JQI-QuICS Special Seminar

We analyse the entanglement structure of states generated by random constant-depth two-dimensional quantum circuits, followed by projective measurements of a subset of sites. By deriving a rigorous lower bound on the average entanglement entropy of such post-measurement states, we prove that macroscopic long-ranged entanglement is generated above some constant critical depth in several natural classes of circuit architectures, which include brickwork circuits and random holographic tensor networks.

To solve problems of practical importance, quantum computers will likely need to incorporate quantum error correction, where a logical qubit is redundantly encoded in many noisy physical qubits. The large physical-qubit overhead typically associated with error correction motivates the search for more hardware-efficient approaches. To this end, in this talk I will describe our recent superconducting circuit experiment realizing a logical qubit memory via the concatenation of encoded bosonic cat qubits with an outer repetition code.

Quantum ergodicity refers to the remarkable ability of quantum systems to explore their entire state space allowed by symmetry. Mechanisms for violating ergodicity are of fundamental interest in statistical and molecular physics and can offer novel insights into decoherence phenomena in complex molecular qubits.  I will discuss the recent experimental observation of ergodicity breaking in rapidly rotating C60 fullerene molecules as a function of rotational angular momentum [1].

The dynamics of quantum many-body systems out-of-equilibrium are pivotal in various fields, ranging from quantum information and the theory of thermalization to impurity physics. Fundamentally, the numerical study of larger quantum systems is challenging due to the exponential number of parameters necessary to describe the wavefunction. If their entanglement is low, wavefunctions can be approximated with relatively few parameters using tensor networks. Since equilibrium wavefunctions have low entanglement, this makes computations viable.

We propose a new sub-Doppler cooling scheme in trapped ion crystals in Paul traps which utilizes a Sisyphus-like cooling mechanism to simultaneously cool all the motional modes of the crystal. We use a hollow tweezer, tuned near resonance with the transition from the qubit manifold to a short-lived excited manifold, to generate a state-dependent tweezer potential. This tweezer also introduces a position dependent quench rate for the qubit states.

Learning properties of unknown quantum systems or processes is of fundamental importance to the development of quantum technologies. While many learning algorithms require access to external ancillary qubits (referred to as quantum memory), the statistical complexity and experimental costs for these algorithms vary considerably due to different sizes of quantum memory. Here, we investigate the transitions for statistical complexity required for learning quantum data with bounded quantum memory.

The search for exotic quantum phases of matter is a central theme in condensed matter physics. The advent of programmable quantum hardware provides an unprecedented access to novel quantum states and represents a new avenue for probing the exotic properties associated with topological order.. In this talk, I will discuss our progress in realization of topologically ordered ground states based on exact efficient quantum circuit representations.

Neutral atoms have emerged as a competitive platform for digital quantum simulations and computing. In this talk, we discuss recent results on the design of time-optimal and robust multi-qubit gates for neutral atoms. We present a family of Rydberg blockade gates that are robust against two common experimental imperfections -- intensity inhomogeneity and Doppler shifts – and demonstrate that these gates outperform existing gates for moderate or large imperfections.

The postulates of quantum mechanics generalize classical probability distributions and thus transmission of information, enabling fundamentally novel protocols for communication and cryptography. These algorithms motivate the deployment of quantum networks, a distributed model of computation where universality and fault-tolerance are often not required. Based on constructions from communication complexity, we design a voting scheme with efficient scaling of quantum communication and computation, and prove its security.

The spin-1/2 nearest-neighbor XXZ model on the pyrochlore lattice is an iconic frustrated three-dimensional spin system with a rich phase diagram on the $\lambda$ axis, where $\lambda$ is the XXZ interaction anisotropy.