@article{21449, abstract = {Three-dimensional (3D) crystals offer a route to scaling up trapped-ion systems for quantum sensing and quantum simulation applications; however, engineering coherent spin-motion couplings and effective spin-spin interactions in large crystals poses technical challenges associated with decoherence and prolonged timescales to generate appreciable entanglement. Here, we explore the possibility of speeding up these interactions in 3D crystals via parametric amplification. For this purpose, we derive a general Hamiltonian for the parametric amplification of spin-motion coupling that is broadly applicable to normal modes with motion transverse to or along the spatial extent of the crystal. Unlike in lower-dimensional crystals, we find that the ability to faithfully (uniformly) amplify the spin-spin interactions in 3D crystals depends on the physical implementation of the spin-motion coupling. We consider the light-shift gate, and the so-called phase-insensitive and phase-sensitive Mølmer-Sørensen (MS) gates, and we find that only the phase-sensitive MS gate can be faithfully amplified in general 3D crystals. We discuss a situation where nonuniform amplification can be advantageous. We also reconsider the effect of counter-rotating terms on parametric amplification and find that they are not as detrimental as previous studies suggest.}, author = {Hawaldar, Samarth and Nikhil, N. and Rey, Ana Maria and Bollinger, John J. and Shankar, Athreya}, issn = {2331-7019}, journal = {Physical Review Applied}, number = {3}, publisher = {American Physical Society}, title = {{Parametric amplification of spin-motion coupling in three-dimensional trapped-ion crystals}}, doi = {10.1103/h1m9-h3yw}, volume = {25}, year = {2026}, } @phdthesis{21863, abstract = {Atoms and photons, two things so different but yet so alike. The former, the building block of matter, something we learn about in school and imagine it as some tiny marbles encircled by other tinier marbles. The latter, an electromagnetic wave, a light particle or an excitation of the electromagnetic field. Quantum mechanics tells us about the properties of these two entities. And even if it sounds, looks and writes counter-intuitive, it has proven right for over a century now. In this work, I elaborate on how we tested the laws of quantum mechanics and how we used them learn more about the tiny building blocks of nature and the fields they use to talk to each other. The atoms we use, are artificial. Superconducting qubits, small electrical circuits with quantized energy levels behave like electrons that transition between different orbitals in an atom. One of the qubits' advantages, is also a big disadvantage. We design the circuits' energy levels and fabricate them in a cleanroom. This allows for arbitrary spaced energy levels but in contrast to real atoms, prevents two superconducting qubits from being alike. Still, this qubit platform is one of the frontrunners for future quantum computing technology and testing fundamental physics due to their scalability. We interface superconducting qubits, which operate in the GHz regime, with microwave photons. We use 3D aluminum cavities as mediators between qubits and photons. The cavities allow for non-destructive readout of the qubit state, they shield the qubits from noise at the qubit frequency and they give us an easy way to frequency-tune these joint systems. We need to operate superconducting qubits and their cavities at millikelvin temperatures in dilution refrigerators. At higher temperatures, superconductivity suffers and even worse, the environment is filled with thermal noise photons. This poses a fundamental limitation on the scalability of superconducting qubit devices. Also connecting multiple devices in different fridges does not work over room temperature links because the microwave photons used for this purpose will be covered in noise and the quantum information they carry, will be unusable. Infrared photons do not suffer from this noise problem since there are close to zero thermal noise photons at their frequencies at room temperature. We cannot simply interface superconducting devices with optical photons due their frequency mismatch and the destructive effect of optical photons on superconductors. Therefore, we use microwave-to-optics transducers that allow to convert microwave photons into optical ones and vice-versa. The transducers that we use are macroscopic electro-optic transducers using the Pockels effect in a disk-shaped Lithium Niobate whispering gallery mode resonator. By using a strong optical pump, photons from the two frequency domains experience a beam-splitter interaction and get converted from one to the other. We measure the generated optical photons using elaborate optical setups, optical heterodyning and single photon detectors to gain knowledge about the qubit state or the converted microwave photons. Bridging the microwave and the optical world allows us to take advantage of both of their strengths but it also requires deep knowledge about both of their working principles. In this work, we describe two experiments that our group conducted to showcase the opportunities that arise from