More Like this

1. Gapless superconductivity from low-frequency electrodynamic response of two-dimensional granular composites

   https://doi.org/10.1103/PhysRevResearch.7.013183  

   Zhang, Xinyang; Wu, Jinze; Palevski, Alexander; Kapitulnik, Aharon (February 2025, Physical Review Research)

   We measured the full complex ac conductance of two-dimensional granular $\mathrm{In}/{\mathrm{InO}}_x$composites using the mutual inductance technique to explore the transition from a “failed superconductor turned anomalous metal” to a robust superconductor. In this system, room-temperature annealing was adopted to tune the ${\mathrm{InO}}_x$-mediated coupling between In grains, allowing for the observation of both a “true” superconductor-to-insulator transition and the emergence of an intervening anomalous metallic state. In this paper, we show that further annealing increases the intergrain coupling, eliminating the anomalous metallic phase but at the same time preventing the emergence of strong Bose-dominated insulating phase. The complex ac conductance revealed a $T\to 0$finite dissipative response in a finite magnetic field, coexisting with a robust superfluid density. The anomalous power-law spectra for the dissipative response suggest quantum critical behavior as probed in the kilohertz range, and point to signatures of gapless superconductivity in our granular superconducting system. Published by the American Physical Society2025 

   more » « less
2. Classical algorithms, correlation decay, and complex zeros of partition functions of quantum many-body systems

   https://doi.org/10.1145/3357713.3384322  

   Harrow, Aram W.; Mehraban, Saeed; Soleimanifar, Mehdi (June 2020, STOC 2020: Proceedings of the 52nd Annual ACM SIGACT Symposium on Theory of Computing)

   null (Ed.)

   We present a quasi-polynomial time classical algorithm that estimates the partition function of quantum many-body systems at temperatures above the thermal phase transition point. It is known that in the worst case, the same problem is NP-hard below this point. Together with our work, this shows that the transition in the phase of a quantum system is also accompanied by a transition in the hardness of approximation. We also show that in a system of n particles above the phase transition point, the correlation between two observables whose distance is at least Ω(logn) decays exponentially. We can improve the factor of logn to a constant when the Hamiltonian has commuting terms or is on a 1D chain. The key to our results is a characterization of the phase transition and the critical behavior of the system in terms of the complex zeros of the partition function. Our work extends a seminal work of Dobrushin and Shlosman on the equivalence between the decay of correlations and the analyticity of the free energy in classical spin models. On the algorithmic side, our result extends the scope of a recent approach due to Barvinok for solving classical counting problems to quantum many-body systems. 

   more » « less
3. Exact results of the one-dimensional repulsive Hubbard model

   https://doi.org/10.1088/1361-6633/ad7b70  

   Luo, Jia-Jia; Pu, Han; Guan, Xi-Wen (October 2024, Reports on Progress in Physics)

   Abstract We present analytical results of the fundamental properties of the one-dimensional (1D) Hubbard model with a repulsive interaction. The new model results with arbitrary external fields include: (I) using the exact solutions of the Bethe ansatz equations of the Hubbard model, we first rigorously calculate the gapless spin and charge excitations, exhibiting exotic features of fractionalized spinons and holons. We then investigate the gapped excitations in terms of the spin string and the $k-\mathrm{\Lambda }$string bound states at arbitrary driving fields, showing subtle differences in spin magnons and charge $\eta$-pair excitations. (II) For a high-density and high spin magnetization region, i.e. near the quadruple critical point, we further analytically obtain the thermodynamic properties, dimensionless ratios and scaling functions near quantum phase transitions. (III) Importantly, we give the general scaling functions at quantum criticality for arbitrary filling and interaction strength. These can directly apply to other integrable models. (IV) Based on the fractional excitations and the scaling laws, the spin-incoherent Luttinger liquid (SILL) with only the charge propagation mode is elucidated by the asymptotic of the two-point correlation functions with the help of conformal field theory. We also, for the first time, obtain the analytical results of the thermodynamics for the SILL. (V) Finally, to capture deeper insights into the Mott insulator and interaction-driven criticality, we further study the double occupancy and propose its associated contact and contact susceptibilities, through which an adiabatic cooling scheme based upon quantum criticality is proposed. In this scenario, we build up general relations among arbitrary external- and internal-potential-driven quantum phase transitions, providing a comprehensive understanding of quantum criticality. Our methods offer rich perspectives of quantum integrability and offer promising guidance for future experiments with interacting electrons and ultracold atoms, both with and without a lattice. 

   more » « less
4. Concomitant Entanglement and Control Criticality Driven by Collective Measurements

   https://doi.org/10.1103/PRXQuantum.6.010351  

   Iadecola, Thomas; Wilson, Justin H; Pixley, JH (March 2025, PRX Quantum)

   Adaptive quantum circuits—where a quantum many-body state is controlled using measurements and conditional unitary operations—are a powerful paradigm for state preparation and quantum error-correction tasks. They can support two types of nonequilibrium quantum phase transitions: measurement-induced transitions between volume- and area-law-entangled steady states and control-induced transitions where the system falls into an absorbing state or, more generally, an orbit visiting several absorbing states. Within this context, nonlocal conditional operations can alter the critical properties of the two transitions and the topology of the phase diagram. Here, we consider the scenario where the measurements are , in order to engineer efficient control onto dynamical trajectories. Motivated by Rydberg-atom arrays, we consider a locally constrained model with global sublattice magnetization measurements and local correction operations to steer the system’s dynamics onto a many-body orbit with finite recurrence time. The model has a well-defined classical limit, which we leverage to aid our analysis of the control transition. As a function of the density of local correction operations, we find control and entanglement transitions with continuously varying critical exponents. For sufficiently high densities of local correction operations, we find that both transitions acquire a dynamical critical exponent $z<1$, reminiscent of criticality in long-range power-law interacting systems. At low correction densities, we find that the criticality reverts to a short-range nature with $z\gtrsim 1$. In the long-range regime, the control and entanglement transitions are indistinguishable to within the resolution of our finite-size numerics, while in the short-range regime we find evidence that the transitions become distinct. We conjecture that the effective long-range criticality mediated by collective measurements is essential in driving the two transitions together. Published by the American Physical Society2025 

   more » « less
5. Nonlinear and non-CP gates for Bloch vector amplification

   https://doi.org/10.1088/1572-9494/acf304  

   Geller, Michael R (September 2023, Communications in Theoretical Physics)

   Abstract Any stater= (x,y,z) of a qubit, written in the Pauli basis and initialized in the pure stater= (0, 0, 1), can be prepared by composing three quantum operations: two unitary rotation gates to reach a pure state $\mathit{r}={\left(x^2+y^2+z^2\right)}^{-\frac{1}{2}}\times (x,y,z)$on the Bloch sphere, followed by a depolarization gate to decrease ∣r∣. Here we discuss the complementary state-preparation protocol for qubits initialized at the center of the Bloch ball,r=0, based on increasing or amplifying ∣r∣ to its desired value, then rotating. Bloch vector amplification increases purity and decreases entropy. Amplification can be achieved with a linear Markovian completely positive trace-preserving (CPTP) channel by placing the channel’s fixed point away fromr=0, making it nonunital, but the resulting gate suffers from a critical slowing down as that fixed point is approached. Here we consider alternative designs based on linear and nonlinear Markovian PTP channels, which offer benefits relative to linear CPTP channels, namely fast Bloch vector amplification without deceleration. These gates simulate a reversal of the thermodynamic arrow of time for the qubit and would provide striking experimental demonstrations of non-CP dynamics. 

   more » « less