Session K69: Landauer-Bennett Award Prize Symposium

Optically accessible donor qubits in ZnO

Donors in semiconductors have been studied as a qubit platform for over 20 years. Here we present our work on synthesizing and isolating single donors in zinc oxide (ZnO). The direct band gap nature of ZnO enables efficient access to the donor electron via the donor-bound exciton. Here we demonstrate synthesis of indium (In) donors via ion implantation and annealing with properties on par with In donors incorporated in situ during growth including: a narrow inhomogeneous linewidths (< 10 GHz), spin initialization, and long longitudinal spin relaxation (> 100 ms). Next we demonstrate single In donor isolation for in situ doped In via plasma-enhanced focused ion beam milling. An outlook and progress toward single implanted In isolation and access to the In nuclear spin will also be presented.   

Measuring quantum correlations in many-body systems of polar molecules

Ultracold molecules have promising applications in the fields of quantum computing, simulation of many-body systems, fundamental precision measurements, and state-controlled chemistry. There has been rapid experimental progress in our ability to prepare and manipulate ultracold molecular gases, either by assembling them in-situ from atomic quantum gases or by direct laser cooling of special molecular species. An outstanding challenge in the field is the detection of quantum correlations between molecules. We have recently developed a novel apparatus for imaging single diatomic molecules in an ultracold gas prepared in well-defined electronic, rovibrational and hyperfine states. I will describe how we used this capability to measure quantum correlations due to the quantum statistics of the molecules or due to entanglement mediated by dipolar interactions. As an example of a potential application, I will discuss our study of out-of-equilibrium dynamics in tunable quantum spin models and our measurements of the evolution of spatial correlations during the ensuing thermalization process.   

New platforms for quantum sensing and quantum computing

The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. I will describe our recent efforts to correlate direct materials characterization with single spin measurements to devise methods to stabilize highly coherent NV centers within nanometers of the surface. We deploy these coherent shallow NV centers for a new nanoscale sensing technique, whereby we use covariance measurements of two or more NV centers to measure two-point magnetic field correlators.

Our approach for correlating surface spectroscopy techniques with single qubit measurements to realize directed improvements is generally applicable to many systems. Separately, I will describe our recent efforts to tackle noise and microwave losses in superconducting qubits. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of planar transmon qubits have remained elusive for several years. We have fabricated planar transmon qubits that have both lifetimes and coherence times exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. Following this discovery, we have parametrized the remaining sources of loss in state-of-the-art devices using systematic measurements of the dependence of loss on temperature, power, and geometry. This parametrization, complemented by direct materials characterization, allows for rational, directed improvement of superconducting qubits.   

Giant Artificial Atoms and Waveguide QED

In this talk, we present a demonstration of "giant artificial atoms" realized with superconducting qubits in a waveguide QED architecture. The superconducting qubits couple to the waveguide at multiple, well-separated locations. In this configuration, the dipole approximation no longer holds, and the giant atom may quantum mechanically self-interfere. This system enables tunable qubit-waveguide couplings with large on-off ratios and a coupling spectrum that can be engineered by design. Multiple, interleaved qubits in this architecture can be switched between protected and emissive configurations, while retaining qubit-qubit interactions mediated by the waveguide. Using this architecture, we generate a Bell state with 94% fidelity, despite both qubits being strongly coupled to the waveguide. We furthermore demonstrate directional emission with an artificial molecule with 97% fidelity. Such waveguide QED technologies are applicable to quantum interconnects and support architectural modularity.   

Testing relativity in the laboratory with optical lattice atomic clocks

The remarkable precision of optical atomic clocks offers sensitivity to new and exotic physics through tests of relativity, searches for dark matter, gravitational wave detection, and probes for beyond Standard Model?particles. While much of optical clock research has focused on improving absolute accuracy, many searches for new physics can be performed with relative comparisons between two clocks. To this end, we have recently realized a “multiplexed” strontium optical lattice clock consisting of two or more clocks in one vacuum chamber. This enables us to bypass many of the limitations present in typical atomic clock comparisons and to achieve new levels of precision.?

In this talk I will explain the motivation, concept, and operating principles of our multiplexed optical lattice clock. I will then present recent experimental results in which?we performed a novel, blinded, precision test of the gravitational redshift with an array of 5 evenly-spaced atomic ensembles spanning a total?height difference of 1?cm. I will present the error budget produced from our systematic evaluation, and the recently unblinded results of our first test. I will also discuss how these results can also be viewed as a proof-of-principle demonstration of relativistic geodesy at the sub-cm scale. Finally, I will discuss the outlook for future searches for new physics with our apparatus, including a novel direct test of the Einstein Equivalence Principle, and explorations of the interplay between general relativity and quantum mechanics.?