interfacing superconducting qubits with optical photons but also the pitfalls, one may encounter on the way. In the first experiment, we managed to all-optically read out a superconducting qubit. We show that the assignment fidelity, the probability that a measurement of the qubit state matches the prepared state, is close to equal for all-optical, microwave-to-optics and conventional microwave readout. We show T1 and T2 measurements for all three readout types and give an analysis of the noise caused by the optics. Finally, we show that the infrared light does not affect the qubit performance in a negative way but that the heating it causes does. This is an important insight that we used in the next experiment. The second experiment is the upconversion of itinerant single microwave photons to the optical domain. We show that we can generate single microwave photons from a qubit-cavity system. We upconvert these single photons, measure them with a single photon detector and reconstruct their shape. By conducting a single photon Rabi measurement, we show correlations between the microwave and the optical domain. And by thorough signal-to-noise measurements and noise analysis, we find that we can generate single infrared photons with high signal-to-noise ratio 5.1 and low transducer added noise (<0.012 quanta). We show that this measurement creates a path towards entanglement of a superconducting qubit and an optical photon and what parameters need to be improved to achieve it. Additionally, this experiment is a proof of principle for an on-demand infrared single photon source. More generally, it allows to link microwave quantum technology in general to the optical domain.}, author = {Werner, Thomas}, issn = {2663-337X}, keywords = {Superconducting qubits, Quantum optics, Single photons and quantum effects, Nonlinear optics}, pages = {97}, publisher = {Institute of Science and Technology Austria}, title = {{Interfacing superconducting qubits with optical photons}}, doi = {10.15479/AT-ISTA-21863}, year = {2026}, } @unpublished{21870, abstract = {Superconducting qubits are a leading candidate for utility-scale quantum computing due to their fast gate speeds and steadily decreasing error rates. The requirement for millikelvin operating temperatures, however, creates a significant scaling bottleneck. Modular architectures using optical fiber links could bridge separate cryogenic nodes, but superconducting circuits do not have coherent optical transitions and microwave-to-optical conversion has not been shown for any non-classical photon state. In this work, we demonstrate the on-demand generation and tomographic reconstruction of itinerant single microwave photons at 8.9 GHz from a superconducting qubit. We upconvert this non-Gaussian state with a transducer added noise below 0.012 quanta and count the converted telecom photons at 193.4 THz with a signal-to-noise ratio of up to 5.1$\pm$1.1. We characterize the trade-offs between throughput and noise, and establish a viable path toward heralded entanglement distribution and gate teleportation. Looking ahead, these results empower existing superconducting devices to take a key role in distributed quantum technologies and heterogeneous quantum systems.}, author = {Werner, Thomas and Riyazi, Erfan and Hawaldar, Samarth and Sahu, Rishabh and Arnold, Georg M and Paul Falthansl-Scheinecker, Paul Falthansl-Scheinecker and Naranjo, Jennifer A. Sánchez and Loi, Dante and Kapoor, Lucky N. and Zemlicka, Martin and Qiu, Liu and Militaru, Andrei and Fink, Johannes M}, booktitle = {arXiv}, title = {{Electro-optic conversion of itinerant Fock states}}, doi = {10.48550/arXiv.2602.00928}, year = {2026}, } @article{19617, abstract = {In this article, we propose a method for generating single microwave photons in superconducting circuits. We theoretically show that pure single microwave photons can be generated on demand and tuned over a large frequency band by making use of Landau-Zener transitions under a rapid sweep of a control parameter. We devise a protocol that enables fast control of the frequency of the emitted photon over two octaves, without requiring extensive calibration. Additionally, we make theoretical estimates of the generation efficiency, tunability, purity, and linewidth of the photons emitted using this method for both charge- and flux-qubit-based architectures. We also provide estimates of the optimal device parameters required for these architectures to realize the device.