14 research outputs found
Quantum interference between single photons from a single atom and a cold atomic ensemble
Master'sMASTER OF SCIENC
Microwave magnon damping in YIG films at millikelvin temperatures
Magnon systems used in quantum devices require low damping if coherence is to
be maintained. The ferrimagnetic electrical insulator yttrium iron garnet (YIG)
has low magnon damping at room temperature and is a strong candidate to host
microwave magnon excitations in future quantum devices. Monocrystalline YIG
films are typically grown on gadolinium gallium garnet (GGG) substrates. In
this work, comparative experiments made on YIG waveguides with and without GGG
substrates indicate that the material plays a significant role in increasing
the damping at low temperatures. Measurements reveal that damping due to
temperature-peak processes is dominant above 1 K. Damping behaviour that we
show can be attributed to coupling to two-level fluctuators (TLFs) is observed
below 1 K. Upon saturating the TLFs in the substrate-free YIG at 20 mK,
linewidths of 1.4 MHz are achievable: lower than those measured at room
temperature.Comment: 5 pages, 4 figure
Mitigation of frequency collisions in superconducting quantum processors
The reproducibility of qubit parameters is a challenge for scaling up
superconducting quantum processors. Signal crosstalk imposes constraints on the
frequency separation between neighboring qubits. The frequency uncertainty of
transmon qubits arising from the fabrication process is attributed to
deviations in the Josephson junction area, tunnel barrier thickness, and the
qubit capacitor. We decrease the sensitivity to these variations by fabricating
larger Josephson junctions and reduce the wafer-level standard deviation in
resistance down to 2%. We characterize 32 identical transmon qubits and
demonstrate the reproducibility of the qubit frequencies with a 40 MHz standard
deviation (i.e. 1%) with qubit quality factors exceeding 2 million. We perform
two-level-system (TLS) spectroscopy and observe no significant increase in the
number of TLSs causing qubit relaxation. We further show by simulation that for
our parametric-gate architecture, and accounting only for errors caused by the
uncertainty of the qubit frequency, we can scale up to 100 qubits with an
average of only 3 collisions between quantum-gate transition frequencies,
assuming 2% crosstalk and 99.9% target gate fidelity.Comment: 8 figures, 18 pages (8 pages main text), units fixed in Fig. 1
Simplified Josephson-junction fabrication process for reproducibly high-performance superconducting qubits
We introduce a simplified fabrication technique for Josephson junctions and demonstrate superconducting Xmon qubits with T1 relaxation times averaging above 50 μs (Q > 1.5
7 1 0 6). Current shadow-evaporation techniques for aluminum-based Josephson junctions require a separate lithography step to deposit a patch that makes a galvanic, superconducting connection between the junction electrodes and the circuit wiring layer. The patch connection eliminates parasitic junctions, which otherwise contribute significantly to dielectric loss. In our patch-integrated cross-type junction technique, we use one lithography step and one vacuum cycle to evaporate both the junction electrodes and the patch. This eliminates a key bottleneck in manufacturing superconducting qubits by reducing the fabrication time and cost. In a study of more than 3600 junctions, we show an average resistance variation of 3.7% on a wafer that contains forty 0.5
7 0.5-cm2 chips, with junction areas ranging between 0.01 and 0.16 μm2. The average on-chip spread in resistance is 2.7%, with 20 chips varying between 1.4% and 2%. For the junction sizes used for transmon qubits, we deduce a wafer-level transition-frequency variation of 1.7%-2.5%. We show that 60%-70% of this variation is attributed to junction-area fluctuations, while the rest is caused by tunnel-junction inhomogeneity. Such high frequency predictability is a requirement for scaling-up the number of qubits in a quantum computer
Signal crosstalk in a flip-chip quantum processor
Quantum processors require a signal-delivery architecture with high
addressability (low crosstalk) to ensure high performance already at the scale
of dozens of qubits. Signal crosstalk causes inadvertent driving of quantum
gates, which will adversely affect quantum-gate fidelities in scaled-up
devices. Here, we demonstrate packaged flip-chip superconducting quantum
processors with signal-crosstalk performance competitive with those reported in
other platforms. For capacitively coupled qubit-drive lines, we find
on-resonant crosstalk better than -27 dB (average -37 dB). For inductively
coupled magnetic-flux-drive lines, we find less than 0.13 % direct-current flux
crosstalk (average 0.05 %). These observed crosstalk levels are adequately
small and indicate a decreasing trend with increasing distance, which is
promising for further scaling up to larger numbers of qubits. We discuss the
implication of our results for the design of a low-crosstalk, on-chip signal
delivery architecture, including the influence of a shielding tunnel structure,
potential sources of crosstalk, and estimation of crosstalk-induced qubit-gate
error in scaled-up quantum processors.Comment: 16 pages, 12 figures, includes appendice
Microwave magnonics at millikelvin temperatures
This thesis reports on three recent experimental studies of microwave magnons in yttrium iron garnet (YIG) systems at millikelvin temperatures.
