical simulations and current quantum devices. Here we experimentally investigate the ergodic-MBL crossover in quasiperiodic Floquet Ising systems using up to 144 qubits on an IBM Quantum processor. By implementing deep Floquet circuits reaching up to 5000 cycles, we access long-time many-body dynamics beyond the regime explored in previous quantum computing experiments. Measurements of autocorrelation functions reveal a smooth crossover from rapid thermalization at weak quasiperiodic potential strength to persistent correlations in the strong-disorder regime. Notably, in addition to the one-dimensional system, we also observe clear signatures consistent with localization behavior in the two-dimensional system. Furthermore, the quantum Fisher information exhibits logarithmic growth over thousands of Floquet cycles, providing evidence for slow entanglement spreading characteristic of the MBL regime. These results demonstrate that programmable quantum processors can serve as experimental platforms for probing nonergodic quantum many-body dynamics and exploring localization phenomena in regimes beyond the reach of classical simulations.
Quantum emulators, owing to their large degree of tunability and control, allow the observation of fine aspects of closed quantum many-body systems, as either the regime where thermalization takes place or when it is halted by the presence of disorder. The latter, dubbed many-body localization (MBL) phenomenon, describes the nonergodic behavior that is dynamically identified by the preservation of local information and slow entanglement growth. Here, we provide a precise observation of this same phenomenology in the case where the quenched on-site energy landscape is not disordered, but rather linearly varied, emulating the Stark MBL. To this end, we construct a quantum device composed of 29 functional superconducting qubits, faithfully reproducing the relaxation dynamics of a nonintegrable spin model. At large Stark potentials, local observables display periodic Bloch oscillations, a manifesting characteristic of the fragmentation of the Hilbert space in sectors that conserve dipole moments. The flexible programmability of our quantum emulator highlights its potential in helping the understanding of nontrivial quantum many-body problems, in direct complement to simulations in classical computers.
The failure of quantum thermalization due to Many-Body Localization (MBL) has evolved from a theoretical concept in spin chains to an experimental reality in synthetic quantum platforms, most notably superconducting circuits based on transmon qubits. Despite its significance, the MBL transition has been studied primarily under purely random disorder and local couplings, leaving more complex and realistic configurations largely unexplored. Here, we investigate the quantum dynamics of transmon networks subject to the unavoidable competing effects of correlated disorder and network-mediated non-local interactions. First, we show how to engineer the physical parameters of the quantum hardware to systematically control the correlated disorder patterns emerging in the system. Then, we demonstrate that the MBL phase transition is robust against such correlations, which is essential for tuning localization properties in realistic transmon devices. This robustness is established through the analysis of the block entanglement entropy variance across disorder realizations, which precisely locates the MBL critical point. Independently, we introduce a local memory parameter, whose dynamics at long evolution times reveals memory retention in the localized phase and yields a critical point consistent with the entropy analysis. These results provide a framework for understanding localization in complex quantum network topologies, with potential implications for multi-qubit processor design.
Recent numerical and experimental works have revealed a disorder-free many-body localization (MBL) in an interacting system subjected to a linear potential, known as the Stark MBL. The conventional MBL, induced by disorder, has been widely studied by using quantum simulations based on superconducting circuits. Here, we consider the Stark MBL in two types of superconducting circuits, i.e., the one-dimensional array of superconducting qubits, and the circuit where nonlocal interactions between qubits are mediated by a resonator bus. We calculate the entanglement entropy and participate entropy of the highly excited eigenstates, and study the nonequilibrium properties of the Stark MBL. In particular, we observe an anomalous relaxation of the imbalance, dominated by the power-law decay ${t}^{\ensuremath{-}\ensuremath{\xi}}$. The exponent $\ensuremath{\xi}$ satisfies $\ensuremath{\xi}\ensuremath{\propto}|\ensuremath{\gamma}\ensuremath{-}{\ensuremath{\gamma}}_{c}{|}^{\ensuremath{\nu}}$ when $\ensuremath{\gamma}<{\ensuremath{\gamma}}_{c}$, and vanishes for $\ensuremath{\gamma}\ensuremath{\ge}{\ensuremath{\gamma}}_{c}$, which can be employed to estimate the ${\ensuremath{\gamma}}_{c}$. Our work indicates that superconducting circuits are a promising platform for investigating the critical properties of the Stark MBL transition.