I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
17
Aug
2020
Stability of superconducting resonators: motional narrowing and the role of Landau-Zener driving of two-level defects
Frequency instability of superconducting resonators and qubits leads to dephasing and time-varying energy-loss and hinders quantum-processor tune-up. Its main source is dielectric noise
originating in surface oxides. Thorough noise studies are needed in order to develop a comprehensive understanding and mitigation strategy of these fluctuations. Here we use a frequency-locked loop to track the resonant-frequency jitter of three different resonator types—one niobium-nitride superinductor, one aluminium coplanar waveguide, and one aluminium cavity—and we observe strikingly similar random-telegraph-signal fluctuations. At low microwave drive power, the resonators exhibit multiple, unstable frequency positions, which for increasing power coalesce into one frequency due to motional narrowing caused by sympathetic driving of individual two-level-system defects by the resonator. In all three devices we probe a dominant fluctuator, finding that its amplitude saturates with increasing drive power, but its characteristic switching rate follows the power-law dependence of quasiclassical Landau-Zener transitions.
A merged-element transmon
Transmon qubits are ubiquitous in the pursuit of quantum computing using superconducting circuits. However, they have some drawbacks that still need to be addressed. Most importantly,
the scalability of transmons is limited by the large device footprint needed to reduce the participation of the lossy capacitive parts of the circuit. In this work, we investigate and evaluate losses in a novel device geometry, namely, the merged-element transmon (mergemon). To this end, we replace the large external shunt capacitor of a traditional transmon with the intrinsic capacitance of a Josephson junction (JJ) and achieve an approximately 100 times reduction in qubit dimensions. We report the implementation of the mergemon using a sputtered Nb/amorphous Si (a-Si)/Nb trilayer film. In an experiment below 10 mK, the frequency of the readout resonator, capacitively coupled to the mergemon, exhibits a qubit-state dependent shift in the low power regime. The device also demonstrates the single- and multi-photon transitions that symbolize a weakly anharmonic system in the two-tone spectroscopy. The transition spectra are explained well with master-equation simulations. A participation ratio analysis identifies the dielectric loss of the a-Si tunnel barrier and its interfaces as the dominant source for qubit relaxation. We expect the mergemon to achieve high coherence in relatively small device dimensions when implemented using a low-loss, epitaxially-grown, and lattice-matched trilayer.
High-fidelity controlled-Z gate with maximal intermediate leakage operating at the speed limit in a superconducting quantum processor
We introduce the sudden variant (SNZ) of the Net Zero scheme realizing controlled-Z (CZ) gates by baseband flux control of transmon frequency. SNZ CZ gates operate at the speed limit
of transverse coupling between computational and non-computational states by maximizing intermediate leakage. The key advantage of SNZ is tuneup simplicity, owing to the regular structure of conditional phase and leakage as a function of two control parameters. We realize SNZ CZ gates in a multi-transmon processor, achieving 99.93±0.24% fidelity and 0.10±0.02% leakage. SNZ is compatible with scalable schemes for quantum error correction and adaptable to generalized conditional-phase gates useful in intermediate-scale applications.
IQ Mixer Calibration for Superconducting Circuits
An important device for modulation and frequency translation in the field of circuit quantum electrodynamics is the IQ mixer, an analog component for which calibration is necessary
to achieve optimal performance. In this paper, we introduce techniques originally developed for wireless communication applications to calibrate upconversion and downconversion mixers. A Kalman filter together with a controllable carrier frequency offset calibrates both mixers without removing them from the embedding measurement infrastructure. These techniques can be embedded into room temperature control electronics and they will find widespread use as circuit QED devices continue to grow in size and complexity.
11
Aug
2020
Bolometer operating at the threshold for circuit quantum electrodynamics
Radiation sensors based on the heating effect of the absorbed radiation are typically relatively simple to operate and flexible in terms of the input frequency. Consequently, they are
widely applied, for example, in gas detection, security, THz imaging, astrophysical observations, and medical applications. A new spectrum of important applications is currently emerging from quantum technology and especially from electrical circuits behaving quantum mechanically. This circuit quantum electrodynamics (cQED) has given rise to unprecedented single-photon detectors and a quantum computer supreme to the classical supercomputers in a certain task. Thermal sensors are appealing in enhancing these devices since they are not plagued by quantum noise and are smaller, simpler, and consume about six orders of magnitude less power than the commonly used traveling-wave parametric amplifiers. However, despite great progress in the speed and noise levels of thermal sensors, no bolometer to date has proven fast and sensitive enough to provide advantages in cQED. Here, we experimentally demonstrate a bolometer surpassing this threshold with a noise equivalent power of 30zW/Hz−−−√ on par with the current record while providing two-orders of magnitude shorter thermal time constant of 500 ns. Importantly, both of these characteristic numbers have been measured directly from the same device, which implies a faithful estimation of the calorimetric energy resolution of a single 30-GHz photon. These improvements stem from the utilization of a graphene monolayer as the active material with extremely low specific heat. The minimum demonstrated time constant of 200 ns falls greatly below the state-of-the-art dephasing times of roughly 100 {\mu}s for superconducting qubits and meets the timescales of contemporary readout schemes thus enabling the utilization of thermal detectors in cQED.
