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
28
Jun
2023
High-Q trenched aluminum coplanar resonators with an ultrasonic edge microcutting for superconducting quantum devices
Dielectric losses are one of the key factors limiting the coherence of superconducting qubits. The impact of materials and fabrication steps on dielectric losses can be evaluated using
coplanar waveguide (CPW) microwave resonators. Here, we report on superconducting CPW microwave resonators with internal quality factors systematically exceeding 5×106 at high powers and 2×106 (with the best value of 4.4×106) at low power. Such performance is demonstrated for 100-nm-thick aluminum resonators with 7-10.5 um center trace on high-resistivity silicon substrates commonly used in quantum Josephson junction circuits. We investigate internal quality factors of the resonators with both dry and wet aluminum etching, as well as deep and isotropic reactive ion etching of silicon substrate. Josephson junction compatible CPW resonators fabrication process with both airbridges and silicon substrate etching is proposed. Finally, we demonstrate the effect of airbridges positions and extra process steps on the overall dielectric losses. The best quality factors are obtained for the wet etched aluminum resonators and isotropically removed substrate with the proposed ultrasonic metal edge microcutting.
27
Jun
2023
Microwave characterization of tantalum superconducting resonators on silicon substrate with niobium buffer layer
Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered
cubic lattice (α-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches 2×107 at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have identified the primary cause of TLS loss to be the amorphous silicon layer at the film-substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS-TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.
26
Jun
2023
Observing Parity Time Symmetry Breaking in a Josephson Parametric Amplifier
A coupled two-mode system with balanced gain and loss is a paradigmatic example of an open quantum system that can exhibit real spectra despite being described by a non-Hermitian Hamiltonian.
We utilize a degenerate parametric amplifier operating in three-wave mixing mode to realize such a system of balanced gain and loss between the two quadrature modes of the amplifier. By examining the time-domain response of the amplifier, we observe a characteristic transition from real-to-imaginary energy eigenvalues associated with the Parity-Time-symmetry-breaking transition.
21
Jun
2023
The Multimode Character of Quantum States Released from a Superconducting Cavity
Quantum state transfer by propagating wave packets of electromagnetic radiation requires tunable couplings between the sending and receiving quantum systems and the propagation channel
or waveguide. The highest fidelity of state transfer in experimental demonstrations so far has been in superconducting circuits. Here, the tunability always comes together with nonlinear interactions, arising from the same Josephson junctions that enable the tunability. The resulting non-linear dynamics correlates the photon number and spatio-temporal degrees of freedom and leads to a multi-mode output state, for any multi-photon state. In this work, we study as a generic example the release of complex quantum states from a superconducting resonator, employing a flux tunable coupler to engineer and control the release process. We quantify the multi-mode character of the output state and discuss how to optimize the fidelity of a quantum state transfer process with this in mind.
Observation and manipulation of quantum interference in a Kerr parametric oscillator
Quantum tunneling is the phenomenon that makes superconducting circuits „quantum“. Recently, there has been a renewed interest in using quantum tunneling in phase space
of a Kerr parametric oscillator as a resource for quantum information processing. Here, we report a direct observation of quantum interference induced by such tunneling in a planar superconducting circuit. We experimentally elucidate all essential properties of this quantum interference, such as mapping from Fock states to cat states, a temporal oscillation induced by the pump detuning, as well as its characteristic Rabi oscillations and Ramsey fringes. Finally, we perform gate operations as manipulations of the observed quantum interference. Our findings lay the groundwork for further studies on quantum properties of Kerr parametric oscillators and their use in quantum information technologies.
20
Jun
2023
Discriminating the Phase of a Coherent Tone with a Flux-Switchable Superconducting Circuit
We propose a new phase detection technique based on a flux-switchable superconducting circuit, the Josephson digital phase detector (JDPD), which is capable of discriminating between
two phase values of a coherent input tone. When properly excited by an external flux, the JDPD is able to switch from a single-minimum to a double-minima potential and, consequently, relax in one of the two stable configurations depending on the phase sign of the input tone. The result of this operation is digitally encoded in the occupation probability of a phase particle in either of the two JDPD wells. In this work, we demonstrate the working principle of the JDPD up to a frequency of 400 MHz with a remarkable agreement with theoretical expectations. As a future scenario, we discuss the implementation of this technique to superconducting qubit readout. We also examine the JDPD compatibility with the single-flux-quantum architecture, employed to fast-drive and measure the device state.
