@article{19280,
  abstract     = {Recent advancements in superconducting circuits have enabled the experimental study of collective behavior of precisely controlled intermediate-scale ensembles of qubits. In this work, we demonstrate an atomic frequency comb formed by individual artificial atoms strongly coupled to a single resonator mode. We observe periodic microwave pulses that originate from a single coherent excitation dynamically interacting with the multiqubit ensemble. We show that this revival dynamics emerges as a consequence of the constructive and periodic rephasing of the five superconducting qubits forming the vacuum Rabi split comb. In the future, similar devices could be used as a memory with in situ tunable storage time or as an on-chip periodic pulse generator with nonclassical photon statistics.},
  author       = {Redchenko, Elena and Zens, M. and Zemlicka, Martin and Peruzzo, Matilda and Hassani, Farid and Sett, Riya and Zielinski, Przemyslaw D and Dhar, H. S. and Krimer, D. O. and Rotter, S. and Fink, Johannes M},
  issn         = {1079-7114},
  journal      = {Physical Review Letters},
  number       = {6},
  publisher    = {American Physical Society},
  title        = {{Observation of collapse and revival in a superconducting atomic frequency comb}},
  doi          = {10.1103/PhysRevLett.134.063601},
  volume       = {134},
  year         = {2025},
}

@phdthesis{17133,
  abstract     = {An ideal quantum computer relies on qubits capable of performing fast gate operations and
maintaining strong interconnections while preserving their quantum coherence. Since the
inception of experimental eforts toward building a quantum computer, the community has
faced challenges in engineering such a system. Among the various methods of implementing a
quantum computer, superconducting qubits have shown fast gates close to tens of nanoseconds,
with the state-of-the-art reaching a coherence of a few milliseconds. However, achieving
simultaneously long lifetimes with fast qubit operations poses an inherent paradox. Qubits
with high coherence require isolation from the environment, while fast operation necessitates
strong coupling of the qubit. This thesis approaches this issue by proposing the idea of
engineering superconducting qubits capable of transitioning between operating in a protected
regime, where the qubit is completely isolated from the environment, and coupling to the
communication channels as needed. In this direction, we use the geometric superinductor to
scan the parameter space of rf-SQUID devices, searching for a regime where we can take the
qubit protection to its extreme.

This leads us to the inductively shunted transmon (IST) regime, characterized by EJ /EC ≫ 1
and EJ /EL ≫ 1, where the circuit potential exhibits a double well with a large barrier
separating the local ground states of each quantum well. In this regime, although it is
anticipated that the two quantum wells would be isolated from each other, we observe single
fuxon tunneling between them. The interplay of the cavity photons and the fuxon transition
forms a rich physical system, containing resonance conditions that allow the preparation of the
fuxon ground or excited states. This enables us to study the relaxation rate of such transition
and show that it can be as large as 3.6 hours. Dynamically controlling the barrier height
between the two quantum wells allows for controllable coupling, which scales exponentially,
for a qubit encoded in two fuxon states.
The 0-π qubit is one of the very few known superconducting circuit types that ofers exponential
protection from both relaxation and dephasing simultaneously. However, this qubit is not
exempt from the fact that such protection comes at the expense of complex readout and
control. In this thesis, we propose a way to controllably break the circuit symmetry, the
key reason for the protection, to momentarily restore the ability to control and manipulate
the qubit. An asymmetry in capacitances and inductances in the 0-π circuit is detrimental
since they lead to coupling of the protected state to the thermally occupied parasitic mode
of the circuit. However, here we try to exploit a controlled asymmetry in Josephson energies
and show that this can be used as a tunable coupler between the protected states. In the
future, this should allow to perform gate operations by dynamically controlling the asymmetry
instead of driving the protected transition with microwave pulses. Therefore, we believe that
the proposed method can make the use of protected qubits more practical in experimental
realizations of quantum computing.},
  author       = {Hassani, Farid},
  isbn         = {978-3-99078-040-4},
  issn         = {2663-337X},
  keywords     = {Quantum information, Qubits, Superconducting devices},
  pages        = {161},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Superconducting qubits capable of dynamic switching between protected and high-speed control regimes}},
  doi          = {10.15479/at:ista:17133},
  year         = {2024},
}

