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1. Quantum communication with itinerant surface acoustic wave phonons

   https://doi.org/10.1038/s41534-021-00511-1  

   Dumur, É.; Satzinger, K. J.; Peairs, G. A.; Chou, M. -H.; Bienfait, A.; Chang, H. -S.; Conner, C. R.; Grebel, J.; Povey, R. G.; Zhong, Y. P.; et al (December 2021, npj Quantum Information)

   Abstract Surface acoustic waves are commonly used in classical electronics applications, and their use in quantum systems is beginning to be explored, as evidenced by recent experiments using acoustic Fabry–Pérot resonators. Here we explore their use for quantum communication, where we demonstrate a single-phonon surface acoustic wave transmission line, which links two physically separated qubit nodes. Each node comprises a microwave phonon transducer, an externally controlled superconducting variable coupler, and a superconducting qubit. Using this system, precisely shaped individual itinerant phonons are used to coherently transfer quantum information between the two physically distinct quantum nodes, enabling the high-fidelity node-to-node transfer of quantum states as well as the generation of a two-node Bell state. We further explore the dispersive interactions between an itinerant phonon emitted from one node and interacting with the superconducting qubit in the remote node. The observed interactions between the phonon and the remote qubit promise future quantum-optics-style experiments with itinerant phonons. 

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2. Dispersive coupling between MoSe ₂ and an integrated zero-dimensional nanocavity

   https://doi.org/10.1364/OME.443536  

   Rosser, David; Gerace, Dario; Chen, Yueyang; Liu, Yifan; Whitehead, James; Ryou, Albert; Andreani, Lucio_C; Majumdar, Arka (December 2021, Optical Materials Express)

   Establishing a coherent interaction between a material resonance and an optical cavity is a necessary first step to study semiconductor quantum optics. Here we report on the signature of a coherent interaction between a two-dimensional excitonic transition in monolayer MoSe₂and a zero-dimensional, ultra-low mode volume (V_m ∼ 2(λ/n)³) on-chip photonic crystal nanocavity. This coherent interaction manifests as a dispersive shift of the cavity transmission spectrum, when the exciton-cavity detuning is decreased via temperature tuning. The exciton-cavity coupling is estimated to be ≈6.5 meV, with a cooperativity of ≈4.0 at 80 K, showing our material system is on the verge of strong coupling. The small mode-volume of the resonator is instrumental in reaching the strongly nonlinear regime, while on-chip cavities will help create a scalable quantum photonic platform. 

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3. Parallel single-shot measurement and coherent control of solid-state spins below the diffraction limit

   https://doi.org/10.1126/science.abc7821  

   Chen, Songtao; Raha, Mouktik; Phenicie, Christopher M.; Ourari, Salim; Thompson, Jeff D. (October 2020, Science)

   Solid-state spin defects are a promising platform for quantum science and technology. The realization of larger-scale quantum systems with solid-state defects will require high-fidelity control over multiple defects with nanoscale separations, with strong spin-spin interactions for multi-qubit logic operations and the creation of entangled states. We demonstrate an optical frequency-domain multiplexing technique, allowing high-fidelity initialization and single-shot spin measurement of six rare-earth (Er³⁺) ions, within the subwavelength volume of a single, silicon photonic crystal cavity. We also demonstrate subwavelength control over coherent spin rotations by using an optical AC Stark shift. Our approach may be scaled to large numbers of ions with arbitrarily small separation and is a step toward realizing strongly interacting atomic defect ensembles with applications to quantum information processing and fundamental studies of many-body dynamics. 

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4. Superconducting metamaterials for waveguide quantum electrodynamics

   https://doi.org/10.1038/s41467-018-06142-z  

   Mirhosseini, Mohammad; Kim, Eunjong; Ferreira, Vinicius S.; Kalaee, Mahmoud; Sipahigil, Alp; Keller, Andrew J.; Painter, Oskar (September 2018, Nature Communications)

   Abstract Embedding tunable quantum emitters in a photonic bandgap structure enables control of dissipative and dispersive interactions between emitters and their photonic bath. Operation in the transmission band, outside the gap, allows for studying waveguide quantum electrodynamics in the slow-light regime. Alternatively, tuning the emitter into the bandgap results in finite-range emitter–emitter interactions via bound photonic states. Here, we couple a transmon qubit to a superconducting metamaterial with a deep sub-wavelength lattice constant (λ/60). The metamaterial is formed by periodically loading a transmission line with compact, low-loss, low-disorder lumped-element microwave resonators. Tuning the qubit frequency in the vicinity of a band-edge with a group index ofn_g = 450, we observe an anomalous Lamb shift of −28 MHz accompanied by a 24-fold enhancement in the qubit lifetime. In addition, we demonstrate selective enhancement and inhibition of spontaneous emission of different transmon transitions, which provide simultaneous access to short-lived radiatively damped and long-lived metastable qubit states. 

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5. Progress toward a capacitively mediated CNOT between two charge qubits in Si/SiGe

   https://doi.org/10.1038/s41534-020-00314-w  

   MacQuarrie, E. R.; Neyens, Samuel F.; Dodson, J. P.; Corrigan, J.; Thorgrimsson, Brandur; Holman, Nathan; Palma, M.; Edge, L. F.; Friesen, Mark; Coppersmith, S. N.; et al (December 2020, npj Quantum Information)

   null (Ed.)

   Abstract Fast operations, an easily tunable Hamiltonian, and a straightforward two-qubit interaction make charge qubits a useful tool for benchmarking device performance and exploring two-qubit dynamics. Here, we tune a linear chain of four Si/SiGe quantum dots to host two double dot charge qubits. Using the capacitance between the double dots to mediate a strong two-qubit interaction, we simultaneously drive coherent transitions to generate correlations between the qubits. We then sequentially pulse the qubits to drive one qubit conditionally on the state of the other. We find that a conditional π -rotation can be driven in just 74 ps with a modest fidelity demonstrating the possibility of two-qubit operations with a 13.5 GHz clockspeed. 

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