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: 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.
- 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. :)
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