@article{17202,
  abstract     = {Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.},
  author       = {Sagi, Oliver and Crippa, Alessandro and Valentini, Marco and Janik, Marian and Baghumyan, Levon and Fabris, Giorgio and Kapoor, Lucky and Hassani, Farid and Fink, Johannes M and Calcaterra, Stefano and Chrastina, Daniel and Isella, Giovanni and Katsaros, Georgios},
  issn         = {2041-1723},
  journal      = {Nature Communications},
  publisher    = {Springer Nature},
  title        = {{A gate tunable transmon qubit in planar Ge}},
  doi          = {10.1038/s41467-024-50763-6},
  volume       = {15},
  year         = {2024},
}

@article{17183,
  abstract     = {The photon blockade breakdown in a continuously driven cavity QED system has been proposed as a prime example for a first-order driven-dissipative quantum phase transition. However, the predicted scaling from a microscopic behavior—dominated by quantum fluctuations—to a macroscopic one—characterized by stable phases—and the associated exponents and phase diagram have not been observed so far. In this work we couple a single transmon qubit with a fixed coupling strength 𝑔 to a superconducting cavity that is in situ bandwidth 𝜅 tunable to controllably approach this thermodynamic limit. Even though the system remains microscopic, we observe its behavior becoming increasingly macroscopic as a function of 𝑔/𝜅. For the highest realized 𝑔/𝜅 of approximately 287, the system switches with a characteristic timescale as long as 6 s between a bright coherent state with approximately 8×103 intracavity photons and the vacuum state. This exceeds the microscopic timescales by 6 orders of magnitude and approaches the perfect hysteresis expected between two macroscopic attractors in the thermodynamic limit. These findings and interpretation are qualitatively supported by neoclassical theory and large-scale quantum-jump Monte Carlo simulations. Besides shedding more light on driven-dissipative physics in the limit of strong light-matter coupling, this system might also find applications in quantum sensing and metrology.},
  author       = {Sett, Riya and Hassani, Farid and Phan, Duc T and Barzanjeh, Shabir and Vukics, Andras and Fink, Johannes M},
  issn         = {2691-3399},
  journal      = {PRX Quantum},
  number       = {1},
  publisher    = {American Physical Society},
  title        = {{Emergent macroscopic bistability induced by a single superconducting qubit}},
  doi          = {10.1103/prxquantum.5.010327},
  volume       = {5},
  year         = {2024},
}

@misc{18978,
  abstract     = {Data analysis files for the manuscript "Emergent Macroscopic Bistability Induced by a Single Superconducting Qubit".

This contains the raw data and the data analysis files for generating the figures in the manuscript.

 Figure1 file - The raw data of cavity transmission spectra for 6 different kappas are there. They are fitted with input-output theory in the python file.
 Figure2 file - The raw data at 8 MHz kappa are included. all hte figures in figure 2 are generated in the python file
 Figure3 file - The raw data of PBB single shot measurements at all kappas are included. The detailed analysis and the Figure3 generated for the paper are all in the python analysis file. Also, thefiles containing the time-evolution of the intensity from Master Equation solution are included.
Figure4 file - The raw data at 2.6 MHz for different drive detunings and the corresponding analyses are included. And the python file includes the analysis of the experimental data as well as approximate neoclassical equations solutions for 2-level and 3-level transmons are included.  },
  author       = {Sett, Riya and Hassani, Farid and Phan, Duc T and Barzanjeh, Shabir and Vukics, Andras and Fink, Johannes M},
  publisher    = {Zenodo},
  title        = {{Data Analysis files for "Emergent Macroscopic Bistability Induced by a Single Superconducting Qubit"}},
  doi          = {10.5281/ZENODO.10518320},
  year         = {2024},
}

