Friday Quantum Seminar

Abstract: Coherent interconnects between gate-defined silicon quantum processors are essential for scalable computation and long-range entanglement. We demonstrate that one-dimensional electron channels in a Si/SiGe quantum well, formed by a resistive topgate, exhibit strong Coulomb interactions, realizing Luttinger liquid physics. At low electron densities, these electrons overcome their kinetic energy to form a one-dimensional Wigner crystal—characterized by dominant 4K_F correlations.

The title and abstract for this talk are forthcoming.

Pizza and drinks will be served after the seminar in ATL 2117.

Topological photonics has emerged as a powerful platform to explore topological physics and design photonic devices that remain robust in the presence of disorder. Early advances centered on photonic crystals and coupled microresonator arrays, but these studies were largely confined to the linear regime. In parallel, microresonator frequency comb technology has advanced dramatically over the past decade, enabling strong on-chip optical nonlinearities and compact frequency comb generation.

Abstract: Gaussian boson sampling (GBS) is a promising protocol for demonstrating quantum computational advantage. One of the key steps for proving classical hardness of GBS is the so-called ``hiding conjecture'', which asserts that one can ``hide'' a complex Gaussian matrix as a submatrix of the outer product of Haar unitary submatrices in total variation distance.

Abstract: The quantum Fisher information (QFI) sets a fundamental limit on how precisely we can measure physical quantities. It plays a role in everything from gravitational wave detection to emerging technologies like quantum lidar. A natural question is: can we always design measurements that actually reach this limit? For single parameters, yes — but for multiple parameters, the story is much trickier. In fact, finding general conditions for when the QFI bound is achievable remains an open problem.

Coherent interconnects between gate-defined silicon quantum processors are essential for scalable computation and long-range entanglement. We demonstrate that one-dimensional electron channels in a Si/SiGe quantum well, formed by a resistive topgate, exhibit strong Coulomb interactions, realizing Luttinger liquid physics. At low electron densities, these electrons overcome their kinetic energy to form a one-dimensional Wigner crystal—characterized by dominant 4K_F correlations.

Device error rates on current quantum computers have improved enough to where demonstrations of error correction below break even are now possible. Still, the circuits required for quantum error correction introduce significant overhead and sometimes inject more errors than they correct. In this work, we introduce adaptive syndrome extraction as a scheme to improve code performance and reduce the quantum error-correction cycle time by measuring only the stabilizer generators that are likely to provide useful syndrome information.

The quantum Fisher information (QFI) sets a fundamental limit on how precisely we can measure physical quantities. It plays a role in everything from gravitational wave detection to emerging technologies like quantum lidar. A natural question is: can we always design measurements that actually reach this limit? For single parameters, yes — but for multiple parameters, the story is much trickier. In fact, finding general conditions for when the QFI bound is achievable remains an open problem.

Gaussian boson sampling (GBS) is a promising protocol for demonstrating quantum computational advantage. One of the key steps for proving classical hardness of GBS is the so-called ``hiding conjecture'', which asserts that one can ``hide'' a complex Gaussian matrix as a submatrix of the outer product of Haar unitary submatrices in total variation distance. In this talk, we will discuss the proof of the hiding conjecture for input states with the maximal number of squeezed states, which is a setup that has recently been realized experimentally [Madsen et al., Nature 606, 75 (2022)].

Abstract: The thermodynamic limit is foundational to statistical mechanics, underlying our understanding of many-body phases. It assumes that, as the system size grows infinitely at fixed density of particles, unambiguous macroscopic phases emerge that are independent of the system's boundary shape. We present explicit quantum spin and dimer Hamiltonians whose ground states violate this principle.