Universal scaling of microwave dissipation in superconducting circuits

  1. Thibault Charpentier,
  2. Anton Khvalyuk,
  3. Lev Ioffe,
  4. Mikhail Feigel'man,
  5. Nicolas Roch,
  6. and Benjamin Sacépé
Improving the coherence of superconducting qubits is essential for advancing quantum technologies. While superconductors are theoretically perfect conductors, they consistently exhibit
residual energy dissipation when driven by microwave currents, limiting coherence times. Here, we report a universal scaling between microwave dissipation and the superfluid density, a bulk property of superconductors related to charge carrier density and disorder. Our analysis spans a wide range of superconducting materials and device geometries, from highly disordered amorphous films to ultra-clean systems with record-high quality factors, including resonators, 3D cavities, and transmon qubits. This scaling reveals an intrinsic bulk dissipation channel, independent of surface dielectric losses, that originates from a universal density of nonequilibrium quasiparticles trapped within disorder-induced spatial variations of the superconducting gap. Our findings define a fundamental limit to coherence set by intrinsic material properties and provide a predictive framework for selecting materials and the design of next-generation superconducting quantum circuits.

High-power readout of a transmon qubit using a nonlinear coupling

  1. Cyril Mori,
  2. Vladimir Milchakov,
  3. Francesca D'Esposito,
  4. Lucas Ruela,
  5. Shelender Kumar,
  6. Vishnu Narayanan Suresh,
  7. Waël Ardati,
  8. Dorian Nicolas,
  9. Quentin Ficheux,
  10. Nicolas Roch,
  11. Tomás Ramos,
  12. and Olivier Buisson
The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly
prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the signal-to-noise ratio (SNR) and the Quantum Non-Demolition (QND) nature of the readout. Here, we explore the high-power regime by engineering a nonlinear coupling between a transmon qubit and its readout mode. Our approach builds upon previous work by Dassonneville et al. [Physical Review X 10, 011045 (2020)], on qubit readout with a non-perturbative cross-Kerr coupling in a transmon molecule. We demonstrate a readout fidelity of 99.21% with 89 photons utilizing a parametric amplifier. At this elevated photon number, the QND nature remains high at 96.7%. Even with up to 300 photons, the QNDness is only reduced by a few percent. This is qualitatively explained by deriving a critical number of photons associated to the nonlinear coupling, yielding a theoretical value of n¯critr=377 photons for our sample’s parameters. These results highlight the promising performance of the transmon molecule in the high-power regime for high-fidelity qubit readout.

Quantitative calibration of a TWPA applied to an optomechanical platform

  1. Alexandre Delattre,
  2. Ilya Golokolenov,
  3. Richard Pedurand,
  4. Nicolas Roch,
  5. Arpit Ranadive,
  6. Martina Esposito,
  7. Luca Planat,
  8. Andrew Fefferman,
  9. Eddy Collin,
  10. Xin Zhou,
  11. Mika A. Sillanpaa,
  12. Laure Mercier de Lepinay,
  13. Andrew D. Armour,
  14. and Jonas Glatthard
In the last decade, the microwave quantum electronics toolbox has been enriched with quantum limited detection devices such as Traveling Wave Parametric Amplifiers (TWPAs). The extreme
sensitivity they provide is not only mandatory for some physics applications within quantum information processing, but is also the key element that will determine the detection limit of quantum sensing setups. In the framework of microwave optomechanical systems, an unprecedented range of small motions and forces is accessible, for which a specific quantitative calibration becomes necessary. We report on near quantum-limited measurements performed with an aluminum drumhead mechanical device within the temperature range 4 mK – 400 mK. The whole setup is carefully calibrated, especially taking into account the power-dependence of microwave absorption in the superconducting optomechanical cavity. This effect is commonly attributed to Two-Level-Systems (TLSs) present in the metal oxide. We demonstrate that a similar feature exists in the TWPA, and can be phenomenologically fit with adapted expressions. The power and temperature dependence is studied over the full parameter range, leading to an absolute definition of phonon population (i.e. Brownian motion amplitude), with an uncertainty +-20 %.

