Friday Quantum Seminar

Abstract: We introduce parallel-sequential (PS) circuits, a family of quantum circuits characterized by a tunable degree of entanglement and maximum correlation length, which interpolates between brickwall and sequential circuits. We provide evidence that on noisy devices, properly chosen PS circuits suppress error proliferation and exhibit superior trainability and evaluation accuracy when employed as variational circuits, thus outperforming brickwall, sequential, and log-depth circuits in [Malz*, Styliaris*, Wei*, Cirac, PRL 2024] across most parameter regimes.

Quantum sensing extends the vast benefits of a quantum advantage to traditional metrology.  A common method of quantum sensing utilizes coherent, crystal defects in semi-conductors (such as nitrogen vacancy centers in diamond) to perform high-precision measurements on a variety of length scales.  Such measurements might span from vectorized magnetometry of macroscopic computer chips to nanoscale strain or temperature mapping in a target matrial.  In exploring new regimes for quantum sensing, we need to model and assess their viability through theoretical or simula

Arrays of gate-defined semiconductor quantum dots are among the leading candidates for building scalable quantum processors.  High-fidelity initialization, control, and readout of spin qubit registers require exquisite and targeted control over key Hamiltonian parameters that define the electrostatic environment.  However, due to the tight gate pitch, capacitive crosstalk between gates hinders independent tuning of chemical potentials and interdot couplings.  While virtual gates offer a practical solution, determining all the required cross-capacitance matrices accurate

 A Landau level (which is a flat band) forms only when a magnetic flux with non-zero total flux threads a system. In fact the degeneracy at the flat band is proportional to the flux. So no flat band can form when the magnetic flux averages to zero. We will discuss this and then show otherwise. This is relevant to time reversal symmetric systems that form flat bands such as magic-angle twisted bilayer graphene. In this talk the magic behind those systems will be revealed through the simplest model that gives rise to magical behaviour.

To implement arbitrary quantum interactions in architectures with restricted topologies, one may simulate all-to-all connectivity by routing quantum information. Therefore, it is of natural interest to find optimal protocols and lower bounds for routing. We consider a connectivity graph, G, of 2 regions connected only through an intermediate region of a small number of qubits that form a vertex bottleneck. Existing results only imply a trivial lower bound on the entangling rate and routing time across a vertex bottleneck.

Fault tolerance is a notion of fundamental importance to the field of quantum information processing. It is one of the central properties a quantum computer must possess in order to enable the achievement of large scale practical quantum computation. While a widely used, general, and intuitive concept, within the literature the term fault tolerant is often applied to specific procedures in an ad-hoc fashion tailored to details of the context or platform under discussion.

Long-range entangled states of matter encompass a variety of exotic quantum phenomena, ranging from topological orders to quantum criticality. In this talk, I will discuss recent advances in leveraging mid-circuit measurements and unitary feedback to efficiently generate these entangled many-body states.

We introduce the “hidden cut problem:” given as input which is product across an unknown bipartition, the goal is to learn precisely where the state is unentangled, i.e. to find the hidden cut. We give a polynomial time quantum algorithm for the hidden cut problem, which consumes O(n/ε^2) many copies of the state, and show that this asymptotic is optimal. In the special case of Haar-random states, the circuits involved are of merely constant depth, which could prove relevant to experimental implementations.

Quantum sensing leverages the principles of quantum mechanics to provide ``quantum-enhanced'' measurement sensitivity, thereby amplifying our ability to observe interesting physical phenomena. It employs a rich arsenal of techniques, including squeezing, photon counting, entanglement assistance, and distributed quantum sensing to achieve unprecedented sensitivity.