We begin with an introduction to the emerging field of quantum magnonics and its underpinning motivations. Basic theory of dipolar spin-wave or magnon dynamics in various sample geometries is presented. This introduction is followed by a brief description of the specificities of our experimental setup and properties of YIG --- the magnetic material used in our studies.
The first experiment involves a hybrid system combining a YIG sphere and a niobium-based superconducting planar resonator. The device is measured at millikelvin temperatures and signals of strong magnon-photon coupling are observed when the excitation energy is at the level of single photons. The superconducting resonator is shown to maintain a good quality factor even under sizeable in-plane magnetic field that is required to support the excitation of magnons in YIG.
The second experiment demonstrates the operation of a magnonic crystal based on an etched YIG film at millikelvin temperatures. A magnonic bandgap is successfully observed both under continuous- and pulsed- microwave excitation. High magnon damping is observed in the device at low temperature.
The third experiment involves the measurement of magnon damping in YIG films. Comparisons between results from different samples and at different temperatures provide insight into the role of the substrate and two-level fluctuators at low temperature. A brief review of the known damping processes in bulk YIG from room temperature down to millikelvin temperature is also presented.
The final chapter summarises results in this thesis and suggests possible future research directions.</p
Microwave magnonics at millikelvin temperatures
This thesis reports on three recent experimental studies of microwave magnons in yttrium iron garnet (YIG) systems at millikelvin temperatures. We begin with an introduction to the emerging field of quantum magnonics and its underpinning motivations. Basic theory of dipolar spin-wave or magnon dynamics in various sample geometries is presented. This introduction is followed by a brief description of the specificities of our experimental setup and properties of YIG --- the magnetic material used in our studies. The first experiment involves a hybrid system combining a YIG sphere and a niobium-based superconducting planar resonator. The device is measured at millikelvin temperatures and signals of strong magnon-photon coupling are observed when the excitation energy is at the level of single photons. The superconducting resonator is shown to maintain a good quality factor even under sizeable in-plane magnetic field that is required to support the excitation of magnons in YIG. The second experiment demonstrates the operation of a magnonic crystal based on an etched YIG film at millikelvin temperatures. A magnonic bandgap is successfully observed both under continuous- and pulsed- microwave excitation. High magnon damping is observed in the device at low temperature. The third experiment involves the measurement of magnon damping in YIG films. Comparisons between results from different samples and at different temperatures provide insight into the role of the substrate and two-level fluctuators at low temperature. A brief review of the known damping processes in bulk YIG from room temperature down to millikelvin temperature is also presented. The final chapter summarises results in this thesis and suggests possible future research directions.</p
Hong-Ou-Mandel interference between triggered and heralded single photons from separate atomic systems
Fast analytic and numerical design of superconducting resonators in flip-chip architectures
In superconducting quantum processors, the predictability of device
parameters is of increasing importance as many labs scale up their systems to
larger sizes in a 3D-integrated architecture. In particular, the properties of
superconducting resonators must be controlled well to ensure high-fidelity
multiplexed readout of qubits. Here we present a method, based on conformal
mapping techniques, to predict a resonator's parameters directly from its 2D
cross-section, without computationally heavy simulation. We demonstrate the
method's validity by comparing the calculated resonator frequency and coupling
quality factor with those obtained through 3D finite-element-method simulation
and by measurement of 15 resonators in a flip-chip-integrated architecture. We
achieve a discrepancy of less than 2% between designed and measured
frequencies, for 6-GHz resonators. We also propose a design method that reduces
the sensitivity of the resonant frequency to variations in the inter-chip
spacing.Comment: 12 pages, 11 figures. Major correction of equation (17), with one
minor correction in Appendix C. This work has been submitted to the IEEE for
possible publication. Copyright may be transferred without notice, after
which this version may no longer be accessibl