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
11
Mrz
2022
Quantum Coherence in Loopless Superconductive Networks
Measurements indicating that planar networks of superconductive islands connected by Josephson junctions display long range quantum coherence are reported. The networks consist of superconducting
islands connected by Josephson junctions and have a tree-like topological structure containing no loops. Enhancements of superconductive gap over specific branches of the networks and sharp increases of pair currents are the main signatures of the coherent states and, in order to unambiguously attribute the observed effects to branches being embedded in the networks, comparisons with geometrically equivalent, but isolated, counterparts are reported. Tuning the Josephson coupling energy by an external magnetic field generates increases of the Josephson currents, along the above mentioned specific branches, which follow a functional dependence typical of phase transitions. Results are presented for double comb and star geometry networks and in both cases the observed effects provide positive quantitative evidence of the predictions of existing theoretical models.
10
Mrz
2022
Probing the Jaynes-Cummings ladder with spin circuit quantum electrodynamics
Circuit quantum electrodynamics with electron spins (spin circuit QED) enables long-range interaction and single-shot readout of spin qubits, which pave the way to large-scale spin
qubit processors. Recent experimental work reported an additional feature in the vacuum Rabi splitting of peaks in the resonator transmission spectrum, which has remained unexplained until now. In this work, we show that this feature originates from transitions between excited states in the Jaynes-Cummings ladder, which are not included in commonly used input-output models for spin circuit QED. We present an input-output framework that does include these effects and is based on a numerical solution of a Lindblad master equation in the stationary limit. In new experiments, we first reproduce previous observations and then reveal both excited-state transitions and multi-photon transitions by increasing the probe power and using two-tone spectroscopy. This ability to probe the Jaynes-Cummings ladder in spin circuit QED is an important new step in the development of the platform. In future work, our input-output framework can be straightforwardly extended to accurately describe resonator-mediated interactions between two remote spins.
08
Mrz
2022
Weakly Flux-Tunable Superconducting Qubit
Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cPhase gates, facilitate fast reset protocols, avoid qubit-frequency collisions
in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly important for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with non-tunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmon-like qubit, which exhibits a frequency tunability range as small as 43 MHz, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise.
Entangling transmons with low-frequency protected superconducting qubits
Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits. However, many protected superconducting
qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging. In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconducting island. By tuning the external flux on the transmon, we show that non-computational states can mediate a two-qubit entangling gate that preserves the Cooper-pair parity independent of the detailed pulse sequence. Interestingly, the entangling gate bears similarities to a controlled-phase gate in conventional transmon devices. Hence, our results suggest that standard high-precision gate calibration protocols could be repurposed for operating hybrid qubit devices.
Weakly Flux-Tunable Superconducting Qubit
Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cPhase gates, facilitate fast reset protocols, avoid qubit-frequency collisions
in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly important for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with non-tunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmon-like qubit, which exhibits a frequency tunability range as small as 43 MHz, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise.
04
Mrz
2022
The transition regime between traveling-wave and resonant parametric amplifier
Traveling wave parametric amplifiers based on kinetic or Josephson nonlinear inductance are known to be microwave quantum limited amplifiers. Usually, a perfectly impedance-matched
model is used to describe their characteristics in terms of standard coupled mode theory. In practice, the amplifiers are unmatched nonlinear devices with finite length, exhibiting ripples in the transmission. Since commonly used models fail to describe the ripples of real parametric amplifiers, here we are introducing a theoretical approach with non-negligible reflections, which provides their gain and bandwidth properly for both 3-wave and 4-wave mixing. Predictions of the model are experimentally demonstrated on two types of TWPA, based on coplanar waveguides with a central wire consisting of i) high kinetic inductance superconductor, and ii) array of 2000 Josephson junctions.
03
Mrz
2022
Generation of control signals using second-Nyquist zone technique for superconducting qubit devices
There is growing interest in developing integrated room temperature control electronics for the control and measurement of superconducting devices for quantum computing applications.
With the availability of faster DACs, it has become possible to generate microwave signals with amplitude and phase controls directly without requiring any analog mixer. In this report, we use the evaluation kit ZCU111 to generate vector microwave pulses using the second-Nyquist zone technique. We characterize the performance of the signal generation and measure amplitude variation across second Nyquist zone, single-sideband phase noise, and spurious-free dynamic range. We further perform various time-domain measurements to characterize a superconducting transmon qubit and benchmark our results against traditionally used analog mixer setups.
02
Mrz
2022
Energetic cost of measurements using quantum, coherent, and thermal light
Quantum measurements are basic operations that play a critical role in the study and application of quantum information. We study how the use of quantum, coherent, and classical thermal
states of light in a circuit quantum electrodynamics setup impacts the performance of quantum measurements, by comparing their respective measurement backaction and measurement signal to noise ratio per photon. In the strong dispersive limit, we find that thermal light is capable of performing quantum measurements with comparable efficiency to coherent light, both being outperformed by single-photon light. We then analyze the thermodynamic cost of each measurement scheme. We show that single-photon light shows an advantage in terms of energy cost per information gain, reaching the fundamental thermodynamic cost.
On-Demand Directional Photon Emission using Waveguide Quantum Electrodynamics
Routing quantum information between non-local computational nodes is a foundation for extensible networks of quantum processors. Quantum information can be transferred between arbitrary
nodes by photons that propagate between them, or by resonantly coupling nearby nodes. Notably, conventional approaches involving propagating photons have limited fidelity due to photon loss and are often unidirectional, whereas architectures that use direct resonant coupling are bidirectional in principle, but can generally accommodate only a few local nodes. Here, we demonstrate high-fidelity, on-demand, bidirectional photon emission using an artificial molecule comprising two superconducting qubits strongly coupled to a waveguide. Quantum interference between the photon emission pathways from the molecule generate single photons that selectively propagate in a chosen direction. This architecture is capable of both photon emission and capture, and can be tiled in series to form an extensible network of quantum processors with all-to-all connectivity.
28
Feb
2022
A superconducting qubit with noise-insensitive plasmon levels and decay-protected fluxon states
The inductively shunted transmon (IST) is a superconducting qubit with exponentially suppressed fluxon transitions and a plasmon spectrum approximating that of the transmon. It shares
many characteristics with the transmon but offers charge offset insensitivity for all levels and precise flux tunability with quadratic flux noise suppression. In this work we propose and realize IST qubits deep in the transmon limit where the large geometric inductance acts as a mere perturbation. With a flux dispersion of only 5.1 MHz we reach the ’sweet-spot everywhere‘ regime of a SQUID device with a stable coherence time over a full flux quantum. Close to the flux degeneracy point the device reveals tunneling physics between the two quasi-degenerate ground states with typical observed lifetimes on the order of minutes. In the future, this qubit regime could be used to avoid leakage to unconfined transmon states in high-power read-out or driven-dissipative bosonic qubit realizations. Moreover, the combination of well controllable plasmon transitions together with stable fluxon states in a single device might offer a way forward towards improved qubit encoding schemes.