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
07
Mai
2025
Traveling-Wave Parametric Amplifier with Passive Reverse Isolation
Traveling-wave parametric amplifiers (TWPAs) have attracted much attention for their broadband amplification and near-quantum-limited noise performance. TWPAs are non-reciprocal by
nature providing gain for forward-propagating signals and transmission line losses for backward traveling waves. This intrinsic non-reciprocity is insufficient to protect sensitive quantum devices from back-action due to noise from warmer amplification stages in practical systems, and thus necessitates the need for bulky cryogenic isolators. We present a multi-stage Traveling-Wave Parametric Amplifier (mTWPA) that addresses this limitation by achieving passive in-band reverse isolation alongside near-quantum-limited noise performance. The multi-stage architecture consists of two, mode conversion stages and a reflectionless high-pass filter which provides the passive isolation. Experimental measurements of a prototype mTWPA demonstrated 20 dB of forward gain across a 1.6 GHz bandwidth and greater than 35 dB of reverse isolation. Noise measurements indicate performance at 1.7 times the quantum limit. This demonstrates that the increased complexity of a multi-stage TWPA design does not lead to significant added noise. The designed distribution of gain across the stages is engineered to minimize internal amplifier noise at the input, and we propose further optimization strategies in redistribution of the gain between the stages. This level of isolation effectively mitigates noise from warmer amplification stages, matching the performance of conventional isolators. The mTWPA approach offers a scalable path forward for more efficient and compact quantum circuit readout systems.
05
Mai
2025
Flux-Trapping Fluxonium Qubit
In pursuit of superconducting quantum computing, fluxonium qubits have recently garnered attention for their large anharmonicity and high coherence at the sweet spot. Towards the large-scale
integration of fluxonium qubits, a major obstacle is the need for precise external magnetic flux bias: To achieve high performance at its sweet spot, each qubit requires a DC bias line. However, such lines inductively coupled to the qubits bring in additional wiring overhead, crosstalk, heating, and decoherence, necessitating measures for mitigating the problems. In this work, we propose a flux-trapping fluxonium qubit, which, by leveraging fluxoid quantization, enables the optimal phase biasing without using external magnetic flux control at the operating temperature. We introduce the design and working principle, and demonstrate the phase biasing achieved through fluxoid quantization.
Lumped-element broadband SNAIL parametric amplifier with on-chip pump filter for multiplexed readout
We present a SNAIL-based parametric amplifier that integrates a lumped-element impedance matching network for increased bandwidth and an on-chip pump-port filter for efficient pump
delivery. The amplifier is fabricated using a single-layer optical lithography step, followed by a single-layer electron beam lithography step. We measure a flat 20 dB gain profile with less than 1 dB ripple across a bandwidth of up to 250 MHz on multiple devices, demonstrating robust performance against variations arising from fabrication and packaging. We characterize the amplifier’s linearity by analyzing gain compression and intermodulation distortion under simultaneous multi-tone excitation. We show that the intermodulation products remain suppressed by more than 23 dB relative to the signal tones, even at the 1 dB gain compression point. We further validate its utility by performing simultaneous high-fidelity readout of two transmon qubits, achieving state assignment fidelities of 99.51% and 98.55%. The combination of compact design, fabrication simplicity, and performance robustness makes this amplifier a practical device for quantum experiments with superconducting circuits.
Hyperinductance based on stacked Josephson junctions
Superinductances are superconducting circuit elements that combine a large inductance with a low parasitic capacitance to ground, resulting in a characteristic impedance exceeding the
resistance quantum RQ=h/(2e)2≃6.45kΩ. In recent years, these components have become key enablers for emerging quantum circuit architectures. However, achieving high characteristic impedance while maintaining scalability and fabrication robustness remains a major challenge. In this work, we present two fabrication techniques for realizing superinductances based on vertically stacked Josephson junctions. Using a multi-angle Manhattan (MAM) process and a zero-angle (ZA) evaporation technique — in which junction stacks are connected pairwise using airbridges — we fabricate one-dimensional chains of stacks that act as high-impedance superconducting transmission lines. Two-tone microwave spectroscopy reveals the expected n‾√ scaling of the impedance with the number of junctions per stack. The chain fabricated using the ZA process, with nine junctions per stack, achieves a characteristic impedance of ∼16kΩ, a total inductance of 5.9μH, and a maximum frequency-dependent impedance of 50kΩ at 1.4 GHz. Our results establish junction stacking as a scalable, robust, and flexible platform for next-generation quantum circuits requiring ultra-high impedance environments.
