Session S01: Hybrid/Macroscopic Quantum Systems, Optomechanics, and Interfacing AMO with Solid State/Nano Systems II

Exploring synthetic quantum matter in superconducting circuits

Superconducting circuits have emerged as a competitive platform for quantum computation, satisfying the challenges of controllability, long coherence and strong interactions. Here we apply this toolbox to the exploration of strongly correlated quantum materials made of microwave photons. I will present our recent results on a new approach for preparing photonic many-body phases, where engineered dissipation is used as a resource to protect the fragile quantum states against intrinsic losses [1]. We build a strongly interacting Bose-Hubbard lattice and realize a dissipatively stabilized Mott insulator of photons. The dynamics of thermalization towards the Mott phase is probed using lattice-site- and time-resolved microscopy. In a separate experiment, we create Chern insulator lattices for microwave photons and observe topologically protected edge states [2]. I will discuss future directions to stabilize strongly correlated photonic states, study many-body dynamics in driven-dissipative settings, and engineer topological lattices to explore strongly interacting topological phases and realize robust encoding and transport of quantum information.

References: [1] R. Ma et al., A dissipatively stabilized Mott insulator of photons, Nature 566, 51–57 (2019). [2] C. Owens et al., Quarter-flux Hofstadter lattice in a qubit-compatible microwave cavity array, Phys. Rev. A 97, 013818 (2018).   

Listening to Bulk Crystalline Vibrations with Superconducting Qubits

Superconducting circuits are a versatile platform for quantum computing owing to their scalability and reconfigurability, integration with high quality microwave resonators, and demonstrated ease in interfacing with hybrid quantum platforms. Recent work has demonstrated strong coupling to long-lived phonons of bulk acoustic waves (BAWs) of pristine crystalline substrates [1]. Despite achieving single-phonon control, a superconducting qubit piezoelectrically coupled to BAWs had a significantly reduced lifetime (T₁ = 7 us) compared to a naked transmon qubit (T₁ = 40 us), suggesting that uncontrolled coupling to BAW modes may be complicit in the qubit’s decoherence.
Here, we use the lifetime of a superconducting qubit to measure the local density of acoustic states in sapphire as a proxy for electro-mechanical decoherence. Varying the crystalline geometry can be used to systematically modify the qubit’s lifetime in order to ascertain how the coupling geometry leads to a mechanical Purcell effect. Our work offers insights into the design of compact quantum memories based on BAW resonators.
[1] Y. Chu, et al. Nature 563, 666-670 (2018)   

Strong coupling of two individually controlled atoms via a nanophotonic cavity

We demonstrate photon-mediated interactions between two individually trapped atoms coupled to a nanophotonic cavity. Specifically, we observe collective enhancement when the atoms are resonant with the cavity, and level repulsion when the cavity is coupled to the atoms in the dispersive regime. Our approach makes use of individual control over the internal states of the atoms, their position with respect to the cavity mode, as well as the light shifts to tune atomic transitions individually, allowing us to directly observe the anti-crossing of the bright and dark two-atom states. These observations open the door for realizing quantum networks and studying quantum many-body physics based on
atom arrays coupled to nanophotonic devices.   

Generating multipartite entangled states with Cavity Rydberg Polaritons

Polaritons are superpositions of matter and photon states, whose effective masses are from the photonic part and their interactions originate from their matter part. While the small mass of the photons allows for observing quantum effects at higher temperatures, even up to the room temperature, the interaction allows creating collective many-body effects.
In this work, I introduce a new type of quasi-particles called cavity-Rydberg polariton, with large interaction inherited from their strongly-interacting Rydberg constituents. In particular, I will show how this dipole-dipole interaction can be utilized to induce large non-linearity between discrete modes of an optical cavity. A property that can be further utilized to generate bipartite and tripartite entangled photons.
  

Parity switching in a semiconductor-based transmon qubit

Unpaired quasiparticles can adversely affect the performance of superconducting devices, including qubits based on Majorana zero modes. We study charge parity switching in a superconductor-semiconductor nanowire-based transmon device that shows Little-Parks oscillations of its frequency as a function of magnetic field. In the recovery regime, where a single flux quantum threads the cross-section of the wire transport measurements recently revealed signatures compatible with Majorana zero modes [1].
We read out the charge parity by dispersive monitoring of a readout resonator to which the transmon qubit is coupled. At zero magnetic field, we measure parity switching times in the range of 10-100 ms. As the magnetic field is increased toward the first closing of the superconducting gap, the switching time is decreased and is consistent with the superconducting gap reduction. In the recovery regime where the gap is re-opened, the switching time is reduced below the sensitivity of our measurement, putting a bound on the minimum observable Majorana hybridization energy in a full-shell nanowire system.