@article{13227,
  abstract     = {Currently available quantum processors are dominated by noise, which severely limits their applicability and motivates the search for new physical qubit encodings. In this work, we introduce the inductively shunted transmon, a weakly flux-tunable superconducting qubit that offers charge offset protection for all levels and a 20-fold reduction in flux dispersion compared to the state-of-the-art resulting in a constant coherence over a full flux quantum. The parabolic confinement provided by the inductive shunt as well as the linearity of the geometric superinductor facilitates a high-power readout that resolves quantum jumps with a fidelity and QND-ness of >90% and without the need for a Josephson parametric amplifier. Moreover, the device reveals quantum tunneling physics between the two prepared fluxon ground states with a measured average decay time of up to 3.5 h. In the future, fast time-domain control of the transition matrix elements could offer a new path forward to also achieve full qubit control in the decay-protected fluxon basis.},
  author       = {Hassani, Farid and Peruzzo, Matilda and Kapoor, Lucky and Trioni, Andrea and Zemlicka, Martin and Fink, Johannes M},
  issn         = {2041-1723},
  journal      = {Nature Communications},
  publisher    = {Springer Nature},
  title        = {{Inductively shunted transmons exhibit noise insensitive plasmon states and a fluxon decay exceeding 3 hours}},
  doi          = {10.1038/s41467-023-39656-2},
  volume       = {14},
  year         = {2023},
}

@article{14517,
  abstract     = {State-of-the-art transmon qubits rely on large capacitors, which systematically improve their coherence due to reduced surface-loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses—a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as 36 × 39 µm2 with  100-nm-wide vacuum-gap capacitors that are micromachined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. We achieve a vacuum participation ratio up to 99.6% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxationtime measurements for small gaps with high zero-point electric field variance of up to 22 V/m reveal a double exponential decay indicating comparably strong qubit interaction with long-lived two-level systems. The exceptionally high selectivity of up to 20 dB to the superconductor-vacuum interface allows us to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide previously exposed to ambient conditions. In terms of future scaling potential, we achieve a ratio of qubit quality factor to a footprint area equal to 20 µm−2, which is comparable with the highest T1 devices relying on larger geometries, a value that could improve substantially for lower surface-loss superconductors. },
  author       = {Zemlicka, Martin and Redchenko, Elena and Peruzzo, Matilda and Hassani, Farid and Trioni, Andrea and Barzanjeh, Shabir and Fink, Johannes M},
  issn         = {2331-7019},
  journal      = {Physical Review Applied},
  number       = {4},
  publisher    = {American Physical Society},
  title        = {{Compact vacuum-gap transmon qubits: Selective and sensitive probes for superconductor surface losses}},
  doi          = {10.1103/PhysRevApplied.20.044054},
  volume       = {20},
  year         = {2023},
}

@misc{14520,
  abstract     = {This dataset comprises all data shown in the figures of the submitted article "Compact vacuum gap transmon qubits: Selective and sensitive probes for superconductor surface losses" at arxiv.org/abs/2206.14104. Additional raw data are available from the corresponding author on reasonable request.},
  author       = {Zemlicka, Martin and Redchenko, Elena and Peruzzo, Matilda and Hassani, Farid and Trioni, Andrea and Barzanjeh, Shabir and Fink, Johannes M},
  publisher    = {Zenodo},
  title        = {{Compact vacuum gap transmon qubits: Selective and sensitive probes for superconductor surface losses}},
  doi          = {10.5281/ZENODO.8408897},
  year         = {2022},
}

@misc{13057,
  abstract     = {This dataset comprises all data shown in the figures of the submitted article "Geometric superinductance qubits: Controlling phase delocalization across a single Josephson junction". Additional raw data are available from the corresponding author on reasonable request.},
  author       = {Peruzzo, Matilda and Hassani, Farid and Szep, Grisha and Trioni, Andrea and Redchenko, Elena and Zemlicka, Martin and Fink, Johannes M},
  publisher    = {Zenodo},
  title        = {{Geometric superinductance qubits: Controlling phase delocalization across a single Josephson junction}},
  doi          = {10.5281/ZENODO.5592103},
  year         = {2021},
}

@misc{10644,
  abstract     = {The purpose of this application note is to demonstrate a working example of a superconducting qubit measurement in a Bluefors cryostat using the Keysight quantum control hardware. Our motivation is twofold. First, we provide pre-qualification data that the Bluefors cryostat, including filtering and wiring, can support long-lived qubits. Second, we demonstrate that the Keysight system (controlled using Labber) provides a straightforward solution to perform these characterization measurements. This document is intended as a brief guide for starting an experimental platform for testing superconducting qubits. The setup described here is an immediate jumping off point for a suite of applications including testing quantum logical gates, quantum optics with microwaves, or even using the qubit itself as a sensitive probe of local electromagnetic fields. Qubit measurements rely on high performance of both the physical sample environment and the measurement electronics. An overview of the cryogenic system is shown in Figure 1, and an overview of the integration between the electronics and cryostat (including wiring details) is shown in Figure 2.},
  author       = {Lake, Russell and Simbierowicz, Slawomir and Krantz, Philip and Hassani, Farid and Fink, Johannes M},
  keywords     = {Application note},
  pages        = {9},
  publisher    = {Bluefors Oy},
  title        = {{The Bluefors dilution refrigerator as an integrated quantum measurement system}},
  year         = {2021},
}

