Like
many other physicists, we spent the past week in the APS March meeting,
sitting in video conference rooms watching PowerPoints, wondering if we
should maybe have joined a

*different*video conference to watch a*different*PowerPoint, all while worrying about how our own presentation would go and hoping that our pet wouldn't make noise during it. But the dust is finally settling. Despite both the chaos of trying to keep track of dozens of quantum computing talks combined with the fatigue that comes with forgetting to get up from our chair for an entire day, we were able to sit in on a host of amazing talks about quantum hardware.We've roped together a few talks
we liked that represent some of the more interesting pieces of work to
come out of the meeting, showing advances in qubit architectures,
control mechanisms, and other hardware topics. We've intentionally left
out any IBM results so as not to look tacky (most of the blog's authors
work there), but feel free to discuss any IBM results in the comments.
This isn't a comprehensive list or a list of "best,"
and given how many talks there were, we definitely left off some other
cool presentations; these are just some of the ones we noticed and
wanted to share. Sound
off in the comments (or on Twitter or elsewhere!) with what you thought
about the meeting, these talks, or any other research that got you
thinking this past week.

Challenges and methodology of assembling Edgeless Four Side Tileable ROICs for a Wafer Scale, Deadzone-less Camera utilizing high density interconnects.*

*Presenter:*

*Farah Fahim (Fermi National Accelerator Laboratory)*

*Abstract:*

*Deadzone-less, large area camera systems can be assembled by connecting wafer scale sensors to an array of almost reticule size, 4-side tileable, edgeless readout integrated circuits (ROIC). The design of truly edgeless ROICs, with active area extending to their edges, has been made possible with the advent of 3D integration technologies with high-density interconnects, which enable new routing and I/O paradigms. Despite their obvious potential, the realization and widespread development of truly edgeless ROICs to create gapless dectors has faced several obstacles including manufacturing processes related to 3D integration, identification of known good dies and edgeless design methodologies. The advancements required in "thru via" approaches and wafer bonding and its impact on developing integrated electronics required for Quantum and AI will be discussed.*

We
thought it was really interesting to see how other physicists have
dealt with scaling up experiments to ridiculous levels of complexity,
and provides some inspiration (and hope!) for the future of quantum
devices.

*Presenter: J*

*acob Blumoff*

*(*

*HRL Laboratories LLC)*

*Abstract:*

*Existing architectures for silicon quantum-dot qubits have enabled high-fidelity state preparation and measurement1, low-error randomized benchmarking2, and millisecond-scale dynamical decoupling3. To facilitate improved control of the underlying electrostatic potential and scaling to larger arrays, we present a more advanced design called Single-Layer Etch-Defined Gate Electrode, or “SLEDGE.” These devices feature a single layer of non-overlapping gate electrodes and employ vias to break the plane to backend routing. Using this process, we demonstrate exchange-only qubit initialization, measurement, and randomized benchmarking with fidelities that compare favorably to the previous design. This architecture provides a path to scalable and high-performance silicon-based quantum devices.*

*Blumoff et al., APS March Meeting 2020, R38.00001**Andrews et al., Nat. Nano. 14, 747 (2019)**Sun et al., APS March Meeting 2020, L17.00008*

This talk was a great intro on exchange-only qubits in Si/SiGe. Blumoff discussed scalability and the fabrication aspect, including improvement made with vias—and the new architecture performs about as well as the architecture it was attempting to improve upon.

*Presenter:*

*Phillipe Campagne-Ibarcq (Quantic Team, Inria Paris)*

*Abstract:*

*In 2001, Gottesman, Kitaev and Preskill (GKP) proposed to encode a fully correctable logical qubit in grid states of a single harmonic oscillator. Although this code was originally designed to correct against shift errors, GKP qubits are robust against virtually all realistic error channels. Since this proposal, other bosonic codes have been extensively investigated, but only recently were the exotic GKP states experimentally synthesized and stabilized. These experiments relied on stroboscopic interactions between a target oscillator and an ancillary two-level system to measure non-destructively the GKP code error syndromes.*

*In this talk, I will review the fascinating properties of the GKP code and the conceptual and experimental tools developed for trapped ions and superconducting circuits, which enabled quantum error correction of a logical GKP qubit encoded in a microwave cavity. I will describe ongoing efforts to suppress further logical errors, and in particular to avoid the apparition of uncorrectable errors stemming from the noisy ancilla involved in error syndrome detection.*

This talk started
with a very clear introduction to GKP states, and the experiments
themselves were amazing. The degree of technical skill that went into
making and manipulating these states was really cool. Plus the states
are really cool looking.