[1] S. Vaitiekenas et al., Flux-induced Majorana modes in full-shell nanowires, arXiv:1809.05513 (2018)   

Suppression of charge dispersion by resonant tunneling in a single-channel transmon qubit

Motivated by the importance of understanding the underlying charge physics in superconducting qubits, we investigate the charge dispersion of a gate-controlled nanowire-based transmon. When approaching the pinch-off regime of the nanowire junction, we observe resonant behavior of the plasma frequency, which we attribute to the formation of a quantum dot in the junction. By measuring the charge dispersion while crossing a resonance, we observe that it is suppressed far below the range expected for a conventional transmon at comparable values of the Josephson and charging energies. The enhanced suppression can be explained and quantitatively modeled by the presence, at resonance, of a single transport channel with near-unity transmission. Our results establish an experimental validation of the theory of Coulomb oscillations in Josephson junctions in a previously unexplored regime. In addition, these results show that charge dispersion can be suppressed without the necessity of large E_J/E_C ratios, potentially allowing a very large qubit anharmonicity.   

Efficient microwave measurement of superconducting optomechanical circuits

Optomechanical interactions between electromagnetic modes and mechanical resonators have proven to be a valuable testbed for measurement and control of linear quantum systems. In this context, the light is considered as a meter that probes and influences the quantum state of the mechanical object. Reaching quantum limits of measurement and control therefore requires both that the optomechanical coupling overwhelms any decoherence and that the light is measured with sufficiently high efficiency. In microwave optomechanical systems, the first requirement has been demonstrated, but the second remains an experimental challenge, with state-of-art continuous linear measurements of microwave fields struggling to exceed 50% efficiency. Here I report recent progress to increase microwave measurement efficiencies to enable new regimes of ponderomotive squeezing and displacement sensing beyond the standard quantum limit.   

A Scalable Nanophotonic Platform for Rare Earth Ions

Rare earth ion ensembles doped in single crystals are a promising platform with widespread applications in optical signal processing, lasing and quantum information processing. The ability to engineer nanoscale patterns surrounding these ions would enable strong light matters interaction and open possibilities for a new generation of photonic devices. We present a new nanophotonic platform with Thulium ions doped in single crystal Lithium niobate thin films on insulator, that supports scalable top down fabrication. The ions in the thin film retain bulk like optical properties. We show spectral hole burning in a nanophotonic waveguide with powers upto 2 orders of magnitude lower than previously reported bulk waveguides. Such a platform paves way for on chip lasers and efficient ensemble quantum memories. The high electro-optic coefficient of lithium niobate coupled with densly patterned electrodes promises large stark tuning which may be useful for ensemble quantum memory protocols like CRIB. Lastly strong light matter interaction can lead to Purcell enhancement and the ability to isolate single ions as a long lived qubit.   

Spin-photon interfaces based on tin-vacancy centers in diamond

Color centers in diamond are quantum systems that can combine long-lived spin degrees of freedom with coherent optical transitions for applications in quantum networks and information processing. The tin-vacancy (SnV) center in diamond in particular combines the characteristic inversion symmetry of the Group IV-vacancy complexes with a large spin-orbit splitting of its ground-state orbitals, enabling the demonstration of long spin coherence times at accessible temperatures. Several challenges remain in the application and understanding of these centers, including a complete theoretical description of the electronic structure, universal spin control, and the engineering of efficient light-matter interaction. Here we discuss theoretical and experimental work towards coherent spin-photon interfaces based on SnV centers at liquid-helium temperatures.
  

Investigating Microwave Raman Transitions Beyond the Rotating Wave Approximation in the Electronic Ground State of the Nitrogen-Vacancy Center

Up to now, spin manipulation experiments with the NV center were mostly limited to applying subsequent monochromatic microwave pulses, manipulating only single electronic transitions at a time. In our work we explore the possibility of applying multitone microwave pulses, allowing a full simultaneous control of all three electronic ground states of the NV center. This here presented spin manipulation scheme opens up new measurement possibilities, which could be used to increase the NV center’s magnetic field sensitivity.
We will present the implementation of a spin-forbidden coherent population swapping between the m_s = -1 and m_s = +1 states, without undergoing the spin allowed transition into the m_s = 0 state via microwave Raman transitions and compare our experimental results with theoretical calculations. These Raman transitions are operated in a regime exceeding the rotating wave approximation (RWA). This has hardly been investigated as the RWA is traditionally applied in the single atom community. Due to the short coherence times, a low detuning has to be chosen, which in turn causes fast Raman transitions and a breakdown of the RWA. An indication for this is an experimentally observed beating signal which we want to utilize for faster and more robust population swapping.   

Solid-state defect based quantum modules

The development of technology required for quantum modules is likely to form the basis of any large-scale quantum information processing device. The question of the appropriate physical implementation of such technology remains, however, unsettled. In this presentation, we compare and contrast a number of different solid-state defects – including the nitrogen-, silicon-, and germanium-vacancy centers in diamond, as well as a variety of defects in silicon carbide – according to their suitability for quantum modules based on strong coupling to optical cavities. The primary aspects forming the basis of this comparison are Hilbert space size and connectivity, optical transition contrast and branching ratios, decay lifetimes, and the capability for robust information storage across multiple failed probabilistic optical operations. Our results show several promising candidates.