@misc{10645,
  abstract     = {Superconducting qubits have emerged as a highly versatile and useful platform for quantum technological applications [1]. Bluefors and Zurich Instruments have supported the growth of this field from the 2010s onwards by providing well-engineered and reliable measurement infrastructure [2]– [6]. Having a long and stable qubit lifetime is a critical system property. Therefore, considerable effort has already gone into measuring qubit energy-relaxation timescales and their fluctuations, see Refs. [7]–[10] among others. Accurately extracting the statistics of a quantum device requires users to perform time consuming measurements. One measurement challenge is that the detection of the state-dependent
response of a superconducting resonator due to a dispersively-coupled qubit requires an inherently low signal level. Consequently, measurements must be performed using a microwave probe that contains only a few microwave photons. Improving the signal-to-noise ratio (SNR) by using near-quantum limited parametric amplifiers as well as the use of optimized signal processing enabled by efficient room temperature instrumentation help to reduce measurement time. An empirical observation for fixed frequency transmons from recent literature is that as the energy-relaxation time 𝑇𝑇1 increases, so do its natural temporal fluctuations [7], [10]. This necessitates many repeated measurements to understand the statistics (see for example, Ref. [10]). In addition, as state-of-the-art qubits increase in lifetime, longer
measurement times are expected to obtain accurate statistics. As described below, the scaling of the widths of the qubit energy-relaxation distributions also reveal clues about the origin of the energy-relaxation.},
  author       = {Simbierowicz, Slawomir and Shi, Chunyan and Collodo, Michele and Kirste, Moritz and Hassani, Farid and Fink, Johannes M and Bylander, Jonas and Perez Lozano, Daniel and Lake, Russell},
  keywords     = {Application note},
  pages        = {8},
  publisher    = {Bluefors Oy},
  title        = {{Qubit energy-relaxation statistics in the Bluefors quantum measurement system}},
  year         = {2021},
}

@article{9928,
  abstract     = {There are two elementary superconducting qubit types that derive directly from the quantum harmonic oscillator. In one, the inductor is replaced by a nonlinear Josephson junction to realize the widely used charge qubits with a compact phase variable and a discrete charge wave function. In the other, the junction is added in parallel, which gives rise to an extended phase variable, continuous wave functions, and a rich energy-level structure due to the loop topology. While the corresponding rf superconducting quantum interference device Hamiltonian was introduced as a quadratic quasi-one-dimensional potential approximation to describe the fluxonium qubit implemented with long Josephson-junction arrays, in this work we implement it directly using a linear superinductor formed by a single uninterrupted aluminum wire. We present a large variety of qubits, all stemming from the same circuit but with drastically different characteristic energy scales. This includes flux and fluxonium qubits but also the recently introduced quasicharge qubit with strongly enhanced zero-point phase fluctuations and a heavily suppressed flux dispersion. The use of a geometric inductor results in high reproducibility of the inductive energy as guaranteed by top-down lithography—a key ingredient for intrinsically protected superconducting qubits.},
  author       = {Peruzzo, Matilda and Hassani, Farid and Szep, Gregory and Trioni, Andrea and Redchenko, Elena and Zemlicka, Martin and Fink, Johannes M},
  issn         = {2691-3399},
  journal      = {PRX Quantum},
  keywords     = {quantum physics, mesoscale and nanoscale physics},
  number       = {4},
  pages        = {040341},
  publisher    = {American Physical Society},
  title        = {{Geometric superinductance qubits: Controlling phase delocalization across a single Josephson junction}},
  doi          = {10.1103/PRXQuantum.2.040341},
  volume       = {2},
  year         = {2021},
}

