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
03
Jun
2024
Driving a Josephson Traveling Wave Parametric Amplifier into chaos: effects of a non-sinusoidal current-phase relation
In this work, we develop a comprehensive numerical analysis of the dynamic response of a Josephson Traveling Wave Parametric Amplifier (JTWPA) by varying the driving parameters, with
a focus on the pathways leading to chaotic behavior. By tuning the working conditions, we capture the broad spectrum of dynamical regimes accessible to JTWPAs, delineating the settings under which transition to chaos occurs. Furthermore, we extend our investigation to device formed by junctions characterized by a non–sinusoidal current phase relation (CPR) and exploring the impact of its shape on the amplifier’s performance. Through the study of gain characteristics, Poincaré sections, and Fourier spectra, we provide an in-depth understanding of how non-linearity and CPR nonsinusoidality influence the JTWPAs‘ operational effectiveness and stability. This investigation offers insights into optimizing the device designs for enhanced performance and robustness against chaotic disruptions, in order to establish a framework for predicting and controlling JTWPA behavior in practical applications. This effort will pave the way for the development of devices with tailored dynamic responses and for advancements in quantum computing and precision measurement technologies, where stability and high fidelity are of paramount importance.
01
Jun
2024
A Near Quantum Limited Sub-GHz TiN Kinetic Inductance Traveling Wave Parametric Amplifier Operating in a Frequency Translating Mode
We present the design and experimental characterization of a kinetic-inductance traveling-wave parametric amplifier (KI-TWPA) for sub-GHz frequencies. KI-TWPAs amplify signals through
nonlinear mixing processes supported by the nonlinear kinetic inductance of a superconducting transmission line. The device described here utilizes a compactly meandered TiN microstrip transmission line to achieve the length needed to amplify sub-GHz signals. It is operated in a frequency translating mode where the amplified signal tone is terminated at the output of the amplifier, and the idler tone at approximately 2.5~GHz is brought out of the cryostat. By varying the pump frequency, a gain of up to 22 dB was achieved in a tunable range from about 450 to 850~MHz. Use of TiN as the nonlinear element allows for a reduction of the required pump power by roughly an order of magnitude relative to NbTiN, which has been used for previous KI-TWPA implementations. This amplifier has the potential to enable high-sensitivity and high-speed measurements in a wide range of applications, such as quantum computing, astrophysics, and dark matter detection.
31
Mai
2024
Josephson junctions, superconducting circuits, and qubit for quantum technologies
In the realm of physics, a pivotal moment occurred six decades ago when Brian Josephson made a groundbreaking prediction, setting in motion a series of events that would eventually
earn him the prestigious Nobel Prize eleven years later. This prediction centered around what is now known as the Josephson effect, a phenomenon with far-reaching implications. At the heart of this effect lies the Josephson junction (JJ), a device that has become a linchpin in various scientific applications. This chapter delves into the foundational principles of the Josephson effect and the remarkable properties of JJs. From their role in metrology to their application in radiation detectors, these junctions have ushered in a new era of electronics. Exploiting the unique features of superconductive devices, such as high speed, low dissipation, and dispersion, JJs find today practical implementation in the development of superconductive qubits and nanotechnology applications.
30
Mai
2024
Investigating pump harmonics generation in a SNAIL-based Traveling Wave Parametric Amplifier
Traveling Wave Parametric Amplifiers (TWPAs) are extensively employed in experiments involving weak microwave signals for their highly desirable quantum-limited and broadband characteristics.
However, TWPAs‘ broadband nature comes with the disadvantage of admitting the activation of spurious nonlinear processes, such as harmonics generation, that can potentially degrade amplification performance. Here we experimentally investigate a Josephson TWPA device with SNAIL (Superconducting Nonlinear Asymmetric Inductive Element)-based unit cells focusing on the amplification behaviour along with the generation of second and third harmonics of the pump. By comparing experimental results with transient numerical simulations, we demonstrate the influence of Josephson junctions‘ fabrication imperfections on the occurrence of harmonics and on the gain behaviour.
29
Mai
2024
Parity-dependent state transfer for direct entanglement generation
As quantum information technologies advance they face challenges in scaling and connectivity. In particular, two necessities remain independent of the technological implementation:
the need for connectivity between distant qubits and the need for efficient generation of entanglement. Perfect State Transfer is a technique which realises the time optimal transfer of a quantum state between distant nodes of qubit lattices with only nearest-neighbour couplings, hence providing an important tool to improve device connectivity. Crucially, the transfer protocol results in effective parity-dependent non-local interactions, extending its utility to the efficient generation of entangled states. Here, we experimentally demonstrate Perfect State Transfer and the generation of multi-qubit entanglement on a chain of superconducting qubits. The system consists of six fixed-frequency transmon qubits connected by tunable couplers, where the couplings are controlled via parametric drives. By simultaneously activating all couplings and engineering their individual amplitudes and frequencies, we implement Perfect State Transfer on up to six qubits and observe the respective single-excitation dynamics for different initial states. We then apply the protocol in the presence of multiple excitations and verify its parity-dependent property, where the number of excitations within the chain controls the phase of the transferred state. Finally, we utilise this property to prepare a multi-qubit Greenberger-Horne-Zeilinger state using only a single transfer operation, demonstrating its application for efficient entanglement generation.
