90 research outputs found
Fabrication and Characterization of Aluminum Airbridges for Superconducting Microwave Circuits
Superconducting microwave circuits based on coplanar waveguides (CPW) are
susceptible to parasitic slotline modes which can lead to loss and decoherence.
We motivate the use of superconducting airbridges as a reliable method for
preventing the propagation of these modes. We describe the fabrication of these
airbridges on superconducting resonators, which we use to measure the loss due
to placing airbridges over CPW lines. We find that the additional loss at
single photon levels is small, and decreases at higher drive powers.Comment: 8 pages and 7 figures including supplementary informatio
Fast Scalable State Measurement with Superconducting Qubits
Progress in superconducting qubit experiments with greater numbers of qubits
or advanced techniques such as feedback requires faster and more accurate state
measurement. We have designed a multiplexed measurement system with a bandpass
filter that allows fast measurement without increasing environmental damping of
the qubits. We use this to demonstrate simultaneous measurement of four qubits
on a single superconducting integrated circuit, the fastest of which can be
measured to 99.8% accuracy in 140ns. This accuracy and speed is suitable for
advanced multi-qubit experiments including surface code error correction.Comment: Five figure
Design and characterization of a lumped element single-ended superconducting microwave parametric amplifier with on-chip flux bias line
We demonstrate a lumped-element Josephson parametric amplifier, using a
single-ended design that includes an on-chip, high-bandwidth flux bias line.
The amplifier can be pumped into its region of parametric gain through either
the input port or through the flux bias line. Broadband amplification is
achieved at a tunable frequency between 5 to 7 GHz with
quantum-limited noise performance, a gain-bandwidth product greater than 500
MHz, and an input saturation power in excess of -120 dBm. The bias line allows
fast frequency tuning of the amplifier, with variations of hundreds of MHz over
time scales shorter than 10 ns
Catching Shaped Microwave Photons with 99.4% Absorption Efficiency
We demonstrate a high efficiency deterministic quantum receiver to convert
flying qubits to logic qubits. We employ a superconducting resonator, which is
driven with a shaped pulse through an adjustable coupler. For the ideal "time
reversed" shape, we measure absorption and receiver fidelities at the single
microwave photon level of, respectively, 99.41% and 97.4%. These fidelities are
comparable with gates and measurement and exceed the deterministic quantum
communication and computation fault tolerant thresholds.Comment: Main paper: 5 pages, 4 figures. Supplement: 11 pages, 12 figures.
Revised abstract and introduction. Minor changes to Figure 1 and figure
caption
Dielectric surface loss in superconducting resonators with flux-trapping holes
Surface distributions of two level system (TLS) defects and magnetic vortices
are limiting dissipation sources in superconducting quantum circuits. Arrays of
flux-trapping holes are commonly used to eliminate loss due to magnetic
vortices, but may increase dielectric TLS loss. We find that dielectric TLS
loss increases by approximately 25% for resonators with a hole array beginning
2 from the resonator edge, while the dielectric loss added by
holes further away was below measurement sensitivity. Other forms of loss were
not affected by the holes. Additionally, we estimate the loss due to residual
magnetic effects to be for resonators
patterned with flux-traps and operated in magnetic fields up to
. This is orders of magnitude below the total loss of the best
superconducting coplanar waveguide resonators.Comment: 7 Pages, 4 Figure
Characterization and Reduction of Capacitive Loss Induced by Sub-Micron Josephson Junction Fabrication in Superconducting Qubits
Josephson junctions form the essential non-linearity for almost all
superconducting qubits. The junction is formed when two superconducting
electrodes come within 1 nm of each other. Although the capacitance of
these electrodes is a small fraction of the total qubit capacitance, the nearby
electric fields are more concentrated in dielectric surfaces and can contribute
substantially to the total dissipation. We have developed a technique to
experimentally investigate the effect of these electrodes on the quality of
superconducting devices. We use /4 coplanar waveguide resonators to
emulate lumped qubit capacitors. We add a variable number of these electrodes
