12 research outputs found
An argon ion beam milling process for native layers enabling coherent superconducting contacts
We present an argon ion beam milling process to remove the native oxide layer
forming on aluminum thin films due to their exposure to atmosphere in between
lithographic steps. Our cleaning process is readily integrable with
conventional fabrication of Josephson junction quantum circuits. From
measurements of the internal quality factors of superconducting microwave
resonators with and without contacts, we place an upper bound on the residual
resistance of an ion beam milled contact of 50 at a frequency of 4.5 GHz. Resonators for which only of the
total foot-print was exposed to the ion beam milling, in areas of low electric
and high magnetic field, showed quality factors above in the single
photon regime, and no degradation compared to single layer samples. We believe
these results will enable the development of increasingly complex
superconducting circuits for quantum information processing.Comment: 4 pages, 4 figures, supplementary materia
State preparation of a fluxonium qubit with feedback from a custom FPGA-based platform
We developed a versatile integrated control and readout instrument for
experiments with superconducting quantum bits (qubits), based on a
field-programmable gate array (FPGA) platform. Using this platform, we perform
measurement-based, closed-loop feedback operations with
platform latency. The feedback capability is instrumental in realizing active
reset initialization of the qubit into the ground state in a time much shorter
than its energy relaxation time . We show experimental results
demonstrating reset of a fluxonium qubit with fidelity, using a
readout-and-drive pulse sequence approximately long.
Compared to passive ground state initialization through thermalization, with
the time constant given by , the use of the
FPGA-based platform allows us to improve both the fidelity and the time of the
qubit initialization by an order of magnitude.Comment: 3 pages, 2 figures. The following article has been submitted to the
AIP Conference Proceedings of the Fifth International Conference on Quantum
Technologies (ICQT-2019
Quantum Nondemolition Dispersive Readout of a Superconducting Artificial Atom Using Large Photon Numbers
Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating photon number in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing is generally observed to also monotonously increase these transition rates. Here we present a fluxonium artificial atom in which, despite the fact that the measured transition rates show nonmonotonous fluctuations within a factor of 6, for photon numbers up to ≈200, the signal-to-noise ratio continuously improves with increasing . Even without the use of a parametric amplifier, at =74, we achieve fidelities of 99% and 93% for feedback-assisted ground and excited state preparations, respectively. At higher , leakage outside the qubit computational space can no longer be neglected and it limits the fidelity of quantum state preparation
Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers
Reading out the state of superconducting artificial atoms typically relies on
dispersive coupling to a readout resonator. For a given system noise
temperature, increasing the circulating photon number in the
resonator enables a shorter measurement time and is therefore expected to
reduce readout errors caused by spontaneous atom transitions. However,
increasing is generally observed to also increase these transition
rates. Here we present a fluxonium artificial atom in which we measure an
overall flat dependence of the transition rates between its first two states as
a function of , up to . Despite the fact that we
observe the expected decrease of the dispersive shift with increasing readout
power, the signal-to-noise ratio continuously improves with increasing
. Even without the use of a parametric amplifier, at , we
measure fidelities of 99% and 93% for feedback-assisted ground and excited
state preparation, respectively.Comment: typos corrected, added figure at p.10 (section IV of the Supplemental
Material), added reference
A quantum Szilard engine for two-level systems coupled to a qubit
The innate complexity of solid state physics exposes superconducting quantum
circuits to interactions with uncontrolled degrees of freedom degrading their
coherence. By using a simple stabilization sequence we show that a
superconducting fluxonium qubit is coupled to a two-level system (TLS)
environment of unknown origin, with a relatively long energy relaxation time
exceeding . Implementing a quantum Szilard engine with an active
feedback control loop allows us to decide whether the qubit heats or cools its
TLS environment. The TLSs can be cooled down resulting in a four times lower
qubit population, or they can be heated to manifest themselves as a negative
temperature environment corresponding to a qubit population of .
We show that the TLSs and the qubit are each other's dominant loss mechanism
and that the qubit relaxation is independent of the TLS populations.
Understanding and mitigating TLS environments is therefore not only crucial to
improve qubit lifetimes but also to avoid non-Markovian qubit dynamics