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
04
Okt
2021
Mediated interactions beyond the nearest neighbor in an array of superconducting qubits
We consider mediated interactions in an array of floating transmons, where each qubit capacitor consists of two superconducting pads galvanically isolated from ground. Each such pair
contributes two quantum degrees of freedom, one of which is used as a qubit, while the other remains fixed. However, these extraneous modes can generate coupling between the qubit modes that extends beyond the nearest neighbor. We present a general formalism describing the formation of this coupling and calculate it for a one-dimensional chain of transmons. We show that the strength of coupling and its range (that is, the exponential falloff) can be tuned independently via circuit design to realize a continuum from nearest-neighbor-only interactions to interactions that extend across the length of the chain. We present designs with capacitance and microwave simulations showing that various interaction configurations can be achieved in realistic circuits. Such coupling could be used in analog simulation of different quantum regimes or to increase connectivity in digital quantum systems. Thus mechanism must also be taken into account in other types of qubits with extraneous modes.
01
Okt
2021
The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors
We introduce a Xilinx RFSoC-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short) which supports the direct synthesis of control pulses with carrier
frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC (RF System-on-Chip) FPGA \cite{zcu111}, custom firmware and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system, as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average Clifford gate fidelity of avg=99.93%. All of the schematics, firmware, and software are open-source \cite{QICKrepo}.
Fast Flux Entangling Gate for Fluxonium Circuits
We analyze a high-fidelity two-qubit gate using fast flux pulses on superconducting fluxonium qubits. The gate is realized by temporarily detuning magnetic flux through fluxonium loop
away from the half flux quantum sweet spot. We simulate dynamics of two capacitively coupled fluxoniums during the flux pulses and optimize the pulse parameters to obtain a highly accurate iswap‾‾‾‾‾‾√-like entangling gate. We also evaluate the effect of the flux noise and qubit relaxation on the gate fidelity. Our results demonstrate that the gate error remains below 10−4 for currently achievable magnitude of the flux noise and qubit relaxation time.
30
Sep
2021
Bimodal Approach for Noise Figures of Merit Evaluation in Quantum-Limited Josephson Traveling Wave Parametric Amplifiers
The advent of ultra-low noise microwave amplifiers revolutionized several research fields demanding quantum-limited technologies. Exploiting a theoretical bimodal description of a linear
phase-preserving amplifier, in this contribution we analyze some of the intrinsic properties of a model architecture (i.e., an rf-SQUID based Josephson Traveling Wave Parametric Amplifier) in terms of amplification and noise generation for key case study input states (Fock and coherents). Furthermore, we present an analysis of the output signals generated by the parametric amplification mechanism when thermal noise fluctuations feed the device.
29
Sep
2021
Predicting Dynamics of Transmon Qubit-Cavity Systems with Recurrent Neural Networks
Developing accurate and computationally inexpensive models for the dynamics of open-quantum systems is critical in designing new qubit platforms by first understanding their mechanisms
of decoherence and dephasing. Current models based on solutions to master equations are not sufficient in capturing the non-Markovian dynamics at play and suffer from large computational costs. Here, we present a method of overcoming this by using a recurrent neural network to obtain effective solutions to the Lindblad master equation for a coupled transmon qubit-cavity system. We present the training and testing performance of the model trained a simulated dataset and demonstrate its ability to map microscopic dissipative mechanisms to quantum observables.
28
Sep
2021
Protocol for temperature sensing using a three-level transmon circuit
We present a method for in situ temperature measurement of superconducting quantum circuits, by using the first three levels of a transmon device to which we apply a sequence of π
gates. Our approach employs projective dispersive readout and utilizes the basic properties of the density matrix associated with thermal states. This method works with an averaging readout scheme and does not require a single-shot readout setup. We validate this protocol by performing thermometry in the range of 50 mK – 200 mK, corresponding to a range of residual populations 1%−20% for the first excited state and 0.02%−3% for the second excited state.
