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
23
Okt
2022
A microwave scattering spectral method to detect the nanomechanical vibrations embedded in a superconducting qubit
Nanomechanical resonators (NMRs), as the quantum mechanical sensing probers, have played the important roles for various high-precision quantum measurements. Differing from the previous
emission spectral probes (i.e., the NMR modified the atomic emission), in this paper we propose an alternative approach, i.e., by probing the scattering spectra of the quantum mechanical prober coupled to the driving microwaves, to characterize the physical features of the NMR embedded in a rf-SQUID based superconducting qubit. It is shown that, from the observed specifical frequency points in the spectra, i.e., either the dips or the peaks, the vibrational features (i.e., they are classical vibration or quantum mechanical one) and the physical parameters (typically such as the vibrational frequency and displacements) of the NMR can be determined effectively. The proposal is feasible with the current technique and should be useful to design the desired NMRs for various quantum metrological applications.
22
Okt
2022
Coherent optical control of a superconducting microwave cavity via electro-optical dynamical back-action
Recent quantum technology advances have established precise quantum control of various microscopic systems involving optical, microwave, spin, and mechanical degrees of freedom. It
is a timely challenge to realize hybrid quantum devices that leverage the full potential of each component. Interfaces based on cryogenic cavity electro-optic systems are particularly promising, due to the direct interaction between microwave and optical fields in the quantum regime. However, low coupling rates and excess back-action from the pump laser have precluded quantum optical control of superconducting circuits. Here we report the coherent control of a microwave cavity mode using laser light in a multimode device at millikelvin temperature with near unity cooperativity, as manifested by the observation of electro-optically induced transparency and absorption due to the electro-optical dynamical back-action. We show that both the stationary and instantaneous pulsed response of the microwave and optical modes comply with the coherent electro-optical interaction and reveal only minuscule amount of excess back-action with an unanticipated time delay. Our demonstration represents a key step to attain full quantum control of microwave circuits using laser light, with possible applications ranging from optical quantum non-demolition measurements of microwave fields beyond the standard quantum limit, optical microwave ground state cooling and squeezing, to quantum transduction, entanglement generation and hybrid quantum networks.
21
Okt
2022
Tuning the inductance of Josephson junction arrays without SQUIDs
It is customary to use arrays of superconducting quantum interference devices (SQUIDs) for implementing magnetic field-tunable inductors. Here, we demonstrate an equivalent tunability
in a (SQUID-free) array of single Al/AlOx/Al Josephson tunnel junctions. With the proper choice of junction geometry, a perpendicularly applied magnetic field bends along the plane of the superconductor and focuses into the tunnel barrier region due to a demagnetization effect. Consequently, the Josephson inductance can be efficiently modulated by the Fraunhoffer-type supercurrent interference. The elimination of SQUIDs not only simplifies the device design and fabrication, but also facilitates a denser packing of junctions and, hence, a higher inductance per unit length. As an example, we demonstrate a transmission line, the wave impedance of which is field-tuned in the range of 4−8 kΩ, centered around the important value of the resistance quantum h/(2e)2≈6.5 kΩ.
19
Okt
2022
Scaling Superconducting Quantum Computers with Chiplet Architectures
Fixed-frequency transmon quantum computers (QCs) have advanced in coherence times, addressability, and gate fidelities. Unfortunately, these devices are restricted by the number of
on-chip qubits, capping processing power and slowing progress toward fault-tolerance. Although emerging transmon devices feature over 100 qubits, building QCs large enough for meaningful demonstrations of quantum advantage requires overcoming many design challenges. For example, today’s transmon qubits suffer from significant variation due to limited precision in fabrication. As a result, barring significant improvements in current fabrication techniques, scaling QCs by building ever larger individual chips with more qubits is hampered by device variation. Severe device variation that degrades QC performance is referred to as a defect. Here, we focus on a specific defect known as a frequency collision.
When transmon frequencies collide, their difference falls within a range that limits two-qubit gate fidelity. Frequency collisions occur with greater probability on larger QCs, causing collision-free yields to decline as the number of on-chip qubits increases. As a solution, we propose exploiting the higher yields associated with smaller QCs by integrating quantum chiplets within quantum multi-chip modules (MCMs). Yield, gate performance, and application-based analysis show the feasibility of QC scaling through modularity.