28
Mai
2024
Evaluating radiation impact on transmon qubits in above and underground facilities
Superconducting qubits can be sensitive to abrupt energy deposits caused by cosmic rays and ambient radioactivity. Previous studies have focused on understanding possible correlated
effects over time and distance due to cosmic rays. In this study, for the first time, we directly compare the response of a transmon qubit measured initially at the Fermilab SQMS above-ground facilities and then at the deep underground Gran Sasso Laboratory (INFN-LNGS, Italy). We observe same average qubit lifetime T1 of roughly 80 microseconds at above and underground facilities. We then apply a fast decay detection protocol and investigate the time structure, sensitivity and relative rates of triggered events due to radiation versus intrinsic noise, comparing above and underground performance of several high-coherence qubits. Using gamma sources of variable activity we calibrate the response of the qubit to different levels of radiation in an environment with minimal background radiation. Results indicate that qubits respond to a strong gamma source and it is possible to detect particle impacts. However, when comparing above and underground results, we do not observe a difference in radiation induced-like events for these sapphire and niobium-based transmon qubits. We conclude that the majority of these events are not radiation related and to be attributed to other noise sources which by far dominate single qubit errors in modern transmon qubits.
24
Mai
2024
Multi-frequency control and measurement of a spin-7/2 system encoded in a transmon qudit
Qudits hold great promise for efficient quantum computation and the simulation of high-dimensional quantum systems. Utilizing a local Hilbert space of dimension d > 2 is known to speed
up certain quantum algorithms relative to their qubit counterparts given efficient local qudit control and measurement. However, the direct realization of high-dimensional rotations and projectors has proved challenging, with most experiments relying on decompositions of SU(d) operations into series of rotations between two-level subspaces of adjacent states and projective readout of a small number of states. Here we employ simultaneous multi-frequency drives to generate rotations and projections in an effective spin-7/2 system by mapping it onto the energy eigenstates of a superconducting circuit. We implement single-shot readout of the 8 states using a multi-tone dispersive readout (F_assignment = 88.3%) and exploit the strong nonlinearity in a high EJ/EC transmon to simultaneously address each transition and realize a spin displacement operator. By combining the displacement operator with a virtual SNAP gate, we realize arbitrary single-qudit unitary operations in O(d) physical pulses and extract spin displacement gate fidelities ranging from 0.997 to 0.989 for virtual spins of size j = 1 to j = 7/2. These native qudit operations could be combined with entangling operations to explore qudit-based error correction or simulations of lattice gauge theories with qudits. Our multi-frequency approach to qudit control and measurement can be readily extended to other physical platforms that realize a multi-level system coupled to a cavity and can become a building block for efficient qudit-based quantum computation and simulation.
23
Mai
2024
Parametrically controlled chiral interface for superconducting quantum devices
Nonreciprocal microwave routing plays a crucial role for measuring quantum circuits, and allows for realizing cascaded quantum systems for generating and stabilizing entanglement between
non-interacting qubits. The most commonly used tools for implementing directionality are ferrite-based circulators. These devices are versatile, but suffer from excess loss, a large footprint, and fixed directionality. For utilizing nonreciprocity in scalable quantum circuits it is desirable to develop efficient integration of low-loss and in-situ controllable directional elements. Here, we report the design and experimental realization of a controllable directional interface that may be integrated directly with superconducting qubits. In the presented device, nonreciprocity is realized through a combination of interference and phase-controlled parametric pumping. We have achieved a maximum directionality of around 30\,dB, and the performance of the device is predicted quantitatively from independent calibration measurements. Using the excellent agreement of model and experiment, we predict that the circuit will be useable as a chiral qubit interface with inefficiencies at the one-percent level or below. Our work provides a route toward isolator-free qubit readout schemes and high-fidelity entanglement generation in all-to-all connected networks of superconducting quantum devices.
21
Mai
2024
Circuit QED theory of direct and dual Shapiro steps with finite-size transmission line resonators
We investigate the occurrence of direct and dual Shapiro steps for a Josephson junction coupled to a finite-size transmission line resonator. We treat both problems through a circuit
QED approach with a large, but finite number of photon modes. For the dual case, we do not assume the (approximate) charge-phase duality, but include the full multi-band dynamics for the Josephson junction. Mean-field equations within such Hamiltonian approach reproduce the result obtained through a dissipative classical equation when the number of transmission line modes is large enough. To account for quantum and thermal fluctuations, we go beyond the mean-field treatment within a truncated Wigner approach. The fluctuations are shown to modify both the direct and the dual steps. We show how the dual steps are very sensitive to these fluctuations and identify the key physical parameters for the junction and the transmission line controlling their robustness, which is essential for applications to close the quantum metrological triangle.
20
Mai
2024
Cryogenic growth of tantalum thin films for low-loss superconducting circuits
Motivated by recent advancements highlighting Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have exploredthe effect of cryogenic temperatures on the growth of Ta and its integration in superconducting circuits. Cryogenic growth of Ta using a low temperature molecular beam epitaxy (MBE) system is found to stabilize single phase α-Ta on several different substrates, which include Al2O3(0001), Si(001), Si(111), SiNx, and GaAs(001). The substrates are actively cooled down to cryogenic temperatures and remain < 20 K during the Ta deposition. X-ray θ-2θ diffraction after warming to room temperature indicates the formation of polycrystalline α-Ta. The 50 nm α-Ta films grown on Al2O3(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity (ρ300K) of 13.4 μΩcm, a residual resistivity ratio (RRR) of 17.3 and a superconducting transition temperature (TC) of 4.14 K, which are comparable to bulk values. In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an RMS roughness of 0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures show higher ρ300K, lower RRR and Tc, and increased β-Ta content. Coplanar waveguide resonators with a gap width of 3 μm fabricated from cryogenically grown Ta on Si(111) and Al2O3(0001) show low power Qi of 1.9 million and 0.7 million, respectively, indicating polycrystalline α-Ta films may be promising for superconducting qubit applications even though they are not fully epitaxial.[/expand]