04
Mai
2025
Analysis of a 3D Integrated Superconducting Quantum Chip Structure
This work presents a combined analytical and simulation-based study of a 3D-integrated quantum chip architecture. We model a flip-chip-inspired structure by stacking two superconducting
qubits fabricated on separate high-resistivity silicon substrates through a dielectric interlayer. Utilizing \emph{rigorous} Ansys High-Frequency Structure Simulator (HFSS) simulations and analytical models from microwave engineering and quantum theory, we evaluate key quantum metrics such as eigenfrequencies, Q-factors, decoherence times, anharmonicity, cross-Kerr, participation ratios, and qubit coupling energy to describe the performance of the quantum device as a function of integration parameters. The integration parameters include the thickness and the quality of the dielectric interlayer. For detuned qubits, these metrics remain mostly invariant with respect to the substrate separation. However, introducing dielectric interlayer loss decreases the qubit quality factor, which consequentially degrades the relaxation time of the qubit. It is found that for the structure studied in this work, the stacked chip distance can be as small as 0.5mm. These findings support the viability of 3D quantum integration as a scalable alternative to planar architectures, while identifying key limitations in qubit coherence preservation due to lossy interlayer materials.
02
Mai
2025
Towards an experimental implementation of entanglement harvesting in superconducting circuits: effect of detector gap variation on entanglement harvesting
Motivated by the prospect of experimental implementations of entanglement harvesting in superconducting circuits, we propose a model of variable-gap particle detector that aims to bridge
some of the gaps between Unruh-DeWitt (UDW) models and realistic implementations. Using parameters tailored to potential experimental setups, we investigate entanglement harvesting in both spacelike-separated and causally connected scenarios. Our findings reveal that while variations in the energy gap reduce the ability to harvest entanglement for spacelike-separated detectors, detectors in causal contact can still become entangled through their interaction with the field. Notably, our analysis shows that (due to the derivative coupling nature of the model) even for causally connected detectors, the entanglement primarily originates from the field’s correlations. This demonstrates the potential for genuine entanglement harvesting in the lab and opens the door to near-future entanglement harvesting experiments in superconducting circuits.
01
Mai
2025
Quasiparticle-induced decoherence of a driven superconducting qubit
We develop a theory for two quasiparticle-induced decoherence mechanisms of a driven superconducting qubit. In the first mechanism, an existing quasiparticle (QP) tunnels across the
qubit’s Josephson junction while simultaneously absorbing a qubit excitation and one (or several) photons from the drive. In the second mechanism, a qubit transition occurs during the non-linear absorption process converting multiple drive quanta into a pair of new QPs. Both mechanisms can remain significant in gap engineered qubits whose coherence is insensitive to QPs without the drive. Our theory establishes a fundamental limitation on fidelity of the microwave qubit operations, such as readout and gates, stemming from QPs.
Theory of Quasiparticle Generation by Microwave Drives in Superconducting Qubits
Microwave drives are commonly employed to control superconducting quantum circuits, enabling qubit gates, readout, and parametric interactions. As the drive frequencies are typically
an order of magnitude smaller than (twice) the superconducting gap, it is generally assumed that such drives do not disturb the BCS ground state. However, sufficiently strong drives can activate multi-photon pair-breaking processes that generate quasiparticles and result in qubit errors. In this work, we present a theoretical framework for calculating the rates of multi-photon-assisted pair-breaking transitions induced by both charge- and flux-coupled microwave drives. Through illustrative examples, we show that drive-induced QP generation may impact novel high-frequency dispersive readout architectures, as well as Floquet-engineered superconducting circuits operating under strong driving conditions.
Offset Charge Dependence of Measurement-Induced Transitions in Transmons
A key challenge in achieving scalable fault tolerance in superconducting quantum processors is readout fidelity, which lags behind one- and two-qubit gate fidelity. A major limitation
in improving qubit readout is measurement-induced transitions, also referred to as qubit ionization, caused by multiphoton qubit-resonator excitation occurring at specific photon numbers. Since ionization can involve highly excited states, it has been predicted that in transmons — the most widely used superconducting qubit — the photon number at which measurement-induced transitions occur is gate charge dependent. This dependence is expected to persist deep in the transmon regime where the qubit frequency is gate charge insensitive. We experimentally confirm this prediction by characterizing measurement-induced transitions with increasing resonator photon population while actively stabilizing the transmon’s gate charge. Furthermore, because highly excited states are involved, achieving quantitative agreement between theory and experiment requires accounting for higher-order harmonics in the transmon Hamiltonian.
Probing excited-state dynamics of transmon ionization
The fidelity and quantum nondemolition character of the dispersive readout in circuit QED are limited by unwanted transitions to highly excited states at specific photon numbers in
the readout resonator. This observation can be explained by multiphoton resonances between computational states and highly excited states in strongly driven nonlinear systems, analogous to multiphoton ionization in atoms and molecules. In this work, we utilize the multilevel nature of high-EJ/EC transmons to probe the excited-state dynamics induced by strong drives during readout. With up to 10 resolvable states, we quantify the critical photon number of ionization, the resulting state after ionization, and the fraction of the population transferred to highly excited states. Moreover, using pulse-shaping to control the photon number in the readout resonator in the high-power regime, we tune the adiabaticity of the transition and verify that transmon ionization is a Landau-Zener-type transition. Our experimental results agree well with the theoretical prediction from a semiclassical driven transmon model and may guide future exploration of strongly driven nonlinear oscillators.