09
Aug
2020
Efficient and low-backaction quantum measurement using a chip-scale detector
Superconducting qubits are a leading platform for scalable quantum computing and quantum error correction. One feature of this platform is the ability to perform projective measurements
orders of magnitude more quickly than qubit decoherence times. Such measurements are enabled by the use of quantum-limited parametric amplifiers in conjunction with ferrite circulators – magnetic devices which provide isolation from noise and decoherence due to amplifier backaction. Because these non-reciprocal elements have limited performance and are not easily integrated on-chip, it has been a longstanding goal to replace them with a scalable alternative. Here, we demonstrate a solution to this problem by using a superconducting switch to control the coupling between a qubit and amplifier. Doing so, we measure a transmon qubit using a single, chip-scale device to provide both parametric amplification and isolation from the bulk of amplifier backaction. This measurement is also fast, high fidelity, and has 70% efficiency, comparable to the best that has been reported in any superconducting qubit measurement. As such, this work constitutes a high-quality platform for the scalable measurement of superconducting qubits.
08
Aug
2020
Orbital tunable 0-π transitions in Josephson junctions with noncentrosymmetric topological superconductors
We investigate the Josephson transport properties in a Josephson junction consisting of a conventional s-wave superconductor coupled to a multi-orbital noncentrosymmetric superconductor
marked by an orbitally driven inversion asymmetry and isotropic interorbital spin-triplet pairing. Contrary to the canonical single band noncentrosymmetric superconductor, we demonstrate that the local interorbital spin-triplet pairing is tied to the occurrence of sign-changing spin-singlet pair amplitude on different bands with d-wave symmetry. Such multi-band d±-wave state is a unique superconducting configuration that drives unexpected Josephson effects with 0-π transitions displaying a high degree of electronic control. Remarkably, we find that the phase state of a noncentrosymmetric/s-wave Josephson junction can be toggled between 0 and π in multiple ways through a variation of electron filling, strength of the spin-orbital coupling, amplitude of the inversion asymmetry interaction, and junction transparency. These results highlight an intrinsic orbital and electrical tunability of the Josephson response and provide unique paths to unveil the nature of unconventional multiorbital superconductivity as well as inspire innovative designs of Josephson quantum devices.
06
Aug
2020
Electron shelving of a superconducting artificial atom
Interfacing stationary qubits with propagating photons is a fundamental problem in quantum technology. Cavity quantum electrodynamics (CQED) invokes a mediator degree of freedom in
the form of a far-detuned cavity mode, the adaptation of which to superconducting circuits (cQED) proved remarkably fruitful. The cavity both blocks the qubit emission and it enables a dispersive readout of the qubit state. Yet, a more direct (cavityless) interface is possible with atomic clocks, in which an orbital cycling transition can scatter photons depending on the state of a hyperfine or quadrupole qubit transition. Originally termed „electron shelving“, such a conditional fluorescence phenomenon is the cornerstone of many quantum information platforms, including trapped ions, solid state defects, and semiconductor quantum dots. Here we apply the shelving idea to circuit atoms and demonstrate a conditional fluorescence readout of fluxonium qubit placed inside a matched one-dimensional waveguide. Cycling the non-computational transition between ground and third excited states produces a microwave photon every 91 ns conditioned on the qubit ground state, while the qubit coherence time exceeds 50 us. The readout has a built-in quantum non-demolition property, allowing over 100 fluorescence cycles in agreement with a four-level optical pumping model. Our result introduces a resource-efficient alternative to cQED. It also adds a state-of-the-art quantum memory to the growing toolbox of waveguide QED.
High-Fidelity Control of Superconducting Qubits Using Direct Microwave Synthesis in Higher Nyquist Zones
Control electronics for superconducting quantum processors have strict requirements for accurate command of the sensitive quantum states of their qubits. Hinging on the purity of ultra-phase-stable
oscillators to upconvert very-low-noise baseband pulses, conventional control systems can become prohibitively complex and expensive when scaling to larger quantum devices, especially as high sampling rates become desirable for fine-grained pulse shaping. Few-GHz radio-frequency digital-to-analog converters (RF DACs) present a more economical avenue for high-fidelity control while simultaneously providing greater command over the spectrum of the synthesized signal. Modern RF DACs with extra-wide bandwidths are able to directly synthesize tones above their sampling rates, thereby keeping the system clock rate at a level compatible with modern digital logic systems while still being able to generate high-frequency pulses with arbitrary profiles. We have incorporated custom superconducting qubit control logic into off-the-shelf hardware capable of low-noise pulse synthesis up to 7.5 GHz using an RF DAC clocked at 5 GHz. Our approach enables highly linear and stable microwave synthesis over a wide bandwidth, giving rise to resource-efficient control and the potential for reducing the required number of cables entering the cryogenic environment. We characterize the performance of the hardware using a five-transmon superconducting device and demonstrate consistently reduced two-qubit gate error (as low as 1.8%) accompanied by superior control chain linearity compared to traditional configurations. The exceptional flexibility and stability further establish a foundation for scalable quantum control beyond intermediate-scale devices.
Granular superconductors for high kinetic inductance and low loss quantum devices
Granular aluminum is a promising material for high kinetic inductance devices such as qubit circuits. It has the advantage over atomically disordered materials such as NbN_x, to maintain
a high kinetic inductance concomitantly with a high quality factor. We show that high quality nano-scale granular aluminum films having a sharp superconducting transition with normal state resistivity values of the order of 1×10^5 \mu\Omega cm and kinetic inductance values of the order of 10 nH/sq can be obtained, surpassing state of the art values. We argue that this is a result of the different nature of the metal-to-insulator transition, being electronic correlations driven (Mott type) in the former and disorder driven (Anderson type) in the latter.