*Presenter:*

*Andras Gyenis (Princeton University)*

*Abstract:*

*Encoding a qubit in logical quantum states with wavefunctions characterized by disjoint support and robust energies can offer simultaneous protection against relaxation and pure dephasing. One of the most promising candidates for such a fully-protected superconducting qubit is the 0-π circuit [Brooks et al., Phys. Rev. A 87, 052306 (2013)]. Here, we realize the proposed circuit topology in an experimentally obtainable parameter regime and show that the device, which we call as the soft 0-π qubit, hosts logical states with disjoint support that are exponentially (first-order) protected against charge (flux) noise. Multi-tone spectroscopy measurements reveal the energy-level structure of the system, which can be precisely described by a simple two-mode Hamiltonian. Using a Raman-type protocol, we exploit a higher-lying charge-insensitive energy level of the device to realize coherent population transfer and logical operations. The measured relaxation (T_1 = 1.6 ms) and dephasing (T_R = 9 μs, T_2E = 25 µs) times demonstrate that the soft 0-π circuit not only broadens the family of superconducting qubits, but also constitutes an important step towards quantum computing with intrinsically protected superconducting qubits.*

The
0-π qubit lives! It was great to see how far along protected qubits
have come. We're also still laughing about the authors claim that the
qubit is "so well protected, even from experimentalists."

*Presenter: Mahdi Naghiloo (MIT)*

*Abstract:*

*We propose a new scheme that combines parametric mode conversion and adiabatic techniques in a pair of coupled nonlinear Josephson junction transmission lines to realize broadband isolation without magnetic elements. The idea is to induce an effective unidirectional parametric coupling between two otherwise orthogonal modes of propagation and engineer the dispersion to have an adiabatic conversion between two modes. Our realistic analysis suggests more than 20 dB isolation over an octave of bandwidth (4-8 GHz) with less than 0.1 dB of insertion loss. Our scheme is compatible with the current superconducting qubit technology. We report on progress toward implementing this device.*

This
was a proposal for making a TRWPA like device to replace a macroscopic
magnetic isolator. This was very exciting to see because the devices
performance looks almost identical to the commercial components. Looks
like it will be a difficult microwave engineering challenge but the
payoff would be enormous.

*Presenter:*

*Teruaki Yoshioka*

*(*

*Tokyo Univ of Science, Kagurazaka)*

*Abstract:*

*We report an experiment of fast initialization of superconducting qubit using SINIS.*

*Active and unconditional initialization is required for NISQ, surface code and quantum computation.*

*By applying a bias voltage to the SINIS, photon assisted tunneling occurs, and the Q value of the resonator can be temporarily deteriorated. A qubit is coupled to the resonator, energy is transferred from the qubit to the resonator by applying two drive pulses which are an existing initialization scheme, and energy is efficiently emitted to the environment by natural relaxation of the resonator. Further, when initialization is not performed, that is, when a bias voltage is not applied to SINIS, the Q value of the resonator returns, so that the Q value does not affect readout and gate operation.*

*In this presentation, we report the experimental results and fabrication of the device.*

The superconductor-insulator-normal metal-insulator-superconductor sandwich
(SINIS) idea has been knocking around for a while. It's a cool attempt
to take a piece of physics we'd normally say was a big problem—exciting
quasiparticles—and turn it into a reset mechanism for resonators.

*Presenter: Chuanhong Liu (University of Wisconsin-Madison)*

*Abstract:*

*The Single Flux Quantum (SFQ) digital logic family has been proposed as a scalable approach for the control of next-generation multiqubit arrays. In an initial implementation, the fidelity of SFQ-based qubit gates was limited by quasiparticle (QP) poisoning induced by the dissipative SFQ driver. Here we introduce superconducting bandgap engineering as a mitigation strategy to suppress QP poisoning in this system. We explore low-gap moats and high-gap fences surrounding the qubit structure, along with a geometry involving extensive coverage of the high-gap groundplane with low-gap traps. We use charge-sensitive transmon qubits to evaluate the effectiveness of the various mitigation strategies in experiments involving direct QP injection.*

This is the first time I've see an interface SFQ logic to qubits without destroying the qubits; they still had good coherence times. This was a cool introduction to superconducting bandgap engineering as a mitigation strategy to suppress quasiparticle poisoning in this system.

*Presenter:*

*Helin Zhang (University of Chicago)*

*Abstract:*

*The heavy-fluxonium qubit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing times at the flux-frustration point. However, the suppressed charge matrix elements and small splitting between computational states have made it challenging to perform fast single and two-qubit gates with conventional methods. In order to achieve high-fidelity initialization and readout, we demonstrate protocols utilizing higher levels beyond the computational subspace. We realize fast qubit control using a universal set of single-cycle flux gates, which are comprised of directly synthesizable pulses, and reach fidelities exceeding 99.8%. Finally, we discuss a set of flux-controlled two-qubit gates for inductively coupled fluxonium qubits. We believe that the fast, flux-based control combined with the coherence properties of the heavy fluxonium make this circuit one of the most promising candidates for next-generation superconducting qubits.*

This
took a good look at extremely low frequency fluxonium qubits at only a
couple hundred MHz. It was really neat to see people control things that
are at or below the thermal limit since they have to cool these qubits
before thy even begin the experiment. Also, the fast flux gates look
similar to something we would see in a spin qubit gate, so its
interesting to see that come together, the control is very atypical.

