69 research outputs found
GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing
We present measurements of the electron temperature using gate defined
quantum dots formed in a GaAs 2D electron gas in both direct transport and
charge sensing mode. Decent agreement with the refrigerator temperature was
observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear
demagnetization stages integrated into the sample wires below 1 mK, the device
electron temperature saturates, remaining close to 10 mK. The extreme
sensitivity of the thermometer to its environment as well as electronic noise
complicates temperature measurements but could potentially provide further
insight into the device characteristics. We discuss thermal coupling
mechanisms, address possible reasons for the temperature saturation and
delineate the prospects of further reducing the device electron temperature.Comment: 8 pages, 3 (color) figure
Voltage-Controlled Superconducting Quantum Bus
We demonstrate the ability of an epitaxial semiconductor-superconductor
nanowire to serve as a field-effect switch to tune a superconducting cavity.
Two superconducting gatemon qubits are coupled to the cavity, which acts as a
quantum bus. Using a gate voltage to control the superconducting switch yields
up to a factor of 8 change in qubit-qubit coupling between the on and off
states without detrimental effect on qubit coherence. High-bandwidth operation
of the coupling switch on nanosecond timescales degrades qubit coherence
Anharmonicity of a Gatemon Qubit with a Few-Mode Josephson Junction
Coherent operation of gate-voltage-controlled hybrid transmon qubits
(gatemons) based on semiconductor nanowires was recently demonstrated. Here we
experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson
junctions, a key parameter for their use as a qubit. Anharmonicity is found to
be reduced by roughly a factor of two compared to conventional metallic
junctions, and dependent on gate voltage. Experimental results are consistent
with a theoretical model, indicating that Josephson coupling is mediated by a
small number of highly transmitting modes in the semiconductor junction
Selective Area Grown Semiconductor-Superconductor Hybrids: A Basis for Topological Networks
We introduce selective area grown hybrid InAs/Al nanowires based on molecular
beam epitaxy, allowing arbitrary semiconductor-superconductor networks
containing loops and branches. Transport reveals a hard induced gap and
unpoisoned 2e-periodic Coulomb blockade, with temperature dependent 1e features
in agreement with theory. Coulomb peak spacing in parallel magnetic field
displays overshoot, indicating an oscillating discrete near-zero subgap state
consistent with device length. Finally, we investigate a loop network, finding
strong spin-orbit coupling and a coherence length of several microns. These
results demonstrate the potential of this platform for scalable topological
networks among other applications.Comment: NBI QDEV 201
Metallic Coulomb Blockade Thermometry down to 10 mK and below
We present an improved nuclear refrigerator reaching 0.3 mK, aimed at
microkelvin nanoelectronic experiments, and use it to investigate metallic
Coulomb blockade thermometers (CBTs) with various resistances R. The high-R
devices cool to slightly lower T, consistent with better isolation from the
noise environment, and exhibit electron-phonon cooling ~ T^5 and a residual
heat-leak of 40 aW. In contrast, the low-R CBTs display cooling with a clearly
weaker T-dependence, deviating from the electronphonon mechanism. The CBTs
agree excellently with the refrigerator temperature above 20 mK and reach a
minimum-T of 7.5 +/- 0.2 mK.Comment: 3 pages, 3 (color) figure
Suppressed Charge Dispersion via Resonant Tunneling in a Single-Channel Transmon
We demonstrate strong suppression of charge dispersion in a
semiconductor-based transmon qubit across Josephson resonances associated with
a quantum dot in the junction. On resonance, dispersion is drastically reduced
compared to conventional transmons with corresponding Josephson and charging
energies. We develop a model of qubit dispersion for a single-channel
resonance, which is in quantitative agreement with experimental data
A Parity-Protected Superconductor-Semiconductor Qubit
Coherence of superconducting qubits can be improved by implementing designs
that protect the parity of Cooper pairs on superconducting islands. Here, we
introduce a parity-protected qubit based on voltage-controlled semiconductor
nanowire Josephson junctions, taking advantage of the higher harmonic content
in the energy-phase relation of few-channel junctions. A symmetric
interferometer formed by two such junctions, gate-tuned into balance and
frustrated by a half-quantum of applied flux, yields a cos(2{\phi}) Josephson
element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate
that relaxation of the qubit can be suppressed tenfold by tuning into the
protected regime
Closing of the Induced Gap in a Hybrid Superconductor-Semiconductor Nanowire
Hybrid superconductor-semiconductor nanowires are predicted to undergo a
field-induced phase transition from a trivial to a topological superconductor,
marked by the closure and re-opening of the excitation gap, followed by the
emergence of Majorana bound states at the nanowire ends. Many local
density-of-states measurements have reported signatures of the topological
phase, however this interpretation has been challenged by alternative
explanations. Here, by measuring nonlocal conductance, we identify the closure
of the excitation gap in the bulk of the semiconductor before the emergence of
zero-bias peaks. This observation is inconsistent with scenarios where
zero-bias peaks occur due to end-states with a trivially gapped bulk, which
have been extensively considered in the theoretical and experimental
literature. We observe that after the gap closes, nonlocal signals fluctuate
strongly and persist irrespective of the presence of local-conductance
zero-bias peaks. Thus, our observations are also incompatible with a simple
picture of clean topological superconductivity. This work presents a new
experimental approach for probing the spatial extent of states in Majorana
wires, and reveals the presence of a regime with a continuum of spatially
extended states and uncorrelated zero-bias peaks
GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing
We present measurements of the electron temperature using gate-defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1mK, the device electron temperature saturates, remaining close to 10mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature
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