Gain compression in Josephson Traveling-Wave Parametric Amplifiers

  1. Gwenael Le Gal,
  2. Guilliam Butseraen,
  3. Arpit Ranadive,
  4. Giulio Cappelli,
  5. Bekim Fazliji,
  6. Edgar Bonet,
  7. Eric Eyraud,
  8. Luca Planat,
  9. and Nicolas Roch
Superconducting traveling-wave parametric amplifiers (TWPAs) are increasingly used in various applications, including quantum computing, quantum sensing, and dark matter detection.
However, one important characteristic of these amplifiers, gain compression, has not received much attention. As a result, there is a lack of comprehensive experimental exploration of this phenomenon in the existing literature. In this study, we present an experimental investigation of gain compression in a Josephson traveling-wave parametric amplifier based on a four-wave mixing process. We have implemented a novel setup to monitor the complex transmission of both the pump and signal tones, which allows us to simultaneously track pump depletion and signal amplification as functions of signal power and frequency across the entire bandwidth of the device. Our findings indicate that, while pump depletion occurs during gain compression, it is not the only mechanism involved in the saturation of a TWPA. Power-induced phase-matching processes also take place within the device. This study provides valuable insights for optimizing TWPAs for applications that require high total input power, such as multiplexed qubit readout or broadband photon emission.

A Traveling Wave Parametric Amplifier Isolator

  1. Arpit Ranadive,
  2. Bekim Fazliji,
  3. Gwenael Le Gal,
  4. Giulio Cappelli,
  5. Guilliam Butseraen,
  6. Edgar Bonet,
  7. Eric Eyraud,
  8. Sina Böhling,
  9. Luca Planat,
  10. A. Metelmann,
  11. and Nicolas Roch
Superconducting traveling-wave parametric amplifiers have emerged as highly promising devices for near-quantum-limited broadband amplification of microwave signals and are essential
for high quantum-efficiency microwave readout lines. Built-in isolation, as well as gain, would address their primary limitation: lack of true directionality due to potential backward travel of electromagnetic radiation to their input port. Here, we demonstrate a Josephson-junction-based traveling-wave parametric amplifier isolator. It utilizes third-order nonlinearity for amplification and second-order nonlinearity for frequency upconversion of backward propagating modes to provide reverse isolation. These parametric processes, enhanced by a novel phase matching mechanism, exhibit gain of up to 20~dB and reverse isolation of up to 30~dB over a static 3~dB bandwidth greater than 500~MHz, while keeping near-quantum limited added noise. This demonstration of a broadband truly directional amplifier ultimately paves the way towards broadband quantum-limited microwave amplification lines without bulky magnetic isolators and with inhibited back-action.

Using bi-fluxon tunneling to protect the Fluxonium qubit

  1. Waël Ardati,
  2. Sébastien Léger,
  3. Shelender Kumar,
  4. Vishnu Narayanan Suresh,
  5. Dorian Nicolas,
  6. Cyril Mori,
  7. Francesca D'Esposito,
  8. Tereza Vakhtel,
  9. Olivier Buisson,
  10. Quentin Ficheux,
  11. and Nicolas Roch
Encoding quantum information in quantum states with disjoint wave-function support and noise insensitive energies is the key behind the idea of qubit protection. While fully protected
qubits are expected to offer exponential protection against both energy relaxation and pure dephasing, simpler circuits may grant partial protection with currently achievable parameters. Here, we study a fluxonium circuit in which the wave-functions are engineered to minimize their overlap while benefiting from a first-order-insensitive flux sweet spot. Taking advantage of a large superinductance (L∼1 μH), our circuit incorporates a resonant tunneling mechanism at zero external flux that couples states with the same fluxon parity, thus enabling bifluxon tunneling. The states |0⟩ and |1⟩ are encoded in wave-functions with parities 0 and 1, respectively, ensuring a minimal form of protection against relaxation. Two-tone spectroscopy reveals the energy level structure of the circuit and the presence of 4π quantum-phase slips between different potential wells corresponding to m=±1 fluxons, which can be precisely described by a simple fluxonium Hamiltonian or by an effective bifluxon Hamiltonian. Despite suboptimal fabrication, the measured relaxation (T1=177±3 μs) and dephasing (TE2=75±5 μs) times not only demonstrate the relevance of our approach but also opens an alternative direction towards quantum computing using partially-protected fluxonium qubits.