*Presenter:*

*Nico Hendrickx*

*(QuTech and Kavli Institute of Nanoscience, Delft University of Technology)*

*Abstract:*

*Quantum dot spin qubits are a promising platform for large-scale quantum computers. Their inherent compatibility with semiconductor fabrication technology promises the ability to scale up to large numbers of qubits. However, all prior experiments are limited to two-qubit logic.*

*Here, we go beyond these demonstrations and operate a four-qubit quantum processor. Furthermore, we define the quantum dots in a two-by-two grid and thereby realize the first two-dimensional qubit array with semiconductor qubits, a crucial step toward quantum error correction and practical quantum algorithms. We achieve these results by defining qubits based on hole states in strained planar germanium quantum wells, enabling a high degree of control, well defined qubit states, and fast, all-electrical qubit driving.*

*We perform one, two, three, and four qubit logic for all qubit combinations, realizing a compact and high-connectivity circuit. Furthermore, we show that the hole coherence can be extended up to 100 ms using refocusing pulses and employ this to perform a quantum circuit executed on the full four-qubit system. These results mark an important step for scaling up spin qubits in two dimensions and position planar germanium as a prime candidate for practical quantum applications.*

This
research represented a big simplification of the germanium spin-qubit
platform. The researchers did so by incorporating enough spin-orbit
coupling such that they didn't need a micromagnet in order to do
microwave manipulations, allowing them to create an array rather than
just a 2-qubit interaction.

*Presenter:*

*Ciaran Ryan-Anderson (Honeywell Intl)*

*Abstract:*

*Mid-circuit measurement and active feed-forward are essential ingredients to fault-tolerant quantum error correction, and the QCCD architecture naturally lends itself to these operational primitives. Ion-transport operations allow for individual qubits to be spatially isolated, where they may be safely interrogated and reinitialized with focused laser beams without damaging idling qubits. Here we present experimental characterizations of these operations including both primitive as well as algorithmic benchmarking results. We will also discuss our results’ implications for the QCCD architecture’s capabilities.*

It
has been really awesome to see the steady progress they have made from
their original H0 device. We appreciated the clear communication of the
effort they have dedicated to methodically solving each problem in turn
and sharing the results.

Presenter: Prof Andrew Houck (Princeton University)

*Abstract: We employ tantalum transmon qubits with coherence times above 0.3 ms to demonstrate the importance of materials engineering in realizing a superconducting quantum processor. In this talk we characterize the regions and mechanisms of loss in state-of-the-art two-dimensional qubits. To do so, we efficiently iterate our fabrication procedure using materials spectroscopy. We correlate the spectroscopic results with time domain measurements to enable rapid screening of new materials and processing techniques. We further elucidate the dominant loss sources by characterizing time, frequency, geometry, and temperature fluctuations of coherence. Our fabrication techniques can be easily employed in standard industry and academic cleanrooms, and integrated into existing quantum processor architectures.*

It's
always great to see new innovations in this field using novel
materials. Prof Houck did a great job outlining why this type of
creative exploration was necessary and the results are not only quite
impressive, they are easily implemented in other labs. We also enjoyed
seeing your co-author, the cat. Unfortunately, he was a little blurry,
but we just assume this means he has very precise momentum.

*Presenter: Uros Delic (University of Vienna)*

*Abstract:*

*Owing to its excellent isolation from the thermal environment, an optically levitated silica nanoparticle in ultra-high vacuum has been proposed to observe quantum behavior of massive objects at room temperature, with applications ranging from sensing to testing fundamental physics. As a first step towards quantum state preparation of the nanoparticle motion, both cavity and feedback cooling methods have been used to attempt cooling to its motional ground state, albeit with many technical difficulties. We have recently developed a new experimental interface, which combines stable (and arbitrary) trapping potentials of optical tweezers with the cooling performance of optical cavities, and demonstrated operation at desired experimental conditions [1]. In order to overcome still existent technical problems we implemented a new cooling method – cavity cooling by coherent scattering – which we employ to demonstrate ground state cooling of the nanoparticle motion [2, 3]. In this talk I will present our latest experimental result on motional ground state cooling of a levitated nanoparticle and discuss next steps toward macroscopic quantum states.*

*Delic, Grass et al., QST 5 (2), 025006**Delic et al., Phys. Rev. Lett. 122, 123602**Delic et al., Science 367, 892-895*

Figuring out why this result is cool is left as an exercise to the reader. :)