18
Okt
2022
Resolving Fock states near the Kerr-free point of a superconducting resonator
We have designed a tunable nonlinear resonator terminated by a SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement). Such a device possesses a sweet spot in which the external
magnetic flux allows to suppress the Kerr interaction. We have excited photons near this Kerr-free point and characterized the device using a transmon qubit. The excitation spectrum of the qubit allows to observe photon-number-dependent frequency shifts about nine times larger than the qubit linewidth. Our study demonstrates a compact integrated platform for continuous-variable quantum processing that combines large couplings, considerable relaxation times and excellent control over the photon mode structure in the microwave domain.
Double Upconversion for Superconducting Qubit Control realised using Microstrip Filters
Superconducting qubits provide a promising platform for physically realising quantum computers at scale. Such devices require precision control at microwave frequencies. Common practice
is to synthesise such control signals using IQ modulation, requiring calibration of a in-phase (I) and quadrature (Q) signals alongside two DC offsets to generate pure tones. This paper presents an economic physical implementation of an alternative method referred to as double upconversion which requires considerably less hardware calibration and physical resources to operate a qubit. A physical circuit was created using standard PCB design techniques for microstrip filters and two common RF mixers. This circuit was then utilised to successfully control a superconducting transmon qubit. When using proper RF shielding, qubit tones were demonstrated with over 70dB of spurious-free dynamic range across the entire operational spectrum of a transmon qubit.
Circuit quantum electrodynamic model of dissipative-dispersive Josephson traveling-wave parametric amplifiers
We present a quantum mechanical model for a four-wave mixing Josephson traveling-wave parametric amplifier including substrate losses and associated thermal fluctuations. Under the
assumption of a strong undepleted classical pump tone, we derive an analytic solution for the bosonic annihilation operator of the weak signal photon field using temporal equations of motion in a reference timeframe, including chromatic dispersion. From this result, we can predict the asymmetric gain spectrum of a Josephson traveling-wave parametric amplifier due to non-zero substrate losses. We also predict the equivalent added input noise including quantum fluctuations as well as thermal noise contributions. Our results are in excellent agreement with recently published experimental data.
14
Okt
2022
One-Dimensional Maxwell-Schrodinger Hybrid Simulation of Transmon Qubits
Transmon quantum bits (qubits) are one of the most popular experimental platforms currently being pursued for developing quantum information processing technologies. In these devices,
applied microwave pulses are used to control and measure the state of the transmon qubit. Currently, the design of the microwave pulses for these purposes is done through simple theoretical and/or numerical models that neglect how the transmon can modify the applied microwave field. In this work, we present the formulation and finite element time domain discretization of a semiclassical Maxwell-Schrodinger hybrid method for describing the dynamics of a transmon qubit capacitively coupled to a transmission line system. Numerical results are presented using this Maxwell-Schrodinger method to characterize the control and measurement of the state of a transmon qubit. We show that our method matches standard theoretical predictions in relevant operating regimes, and also show that our method produces physically meaningful results in situations where the theoretical models break down. In the future, our method can be used to explore broader operating regimes to search for more effective control and measurement protocols for transmon qubits.
10
Okt
2022
Efficient qutrit gate-set tomography on a transmon
Gate-set tomography enables the determination of the process matrix of a set of quantum logic gates, including measurement and state preparation errors. Here we propose an efficient
method to implement such tomography on qutrits, using only gates in the qutrit Clifford group to construct preparation and measurement fiducials. Our method significantly reduces computational overhead by using the theoretical minimum number of measurements and directly parametrizing qutrit Hadamard gates. We demonstrate qutrit gate-set tomography on a superconducting transmon, and find good agreement of average gate infidelity with qutrit randomized benchmarking.
Intermodulation Distortion in a Josephson Traveling Wave Parametric Amplifier
Josephson traveling wave parametric amplifiers enable the amplification of weak microwave signals close to the quantum limit with large bandwidth, which has a broad range of applications
in superconducting quantum computing and in the operation of single-photon detectors. While the large bandwidth allows for their use in frequency-multiplexed detection architectures, an increased number of readout tones per amplifier puts more stringent requirements on the dynamic range to avoid saturation. Here, we characterize the undesired mixing processes between the different frequency-multiplexed tones applied to a Josephson traveling wave parametric amplifier, a phenomenon also known as intermodulation distortion. The effect becomes particularly significant when the amplifier is operated close to its saturation power. Furthermore, we demonstrate that intermodulation distortion can lead to significant crosstalk and reduction of fidelity for multiplexed readout of superconducting qubits. We suggest using large detunings between the pump and signal frequencies to mitigate crosstalk. Our work provides insights into the limitations of current Josephson traveling wave parametric amplifiers and highlights the importance of performing further research on these devices.