QuICS Special Seminar

Huang, Kueng, Preskill introduced the learning task now known as “classical shadows”: given few copies of an unknown state ρ, construct a classical description of the state from independent measurements that can be used to predict certain properties of the state. Specifically, they show Θ(B/epsilon^2) samples of ρ suffice to approximate the expectation value Tr(Oρ) of any Hermitian observable O to within additive error epsilon provided Tr(O^2) ≤ B and the eigenvalues of O are contained in [-1,1].

The ability to perform fast and robust operations on multi-qubit quantum systems is a necessity for realizing reliable quantum computation. Unfortunately, the inevitable interaction between a quantum system and its environment presents an obstacle for achieving such operations. Despite this challenge, when used in tandem, quantum noise characterization and quantum control provide a means for engineering targeted control protocols that achieve noise-robust quantum logic operations informed by knowledge of the underlying noise properties.

In this talk, I will focus on topological order and error correction on fractal geometries.  Firstly, I will present a no-go theorem that Z_N topological order cannot survive on any fractal embedded in two spatial dimensions and then show that for fractal lattice models embedded in 3D or higher spatial dimensions, Z_N topological order survives if the boundaries on the holes condense only loop or membrane excitations.  Next, I will discuss fault-tolerant logical gates in the Z_2 version of these fractal models, which we name as fractal surface codes, using their c

Nonequilibrium fluctuation theorems provide a correspondence between properties of quantum systems in thermal equilibrium and work distributions arising in nonequilibrium processes. Building upon these theorems, we present a quantum algorithm to prepare a purification of the thermal state of a quantum system of interest.

Quantum technology has the potential to revolutionize how we acquire and process experimental data to learn about the physical world. An experimental setup that transduces data from a physical system to a stable quantum memory, and processes that data using a quantum computer, could have significant advantages over conventional experiments in which the physical system is measured and the outcomes are processed using a classical computer.

Current semiconductor-based quantum computing approaches rely upon achieving control of nanocircuits at the single-electron level and using them as quantum bits (qubits). Establishing a stable configuration of spins in quantum dot (QD) devices is accomplished by a combination of electrostatic confinement, bandgap engineering, and dynamical control via nearby electrical gates. However, with an increasing number of QD qubits, the relevant parameter space grows exponentially, making heuristic control unfeasible.

Quantum circuits have been widely used as a platform to explore universal properties of generic quantum many-body systems. In this talk, I will present our work in which we construct a field theoretical approach to study quantum circuits. We reformulate the sigma model for time periodic Floquet systems using the replica method, and apply it to the study of spectral statistics of the evolution operator of quantum circuits.

Variational Quantum Algorithms (VQAs) are viewed as amongst the best hope for near-term quantum advantage. A natural question is whether noise places fundamental limitations on VQA performance. In the first part of this talk, we show that noise can severely limit the trainability of VQAs by exponentially flattening the optimization landscape and suppressing the magnitudes of cost gradients.

In this talk I will study the extension of fault tolerance techniques to holographic quantum error correcting codes in the context of the ads/cft correspondence. I will seek to argue that the threshold here corresponds to that of the confinement/de confinement phase transition here, analogously to the situation in topological quantum error correcting codes based on Tqft’s.

With the development of delegated quantum computation, clients will want to ensure confidentiality of their data and algorithms, and the integrity of their computations. In this talk, I present recent work on two directions of research related to blind and verified quantum computing.