27
Sep
2021
Circuit QED simulator of two-dimensional Su-Schrieffer-Hegger model: magnetic field induced topological phase transition in high-order topological insulators
High-order topological insulator (HOTI) occupies an important position in topological band theory due to its exotic bulk-edge correspondence. Recently, it has been predicted that external
magnetic field can introduce rich physics into two-dimensional (2D) HOTIs. However, up to now the theoretical description is still incomplete and the experimental realization is still lacking. Here we investigate the influence of continuously varying magnetic field on 2D Su-Schriffer-Heeger lattice, which is one of the most celebrated HOTI models, and proposed a corresponding circuit quantum electrodynamics (cQED) simulator. Our numerical calculation shows that the zero energy corner modes (ZECMs), which can serve as evidence of the high order topology of the lattice, exhibit exotic and rich dependence on the imposed magnetic field and the inhomogeneous hopping strength. Moreover, by exploiting the parametric conversion method, we can establish time- and site-resolved tunable hopping constants in the proposed cQED simulator, thus providing an ideal platform for simulating the magnetic field induced topological phase transitions in 2D HOTIs. Since the high-order topological phases of the proposed model can be characterized by the existence of the ZECMs on the lattice, we further investigate the corner site excitation of the lattice in the steady state limit. Our numerical results imply that the predicted topological phase transitions can be unambiguously identified by the steady-state photon number measurement of the corner sites and their few neighbors. Requiring only current level of technology, our scheme can be readily tested in experiment and may pave an alternative way towards the future investigation of HOTIs in the presence of magnetic field, disorder, and strong correlation.
26
Sep
2021
A General Circuit Analysis and Simulation Method for Superconducting Quantum Interference Devices
The Superconducting Quantum Interference Device (SQUID) is an extremely sensitive flux-to-voltage converter widely used in weak magnetic signal detection systems. It is the superconducting
circuit composed of Josephson junctions and superconducting loops. However, the analysis is usually based on superconducting physics rather than the conventional circuit theorems. This article presents a general circuit analysis method using only the conventional circuit variables and laws to simplify the analysis and simulation of SQUID circuits. The method unifies the descriptions of Josephson junctions and non-superconducting elements with a general non-linear inductance concept; and derives the uniform SQUID circuit equations with the common circuit laws used for both superconducting and normal circuits. The uniform circuit equation and dynamic model show that the only element making the SQUID distinct from the non-superconducting circuits is the cosine potential introduced by the Josephson current. This general analysis method bridges the gap between the superconductive SQUID circuits and the conventional normal circuits for the electronics engineers trained with the conventional circuit theory.
24
Sep
2021
Qubit alive thanks to the anomaly
We present an exact full symmetry analysis of the 0-π superconducting circuit. We identify points in control parameter space of enhanced anomalous symmetry, which imposes robust twofold
degeneracy of its ground state, that is for all values of the energy parameters of the model. We show, both analytically and numerically, how this anomalous symmetry is maintained in the low-energy sector, thus providing us with a strong candidate for robust qubit engineering.
Coherence Time Enhancement of Interacting Two-Level Systems in Aluminum Superconducting Resonators
Superconducting resonators are widely used in many applications such as qubit readout for quantum computing applications, and kinetic inductance detectors. These resonators are susceptible
to numerous loss and noise mechanisms under microwave excitation, especially the dissipation due to non-equilibrium quasi-particles and two-level systems (TLS), which can result in a decrease of the superconducting intrinsic quality factor (Qi) in high quality superconducting resonators. Particularly in the few-photon and low temperature (T) regime, TLS losses can become a dominant loss mechanism. In this study, novel aluminum half-wavelength resonators are investigated, focusing on the loss properties at extra-low power and low temperature. An unusual increase of Qi(T) with deceasing temperature is observed. This behavior is attributed to the increase of TLS coherence time (T2) at ultra-low temperatures and powers. This T2 increase is consistent with other work on resonant frequency noise in resonators and measurements of individual TLS, and likely arises from interacting TLS in the aluminum half-wavelength resonators.