Entanglement assisted probe of the non-Markovian to Markovian transition in open quantum system dynamics

  1. Chandrashekhar Gaikwad,
  2. Daria Kowsari,
  3. Carson Brame,
  4. Xingrui Song,
  5. Haimeng Zhang,
  6. Martina Esposito,
  7. Arpit Ranadive,
  8. Giulio Cappelli,
  9. Nicolas Roch,
  10. Eli M. Levenson-Falk,
  11. and Kater W. Murch
We utilize a superconducting qubit processor to experimentally probe the transition from non-Markovian to Markovian dynamics of an entangled qubit pair. We prepare an entangled state
between two qubits and monitor the evolution of entanglement over time as one of the qubits interacts with a small quantum environment consisting of an auxiliary transmon qubit coupled to its readout cavity. We observe the collapse and revival of the entanglement as a signature of quantum memory effects in the environment. We then engineer the non-Markovianity of the environment by populating its readout cavity with thermal photons to show a transition from non-Markovian to Markovian dynamics, reaching a regime where the quantum Zeno effect creates a decoherence-free subspace that effectively stabilizes the entanglement between the qubits.

A gate-tunable graphene Josephson parametric amplifier

  1. Guilliam Butseraen,
  2. Arpit Ranadive,
  3. Nicolas Aparicio,
  4. Kazi Rafsanjani Amin,
  5. Abhishek Juyal,
  6. Martina Esposito,
  7. Kenji Watanabe,
  8. Takashi Taniguchi,
  9. Nicolas Roch,
  10. François Lefloch,
  11. and Julien Renard
With a large portfolio of elemental quantum components, superconducting quantum circuits have contributed to dramatic advances in microwave quantum optics. Of these elements, quantum-limited
parametric amplifiers have proven to be essential for low noise readout of quantum systems whose energy range is intrinsically low (tens of μeV ). They are also used to generate non classical states of light that can be a resource for quantum enhanced detection. Superconducting parametric amplifiers, like quantum bits, typically utilize a Josephson junction as a source of magnetically tunable and dissipation-free nonlinearity. In recent years, efforts have been made to introduce semiconductor weak links as electrically tunable nonlinear elements, with demonstrations of microwave resonators and quantum bits using semiconductor nanowires, a two dimensional electron gas, carbon nanotubes and graphene. However, given the challenge of balancing nonlinearity, dissipation, participation, and energy scale, parametric amplifiers have not yet been implemented with a semiconductor weak link. Here we demonstrate a parametric amplifier leveraging a graphene Josephson junction and show that its working frequency is widely tunable with a gate voltage. We report gain exceeding 20 dB and noise performance close to the standard quantum limit. Our results complete the toolset for electrically tunable superconducting quantum circuits and offer new opportunities for the development of quantum technologies such as quantum computing, quantum sensing and fundamental science.

Observation of two-mode squeezing in a traveling wave parametric amplifier

  1. Martina Esposito,
  2. Arpit Ranadive,
  3. Luca Planat,
  4. Sebastian Leger,
  5. Dorian Fraudet,
  6. Vincent Jouanny,
  7. Olivier Buisson,
  8. Wiebke Guichard,
  9. Cécile Naud,
  10. José Aumentado,
  11. Florent Lecocq,
  12. and Nicolas Roch
Traveling wave parametric amplifiers (TWPAs) have recently emerged as essential tools for broadband near quantum-limited amplification. However, their use to generate microwave quantum
states still misses an experimental demonstration. In this letter, we report operation of a TWPA as a source of two-mode squeezed microwave radiation. We demonstrate broadband entanglement generation between two modes separated by up to 400 MHz by measuring logarithmic negativity between 0.27 and 0.51 and collective quadrature squeezing below the vacuum limit between 1.5 and 2.1 dB. This work opens interesting perspectives for the exploration of novel microwave photonics experiments with possible applications in quantum sensing and continuous variable quantum computing.

A CMOS compatible platform for high impedance superconducting quantum circuits

  1. Kazi Rafsanjani Amin,
  2. Carine Ladner,
  3. Guillaume Jourdan,
  4. Sebastien Hentz,
  5. Nicolas Roch,
  6. and Julien Renard
Aluminium based platforms have allowed to reach major milestones for superconducting quantum circuits. For the next generation of devices, materials that are able to maintain low microwave
losses while providing new functionalities, such as large kinetic inductance or compatibility with CMOS platform are sought for. Here we report on a combined direct current (DC) and microwave investigation of titanium nitride lms of dierent thicknesses grown using CMOS compatible methods. For microwave resonators made of TiN lm of thickness ∼3 nm, we measured large kinetic inductance LK ∼ 240 pH/sq, high mode impedance of ∼ 4.2 kΩ while maintaining microwave quality factor ∼ 10^5 in the single photon limit. We present an in-depth study of the microwave loss mechanisms in these devices that indicates the importance of quasiparticles and provide insights for further improvement.