@misc{13056,
  abstract     = {This datasets comprises all data shown in plots of the submitted article "Converting microwave and telecom photons with a silicon photonic nanomechanical interface". Additional raw data are available from the corresponding author on reasonable request.},
  author       = {Arnold, Georg M and Wulf, Matthias and Barzanjeh, Shabir and Redchenko, Elena and Rueda Sanchez, Alfredo R and Hease, William J and Hassani, Farid and Fink, Johannes M},
  publisher    = {Zenodo},
  title        = {{Converting microwave and telecom photons with a silicon photonic nanomechanical interface}},
  doi          = {10.5281/ZENODO.3961561},
  year         = {2020},
}

@misc{13070,
  abstract     = {This dataset comprises all data shown in the figures of the submitted article "Surpassing the resistance quantum with a geometric superinductor". Additional raw data are available from the corresponding author on reasonable request.},
  author       = {Peruzzo, Matilda and Trioni, Andrea and Hassani, Farid and Zemlicka, Martin and Fink, Johannes M},
  publisher    = {Zenodo},
  title        = {{Surpassing the resistance quantum with a geometric superinductor}},
  doi          = {10.5281/ZENODO.4052882},
  year         = {2020},
}

@article{8755,
  abstract     = {The superconducting circuit community has recently discovered the promising potential of superinductors. These circuit elements have a characteristic impedance exceeding the resistance quantum RQ ≈ 6.45 kΩ which leads to a suppression of ground state charge fluctuations. Applications include the realization of hardware protected qubits for fault tolerant quantum computing, improved coupling to small dipole moment objects and defining a new quantum metrology standard for the ampere. In this work we refute the widespread notion that superinductors can only be implemented based on kinetic inductance, i.e. using disordered superconductors or Josephson junction arrays. We present modeling, fabrication and characterization of 104 planar aluminum coil resonators with a characteristic impedance up to 30.9 kΩ at 5.6 GHz and a capacitance down to ≤ 1 fF, with lowloss and a power handling reaching 108 intra-cavity photons. Geometric superinductors are free of uncontrolled tunneling events and offer high reproducibility, linearity and the ability to couple magnetically - properties that significantly broaden the scope of future quantum circuits. },
  author       = {Peruzzo, Matilda and Trioni, Andrea and Hassani, Farid and Zemlicka, Martin and Fink, Johannes M},
  issn         = {2331-7019},
  journal      = {Physical Review Applied},
  number       = {4},
  publisher    = {American Physical Society},
  title        = {{Surpassing the resistance quantum with a geometric superinductor}},
  doi          = {10.1103/PhysRevApplied.14.044055},
  volume       = {14},
  year         = {2020},
}

@article{8529,
  abstract     = {Practical quantum networks require low-loss and noise-resilient optical interconnects as well as non-Gaussian resources for entanglement distillation and distributed quantum computation. The latter could be provided by superconducting circuits but existing solutions to interface the microwave and optical domains lack either scalability or efficiency, and in most cases the conversion noise is not known. In this work we utilize the unique opportunities of silicon photonics, cavity optomechanics and superconducting circuits to demonstrate a fully integrated, coherent transducer interfacing the microwave X and the telecom S bands with a total (internal) bidirectional transduction efficiency of 1.2% (135%) at millikelvin temperatures. The coupling relies solely on the radiation pressure interaction mediated by the femtometer-scale motion of two silicon nanobeams reaching a <jats:italic>V</jats:italic><jats:sub><jats:italic>π</jats:italic></jats:sub> as low as 16 μV for sub-nanowatt pump powers. Without the associated optomechanical gain, we achieve a total (internal) pure conversion efficiency of up to 0.019% (1.6%), relevant for future noise-free operation on this qubit-compatible platform.},
  author       = {Arnold, Georg M and Wulf, Matthias and Barzanjeh, Shabir and Redchenko, Elena and Rueda Sanchez, Alfredo R and Hease, William J and Hassani, Farid and Fink, Johannes M},
  issn         = {2041-1723},
  journal      = {Nature Communications},
  keywords     = {General Biochemistry, Genetics and Molecular Biology, General Physics and Astronomy, General Chemistry},
  publisher    = {Springer Nature},
  title        = {{Converting microwave and telecom photons with a silicon photonic nanomechanical interface}},
  doi          = {10.1038/s41467-020-18269-z},
  volume       = {11},
  year         = {2020},
}