What Were Your Favorite APS March Talks?

 

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. 

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: Jacob 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.
  1. Blumoff et al., APS March Meeting 2020, R38.00001
  2. Andrews et al., Nat. Nano. 14, 747 (2019)
  3. 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.
  1. Delic, Grass et al., QST 5 (2), 025006
  2. Delic et al., Phys. Rev. Lett. 122, 123602
  3. Delic et al., Science 367, 892-895
Figuring out why this result is cool is left as an exercise to the reader. :)



 

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Here's How Ion Trap Quantum Computers Work

By Petar Jurcevic and Ryan Mandelbaum

It's easy, in principle, to build a quantum computer: just find a system that obeys the laws of quantum mechanics with properties that you can exploit in order to perform your computations. In practice, it's extremely hard to build a quantum computer; you need to be able to control that system.

Several architectures have arisen as viable controllable quantum systems; perhaps the "leaders" are the superconducting qubits that IBM and Google researches, and the trapped ion approach pursued by companies such as Honeywell and IonQ. You can read about how superconducting qubits work here. As for me, I did my Ph.D at the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria, studying quantum simulations of many-body interacting systems using trapped ions under professor Rainer Blatt and Dr. Christian Roos. I didn't join the team because I had a specific preference for ions; it was an opportunity in entering the field of experimental quantum computation at a time when programs were generally pursued by academic groups with a few notable exceptions. I was lucky to get the opportunity to join one of the major trapped-ion research groups in the world and to experience the evolution of quantum computation early on. 

Based on my experience, I can fairly say that neither technology is "better" or "worse" than the other, but both approaches have strengths as well as challenges that their respective engineers must overcome (if they hope for their devices to become useful in in the future.)

A macroscopic ion trap (via Blatt lab)
 

Ion traps are among the more  "natural" quantum computers; we represent the computer's two bit values with two quantum states of an electron around an atom. We begin setting up our device by loading from a neutral atomic source inside the computer's vacuum chamber, and then add energy with heat or lasers in order to create a tiny stream of neutral atoms. There are many choices of species we could use, calcium and ytterbium being some of the more popular, all of which have their own pros and cons— which choice is based on the engineering path you'd like to go down. Once we have our neutral stream of atoms, we use a laser to rip off a single electron, charging the atom and turning it into an ion. This charged ion then falls into the trapping potential, a specially-generated rotating electromagnetic field; which you can think of as a ball remaining at the base of a quickly-spinning saddle. The longer you wait with this process, the more ions you trap, and therefore, the more qubits you have to work with; each ion represents one qubit.

In order to actually compute with these qubits, we begin by Doppler cooling the ions with lasers of a specially tuned frequency slightly below the electron transition frequency, halting their motion. We use optical light to pump the atom into its ground state, and use sideband cooling, another laser cooling technique, to further cool the atom's motional modes into the ground state.
 
Once the system is initialized, we use laser fields to apply single or two-qubit gates. Single-qubit gates "move" the electron from one state to another, either leaving the electron in the ground, or 0 state, exciting it into the 1 state, or generating a superposition of 0 and 1. We also use lasers to couple these internal degrees of freedom, i.e. qubit states,  to the external motional degrees of freedom in order to generate two qubit gates; the common motion of the ions, due to the harmonic trapping potential, can be thought of as a bus system coupling all the qubits together at once. The laser fields are tuned in a particular way such that the ions' motion gets excited and de-excited only if the qubits are in a particular state; this state-dependent force is know as the Mølmer-Sørensen gate, the most commonly used two-qubit gate scheme in trapped ion architectures. 
 
After we run our circuit of one- and two- qubit gates, it's time to make our final measurements in order to get our readout, the bitstring that represents the computation's results. We use fluorescence; we couple one of the two qubit states to a short-lived transition that scatters a lot of light which we can collect with a CCD camera or a photo multiplier tube, so if the qubit is in this state we see light, and if the qubit is not in this state, we don't. The resulting measurements is therefore represented by a series of dark or light spots—the bitstring's zeros and ones.
 

These systems have natural advantages—depending on which ion you use, ion trap computers can have qubit T1s (the measure of how long until the ions return to their ground state) of several minutes or longer, and T2s (the measure of how long until ions in superposition dephase) of several seconds. All ions of the same species of atom are identical, so there's no variation in your qubits introduced by fabrication. Ions have extremely high gate fidelities, and in fact, reported record fidelities for single- and two-qubit gates have so far come from trapped ion systems. Moreover, state preparation/measurement errors can be orders of magnitude smaller compared to superconducting qubits and are rarely a real concern. Finally, ions in the same to trap have all-to-all connectivity, meaning you can drive gates between any pair of ions in the system. This is a direct consequence of using the common-motional modes as a bus system. Sparsely-connected superconducting qubits instead utilize SWAP gates or teleportation schemes to generate quantum entanglement between far-distant qubits which do not have direct connection between each other. While superconducting computers benefit from the ability to perform a variety of circuits, connectivity gives ion-trapped based platforms an edge in the early stages of quantum computing development, for now.
 
A setup in an ion trap lab (via Blatt lab)


But ions have their own challenges. It can take a long time to cool the ions in the trap, and these system require a lot more hands-on work that makes them more difficult to automate. Selectively performing two-qubit gates on specific and/or individual qubit pairs is a harder engineering challenge on ions than it is on superconducting qubits. There are various approaches being pursued to improve these gates, all with their own set of advantages and challenges, but all of them add an additional layer of complexity, with scalability and performance being a focus of current research. Gates can also take a few orders of magnitude more time to run than in superconducting quantum computers, a pain that can be felt especially in cases where we must perform many iterations on a parametrized quantum circuit communicating with classical computational resources. Building devices with a larger number of qubits can prove especially challenging.
 
There are proposals under development in order to surpass these scaling challenges, of course. One is the quantum charge-coupled device architecture proposal, where a micro-fabricated trap contains various separate trapping regions, including a region where loading happens, a region where computing happens, or a storage region, and the system shuffles ions around depending on how they're being used. Another proposal consists of multiple individual ion traps linked together via optical links, where photons mapped to qubit states can exchange information with ions in another trap. Each of these solutions come with their own considerations; quantum CCD requires many control knobs that all need to be stable themselves, and linking ion traps is (for now) a very inefficient process.
Petar in the lab (via Blatt lab)


But during my seven years researching ions (five as a Ph.D student, two as a postdoctoral researcher), I gained a lot of respect for this architecture that I've carried with me through my career. I think that ions made me really aware of the tiny effects that might not be a big deal now, but will be important to any quantum computing architecture as we scale up. After we made major improvements to our trapped ion setup, we suddenly could measure the elevator movements in our building, for example! And though I cherish my time with ions and I love teasing my fellow transmon-qubit researcher about how easy ion-qubits are made, I'm also very happy I don't have to spend time fiddling with lasers and optics anymore.
 
Quantum technology is certainly further along than ever before, and given the attention, both ion traps and superconducting qubits are advancing quickly. And while there's certainly some business competition, many in the fields have worked in both systems—and it's likely that the quantum ecosystem of the future will incorporate knowledge or hardware from both of these architectures.


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