Session B28: Hybrid Quantum Systems

Quantum sensing of magnons with a superconducting qubit

Magnons are quanta of collective spin excitations, such as in magnetostatic modes and spin waves, ubiquitously found in magnetic materials. While conventional experimental techniques in magnetic resonance and spintronics only deal with large amplitude signals, the recent progress in quantum magnonics has enabled us to investigate the single magnon limit [1]. In this talk, we demonstrate how to control and detect single magnons in a macroscopic-scale ferromagnetic crystal. We strongly couple a superconducting transmon qubit, an artificial two-level system realized in a superconducting circuit, to a magnon in a millimeter-sized single-crystalline sphere of yttrium-gallium garnet (YIG) via a virtual excitation of a photon in a copper microwave cavity. In the strong dispersive regime, the interaction results in a magnon-number-resolved qubit spectrum, through which we can generate entanglement between the qubit and the magnon mode by applying a qubit π-rotation conditioned on the magnon number. Subsequent readout of the qubit leads to the detection of a magnon [2]. In another scheme, we apply the so-called T₂^* relaxometry and measure the tiny steady-state time-averaged population of magnons in the YIG sphere through Ramsey interference of the qubit. Counterintuitively, the sensitivity is enhanced by the dissipation of magnons [3]. We will discuss the current limitations and possible improvements of the techniques.   

Quantum information processing at telecom wavelengths using mechanical resonators

Mechanical resonators have attracted significant attention for their potential use in quantum information processing tasks. Their unique ability to couple to a large variety of other quantum systems and the freedom to design their properties makes them ideally suited for tasks such as for compact quantum memories or as transducers between the microwave and optical domains. Their massive nature also makes them interesting candidates to study quantum physics on a new scale.

Here, we would like to discuss several experiments demonstrating non-classical behavior of mechanical motion by coupling a micro-fabricated acoustic resonator to single optical photons. Our approach is based on optomechanical crystals, with mechanical resonances in the Gigahertz regime that can be addressed optically from the conventional telecom band. In our measurements we show how these structures can be used as a mechanical quantum memory for photons and for quantum repeater tasks through the realization of a quantum teleportation protocol.   

Nonreciprocity beyond quantum information processing

The concept of dissipation engineering has enriched the methods available for state preparation, dissipative quantum computing and quantum information processing. Combining such engineered dissipative processes with coherent dynamics allows for new effects to emerge. For example, we found that any factorisable (coherent) Hamiltonian interaction can be rendered nonreciprocal if balanced with the corresponding dissipative interaction. This powerful concept can be exploited to engineer nonreciprocal devices for quantum information processing, computation and communication protocols, e.g., to achieve control over the direction of propagation of photonic signals. However, although nonreciprocal concepts are realizable in quantum architectures, nonreciprocity itself holds up to the classical level and is not inherently quantum; the fundamental aspects of nonreciprocity in the quantum regime have yet to be fully investigated. In this talk we will address these aspects and discuss routes of how nonreciprocal concepts can find application beyond quantum information processing.    

Entangling remote quantum repeater nodes

The distribution of entanglement between the nodes of a quantum network will allow new advances e.g. in long distance quantum communication, distributed quantum computing and quantum sensing. On the ground, quantum information can be distributed across the nodes using single photons at telecommunication wavelengths traveling in optical fibers. To bridge distances much longer than the fiber attenuation length, quantum repeaters can be used. The nodes of a quantum repeater are matter systems that should efficiently interact with quantum light, allow entanglement with photons (ideally at telecommunication wavelengths) and serve as a quantum memory allowing long-lived and faithful storage of (entangled) quantum bits. In addition, for efficient distribution of entanglement, the nodes should allow multiplexed operation and ideally enable quantum processing capabilities between stored qubits.

In this talk, after introducing the context I will describe our recent progress towards the realization of quantum repeater nodes with multiplexed ensemble-based quantum memories, using cryogenically cooled rare-earth ion doped solids. I will show how solid-state quantum memories can be entangled with telecom photons using non-degenerate photon pair sources.  I will also describe our efforts to scale up quantum repeater links. In particular, I will report a recent experiment demonstrating entanglement between remote multiplexed solid-state quantum memories, heralded by a telecom photon. Finally, I will explain our current work to build quantum processing nodes using e.g. single rare-earth ions in microcavities or ensembles of cold Rydberg atoms.