to the capacitive end of these resonators and measure how the additional loss
scales with number of electrodes. We then reduce this loss with fabrication
techniques that limit the amount of lossy dielectrics. We then apply these
techniques to the fabrication of Xmon qubits on a silicon substrate to improve
their energy relaxation times by a factor of 5
Traveling wave parametric amplifier with Josephson junctions using minimal resonator phase matching
Josephson parametric amplifiers have become a critical tool in
superconducting device physics due to their high gain and quantum-limited
noise. Traveling wave parametric amplifiers (TWPAs) promise similar noise
performance while allowing for significant increases in both bandwidth and
dynamic range. We present a TWPA device based on an LC-ladder transmission line
of Josephson junctions and parallel plate capacitors using low-loss amorphous
silicon dielectric. Crucially, we have inserted resonators at
regular intervals along the transmission line in order to maintain the phase
matching condition between pump, signal, and idler and increase gain. We
achieve an average gain of 12\,dB across a 4\,GHz span, along with an average
saturation power of -92\,dBm with noise approaching the quantum limit
Characterization and reduction of microfabrication-induced decoherence in superconducting quantum circuits
Many superconducting qubits are highly sensitive to dielectric loss, making
the fabrication of coherent quantum circuits challenging. To elucidate this
issue, we characterize the interfaces and surfaces of superconducting coplanar
waveguide resonators and study the associated microwave loss. We show that
contamination induced by traditional qubit lift-off processing is particularly
detrimental to quality factors without proper substrate cleaning, while
roughness plays at most a small role. Aggressive surface treatment is shown to
damage the crystalline substrate and degrade resonator quality. We also
introduce methods to characterize and remove ultra-thin resist residue,
providing a way to quantify and minimize remnant sources of loss on device
surfaces
Measuring and Suppressing Quantum State Leakage in a Superconducting Qubit
Leakage errors occur when a quantum system leaves the two-level qubit
subspace. Reducing these errors is critically important for quantum error
correction to be viable. To quantify leakage errors, we use randomized
benchmarking in conjunction with measurement of the leakage population. We
characterize single qubit gates in a superconducting qubit, and by refining our
use of Derivative Reduction by Adiabatic Gate (DRAG) pulse shaping along with
detuning of the pulses, we obtain gate errors consistently below and
leakage rates at the level. With the control optimized, we find that
a significant portion of the remaining leakage is due to incoherent heating of
the qubit.Comment: 10 pages, 10 figures including supplement; fixed typos in metadat
Logic gates at the surface code threshold: Superconducting qubits poised for fault-tolerant quantum computing
A quantum computer can solve hard problems - such as prime factoring,
database searching, and quantum simulation - at the cost of needing to protect
fragile quantum states from error. Quantum error correction provides this
protection, by distributing a logical state among many physical qubits via
quantum entanglement. Superconductivity is an appealing platform, as it allows
for constructing large quantum circuits, and is compatible with
microfabrication. For superconducting qubits the surface code is a natural
choice for error correction, as it uses only nearest-neighbour coupling and
rapidly-cycled entangling gates. The gate fidelity requirements are modest: The
per-step fidelity threshold is only about 99%. Here, we demonstrate a universal
set of logic gates in a superconducting multi-qubit processor, achieving an
average single-qubit gate fidelity of 99.92% and a two-qubit gate fidelity up
to 99.4%. This places Josephson quantum computing at the fault-tolerant
threshold for surface code error correction. Our quantum processor is a first
step towards the surface code, using five qubits arranged in a linear array
with nearest-neighbour coupling. As a further demonstration, we construct a
five-qubit Greenberger-Horne-Zeilinger (GHZ) state using the complete circuit
and full set of gates. The results demonstrate that Josephson quantum computing
is a high-fidelity technology, with a clear path to scaling up to large-scale,
fault-tolerant quantum circuits.Comment: 15 pages, 13 figures, including supplementary materia
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