Physikalisches Kolloquium: Prof. Dr. Robert König – Quantum fault-tolerance with noisy circuits and entanglement generation

Titel: Quantum fault-tolerance with noisy circuits and entanglement generation

Abstract:
A fundamental question in quantum information science is: what can noisy quantum circuits actually do? Understanding the computational and communication power of realistic, imperfect quantum hardware is essential for both near-term applications and the long-term development of quantum technologies. A paradigmatic problem in this context is long-range entanglement generation, i.e., creating high-quality entangled states between distant qubits using only noisy local operations. This task is central to quantum networking and distributed quantum computing, where shared entanglement across large distances serves as the key resource.
We study this problem on a rectangular grid of qubits in 2D, subject to local stochastic Pauli noise. This is a setup that, depending on the length scale, describes both a single quantum device with geometrically limited connectivity and a planar network of constant-sized quantum stations. We give a protocol that, for noise below a constant threshold, generates a constant-fidelity Bell pair between qubits separated by an arbitrarily large distance R. The required grid dimensions scale as Θ(R) × Θ(poly(log R)), and the protocol runs in constant time (single-shot), producing a Bell state up to a known Pauli correction. This contrasts with existing approaches, which either require local devices whose size grows with the target distance, or need a distance-dependent number of rounds.
Our work provides the first example of a short-range entangled state in 2D from which long-range Bell pairs can be reliably extracted despite noise, as well as a 2D-local stabilizer Hamiltonian whose thermal states retain this property at constant positive temperature.

The talk is based on joint work with Dylan Harley, arXiv:2604.05870.

Sprecher / Speaker: Prof. Dr. Robert König, TU München
Kontakt / Contact: Prof. Dr. Daniel Burgarth