QFarm

Photons play a central role in many areas of quantum information science, either as qubit themselves or to mediate interactions between long-lived matter based qubits. Techniques for (1) high-fidelity generation, (2) precise manipulation and (3) ultra-efficient detection of quantum states of light are therefore a prerequisite for virtually all quantum technologies. A quantum photonic processor is the union of these three core technologies into a single system, and, bolstered by advances in integrated photonics, promises to be a versatile platform for quantum information science and beyond. In this talk I present recent progress towards large-scale quantum photonic processors and demonstrate how such systems enable new applications at the nexus of quantum mechanics and machine learning.

I will review the most recent advances in the theoretical methods of quantum metrology, focusing on the quantum information related concepts such as quantum error-correction and matrix product states formalism. The aim of the talk is to show how the two fields, quantum metrology and quantum information, are closely connected and how they can benefit from each other. The talk will be based on three papers:

[1] R. Demkowicz-Dobrzanski, J. Czajkowski, P. Sekatski, Adaptive quantum metrology under general Markovian noise, Phys. Rev. X 7, 041009 (2017)

[2] K. Chabuda, J. Dziarmaga, T. J. Osborne, R.Demkowicz-Dobrzanski, Tensor-Network Approach for Quantum Metrology in Many-Body Quantum Systems, Nat. Commun. 11, 250 (2020)

[3] A. Kubica, R. Demkowicz-Dobrzanski, Using Quantum Metrological Bounds in Quantum Error Correction: A Simple Proof of the Approximate Eastin-Knill Theorem, arXiv:2004.11893 (2020)

Password via qfarm-contact@stanford.edu

Despite their many applications over the last decades, entangled and single-photon sources from probabilistic sources such as spontaneous parametric downconversion have limited performance due to the presence of unwanted multi-pair events. For example, it is not possible to produce a heralded single photon with a probability exceeding 25%. However, by incorporating low-loss multiplexing (in our case, time-bin multiplexing), one can greatly enhance device performance. We will review our recent work in this area, and speculate about the future.

With the advances of quantum simulators in implementing various quantum many-body states, it is important to find efficient ways to characterize and measure many-body states, without resorting to full quantum state tomography. Specifically, in contrast to electronic materials, where the measurements are mainly within the linear-response paradigm, quantum simulators offer unique access to the full wave function that inspires novel probing approaches. In this talk, I discuss how various quantities, such as entanglement spectrum, symmetry-protected topological invariants, and fractional many-body Chern number could be extracted. In the latter case, we show how such an invariant can be measured, using a single wave function, without the knowledge of the Hamiltonian. This should be contrasted to the conventional way, where on requires a family of many-body wave functions parameterized by twist angles in order to calculate the Berry curvature.

Science Advance 6, 3666 (2020)
arxiv 2005.13543
arxiv 2005.13677

Bosonic modes are widely used for quantum communication and information processing. Recent developments in superconducting circuits enable us to control bosonic microwave cavity modes and implement arbitrary operations allowed by quantum mechanics, such as quantum error correction against excitation loss errors. We investigate various bosonic codes, error correction schemes, and potential applications.

In linear optical quantum computing qubits do not fundamentally interact, and yet via measurement complex entanglement can be constructed to implement quantum error corrected computation via topological codes. As a hardware platform for quantum computation linear optics offers unique flexibility in the options for building up topological error correcting schemes. Some interesting examples are the long range connectivity which is straightforward in a photonic architecture, and the ability to move qubits around in temporal as well as spatial dimensions. I will give an overview of quantum computing with silicon photonics and demonstrate how these physical features of the photonic approach can inspire novel schemes for fault tolerance.

In this talk, I explore two examples of how the symmetry of a microscopic lattice or topology of an underlying non-interacting band structure can be imprinted in the excitations of strongly-correlated insulators. First, I show that previously-unnoticed crystal symmetry constraints drastically alter the understanding of Ising quantum criticality in the quasi-1D magnetic insulator CoNb2O6, resolving decade-old puzzles related to the dispersion of confined 'kinks' in the ordered phase and the decay of spin-flip quasiparticles in the paramagnetic phase [1]. Turning next to the correlated quantum anomalous Hall insulator recently observed in twisted bilayer graphene near the magic angle, I show that this state supports stable, weakly-dispersing flat bands of neutral bosonic excitons with substantial Berry curvature [2], and argue that these can under certain conditions form a new hierarchy of bosonic fractional quantum Hall liquids [3].

References:
[1] M. Fava, R. Coldea, and S.A. Parameswaran, arXiv:2004.04169 (2020).
[2] Y.H. Kwan, Y. Hu, S.H. Simon, and S.A. Parameswaran, arXiv:2003.11560 (2020).
[3] Y.H. Kwan, Y. Hu, S.H. Simon, and S.A. Parameswaran, arXiv:2003.11559 (2020).

I first introduce the concept of void formation in Heisenberg evolution of operators and provide support that it can be used to distinguish chaotic and integrable systems.

After a brief discussion of applications of void formation to entanglement growths and generation of multi-partite entanglement, I will explain how it can provide insights into evolution of black holes by considering some simple quantum mechanical toy models of black hole evaporation.

Abstract TBA - speaker and event details are subject to change.

Starting with a lattice system at short distances, its long-distance behavior is captured by a continuum Quantum Field Theory (QFT). This description is universal, i.e. it is independent of most of the details of the microscopic system. Surprisingly, certain recently discovered lattice systems, and in particular models of fractons, seem to violate this general dogma. Motivated by this apparent contradiction, we will present exotic continuum QFTs that describe these systems.