19
Jun
2023
Demonstration of a Quantum Noise Limited Traveling-Wave Parametric Amplifier
Recent progress in quantum computing and the development of novel detector technologies for astrophysics is driving the need for high-gain, broadband, and quantum-limited amplifiers.
We present a purely traveling-wave parametric amplifier (TWPA) using an inverted NbTiN microstrip and amorphous Silicon dielectric. Through dispersion engineering, we are able to obtain 50 Ω impedance matching and suppress undesired parametric processes while phase matching the three-wave-mixing amplification across a large range of frequencies. The result is a broadband amplifier operating with 20 dB gain and quantum-limited noise performance at 20 mK. At the single frequency where the amplifier is phase sensitive, we further demonstrate 8 dB of vacuum noise squeezing.
16
Jun
2023
Epitaxial α-Ta (110) film on a-plane sapphire substrate for superconducting qubits with long coherence times
Realization of practical superconducting quantum computing requires large-scale integration of qubits with long coherence times. It has been reported that {lpha}-Ta (110) film can
greatly improve the coherence times of qubits. Compared to the commonly used {\alpha}-Ta (110) film deposited on c-plane sapphire, {\alpha}-Ta (110) film can be epitaxially grown on a-plane sapphire because of the atomic relationships at their interface. Here, we demonstrate the growth of a large-scale well-ordered quasi-single crystal {\alpha}-Ta (110) film with a low density of defects on a-plane sapphire. The root mean square of the film with thickness of 200 nm is below 0.7 nm over a 10 {\mu}m \times 10 {\mu}m area and the residual resistance ratio is as high as 15.5. Transmon qubits are also fabricated using this kind of film and show relaxation times exceeding 150 {\mu}s. These results suggest {\alpha}-Ta (110) film grown on a-plane sapphire is an alternative promising choice for large-scale superconducting circuits with long coherence times of qubits.
Deterministic generation and tomography of a macroscopic Bell state between a millimeter-sized spin system and a superconducting qubit
Entanglement is a fundamental property in quantum mechanics that systems share inseparable quantum correlation regardless of their mutual distances. Owing to the fundamental significance
and versatile applications, the generation of quantum entanglement between {\it macroscopic} systems has been a focus of current research. Here we report on the deterministic generation and tomography of the macroscopically entangled Bell state in a hybrid quantum system containing a millimeter-sized spin system and a micrometer-sized superconducting qubit. The deterministic generation is realized by coupling the macroscopic spin system and the qubit via a microwave cavity. Also, we develop a joint tomography approach to confirming the deterministic generation of the Bell state, which gives a generation fidelity of 0.90±0.01. Our work makes the macroscopic spin system the largest system capable of generating the maximally entangled quantum state.
Fast superconducting qubit control with sub-harmonic drives
Increasing the fidelity of single-qubit gates requires a combination of faster pulses and increased qubit coherence. However, with resonant qubit drive via a capacitively coupled port,
these two objectives are mutually contradictory, as higher qubit quality factor requires a weaker coupling, necessitating longer pulses for the same applied power. Increasing drive power, on the other hand, can heat the qubit’s environment and degrade coherence. In this work, by using the inherent non-linearity of the transmon qubit, we circumvent this issue by introducing a new parametric driving scheme to perform single-qubit control. Specifically, we achieve rapid gate speed by pumping the transmon’s native Kerr term at approximately one third of the qubit’s resonant frequency. Given that transmons typically operate within a fairly narrow range of anharmonicity, this technique is applicable to all transmons. In both theory and experiment, we show that the Rabi rate of the process is proportional to applied drive amplitude cubed, allowing for rapid gate speed with only modest increases in applied power. In addition, we demonstrate that filtering can be used to protect the qubit’s coherence while performing rapid gates, and present theoretical calculations indicating that decay due to multi-photon losses, even in very strongly coupled drive lines, will not limit qubit lifetime. We demonstrate π/2 pulses as short as tens of nanoseconds with fidelity as high as 99.7\%, limited by the modest coherence of our transmon. We also present calculations indicating that this technique could reduce cryostat heating for fast gates, a vital requirement for large-scale quantum computers.