Program with Abstracts > Tuesday AM

Individual solid-state nuclear spin qubits with coherence exceeding seconds

James O'Sullivan  (CEA)   —  Tue 10:00-10:30

I will present a new platform for quantum information processing, consisting of ¹⁸³W nuclear spin qubits adjacent to an Er³⁺ impurity in a CaWO₄ crystal, interfaced via a superconducting resonator and detected using a microwave photon counter at 10 mK. We study two nuclear spin qubits with 𝑇₂* of 0.8(2) s and 1.2(3) s, 𝑇₂ of 3.4(4) s and 4.4(6) s, respectively. We demonstrate single-shot quantum non-demolition readout of each nuclear spin qubit using the Er³⁺ spin as an ancilla. We introduce a new scheme for all-microwave single- and two-qubit gates, based on stimulated Raman driving of the coupled electron-nuclear spin system. We realize single- and two-qubit gates on a timescale of a few milliseconds, and prepare a decoherence-protected Bell state with 88% fidelity and 𝑇₂* of 1.7(2) s. I will also present recent results investigating a spin-9/2 niobium nucleus in the same CaWO₄ crystal, controlled and read out via an Er ancilla.

Quantum cellular automata: structure & quantum simulation of QED 

Pablo Arrighi  (INRIA Saclay)     —   Tue 11:00-11:30

Quantum cellular automata (QCA) consist in arrays of identical finite-dimensional quantum systems, evolving in discrete-time steps by iterating a finite-depth, translation-invariant quantum circuit. I will give an overview of their mathematical structure, showing that all non-signalling unitary operators in discrete space are of that form. I will then showcase what quantum cellular automata can achieve in terms of reformulation and quantum simulation of quantum field theories. In particular, I will explain the core ideas behind QCA-based quantum algorithms for the quantum simulation of the Dirac Equation, the Dirac Equations in Curved spacetime, Fermions as qubits even in 2D and 3D, and quantum electrodynamics.

Auxiliary-assisted cooling of many-body systems via a stochastic mechanism

George Mouloudakis et al.  (FU Berlin)    — Tue 11:30-12:00

Many important optimization tasks can be mapped to the search of ground states of effective Hamiltonians, allowing for the solution of computationally hard problems in physics, finance, logistics and other fields. In quantum annealing, an initial easy-to-prepare Hamiltonian, cooled down to its ground state, gets gradually changed towards a more complex one, whose ground state represents the solution to a specific optimization problem. Despite its evident success, quantum annealing faces the challenge of diabatic transitions that tend to excite the systems towards energetically higher states, ruining the performance of adiabatic protocols. In this work we propose a new mechanism to cool down diabatic transitions in many-body chains by coupling one of their ends to an auxiliary qubit that is frequently being reset to its ground state. In contrast to previously existing protocols that rely on knowledge of the time-dependent spectrum, our protocol relies on stochastic choices of the auxiliary qubit’s energy at each reset. Using our protocol, we demonstrate the ability to effectively cool static chains down to their ground states, independent of their initial state or size. Our results are also generalized to time-dependent Hamiltonians, showing the efficiency of our protocol in improving the performance of several quantum annealing tasks.

Roundtable  featuring session speakers, Paris quantum company C12   —  Tue  12:00-12:30