}, author = {Hawaldar, Samarth and Khaire, Siddhi Satish and Delsing, Per and Suri, Baladitya}, issn = {2331-7019}, journal = {Physical Review Applied}, number = {4}, publisher = {American Physical Society}, title = {{On-demand single-microwave-photon source in a superconducting circuit with wideband frequency tunability}}, doi = {10.1103/physrevapplied.23.044042}, volume = {23}, year = {2025}, } @article{20927, abstract = {Cavity-magnon polaritons are hybrid excitations from the interaction between cavity photons and magnons, the quanta of collective spin oscillations. Along with the tunability of the magnon-photon coupling strength, fast information transfer and conversion speed are desired in hybrid devices. This can be achieved utilizing the propagating nature of spin waves with nonzero momentum for their ultrafast time dynamics and reduced ohmic dissipation. Antiferromagnets are particularly interesting as hosts for magnons since stray-field interactions are minimized and they support multiple modes with distinctive magnetic-field behavior across the phase diagram. Chromium trichloride (CrCl3) is a van der Waals layered antiferromagnet having a strong easy-plane anisotropy and a weak in-plane easy-axis anisotropy. Despite some magnetic resonance studies, the impact of magnetic reorientation of spins in CrCl3 on the cavity-magnon-polariton interaction strength as a function of magnetic field remains largely unexplored. In this study, we investigate the coupling between magnons in CrCl3 and photons in a coplanar waveguide resonator as a function of magnetic field. In particular, we find that the magnon-photon coupling strength varies nonmonotonically and distinctly with the magnetic field for the acoustic and the optical magnons, which can be utilized to tune the magnon-photon coupling strength using an external magnetic field as a knob. We find the signature of spin-flop transition in the two harmonics of the cavity due to a stronger dispersive coupling between optical magnons and cavity photons at lower fields. Additionally, we find standing modes formed by spin waves with nonzero momentum associated with the two hybrid magnons when the external field is applied at an angle with the crystal plane. These modes do not undergo substantial coupling with the cavity mode unlike the antiferromagnetic modes and can be used as low-loss propagation channels in hybrid devices.}, author = {Mandal, Supriya and Maji, Krishnendu and Kapoor, Lucky and Sasmal, Souvik and Manni, Soham and Jesudasan, John and Raychaudhuri, Pratap and Thamizhavel, Arumugam and Deshmukh, Mandar M.}, issn = {2469-9969}, journal = {Physical Review B}, number = {21}, publisher = {American Physical Society}, title = {{Cavity based sensing of antiferromagnetic canting and nonzero-momentum spin waves in a van der Waals cavity-magnon-polariton system}}, doi = {10.1103/bdd1-b8ys}, volume = {112}, year = {2025}, } @misc{20940, abstract = {These are the raw data files that supplement our study of mode dispersion with magnetic field of a cavity-magnonics system containing chromium trichloride on coplanar waveguide resonator.}, author = {Mandal, Supriya and Maji, Krishnendu and Kapoor, Lucky and Sasmal, Souvik and Manni, Soham and Jesudasan, John and Raychaudhuri, Pratap and Thamizhavel, Arumugam and Deshmukh, Mandar M.}, publisher = {Zenodo}, title = {{Mode dispersion with magnetic field in a cavity-magnonics system}}, doi = {10.5281/ZENODO.15321721}, year = {2025}, } @article{20976, abstract = {We present an experimental demonstration of an impedance-engineered Josephson parametric amplifier (IEJPA) fabricated in a single-step lithography process. Impedance-engineering is implemented using a lumped-element series LC circuit. We use a simpler lithography process where the entire device—impedance transformer and Josephson parametric amplifier (JPA)—is patterned in a single electron beam lithography step, followed by a double-angle Dolan-bridge technique for Al–AlOx–Al deposition. We observe amplification with 18 dB gain over a wide 400 MHz bandwidth centered around 5.3 GHz with added noise approaching the quantum limit, and a saturation power of −114 dBm. To accurately explain our experimental results, we extend existing theories for IEJPAs to incorporate the full sine nonlinearity of both the JPA and the transformer. Our work provides a route to simpler realization of broadband JPAs and a theoretical foundation for a regime of JPA operation that has been less explored in literature.}, author = {Patel, Lipi and Hawaldar, Samarth and Panikkar, Aditya and Shankar, Athreya and Suri, Baladitya}, issn = {1077-3118}, journal = {Applied Physics Letters}, number = {25}, publisher = {AIP Publishing}, title = {{Impedance-engineered Josephson parametric amplifier with single-step lithography}}, doi = {10.1063/5.0290636}, volume = {127}, year = {2025}, } @article{21318, abstract = {Matter waves have been observed in double-slit experiments with microscopic objects, such as atoms or molecules. The wave function describing the motion of these objects must extend over a distance comparable to the slit separation, much larger than the characteristic size of the objects. Preparing such states for more massive objects, such as mechanical oscillators, remains an outstanding challenge. Here we delocalize the quantum ground state of an optically levitated nanosphere by modulating the stiffness of the confining potential. We show a more than threefold increase of the initial coherence length, which corresponds to mechanical momentum squeezing of more than 7 dB. Our work is a stepping stone toward the generation of coherence lengths comparable to the object size, a crucial regime for macroscopic quantum experiments.}, author = {Rossi, M. and Militaru, Andrei and Carlon Zambon, N. and Riera-Campeny, A. and Romero-Isart, O. and Frimmer, M. and Novotny, L.}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {8}, publisher = {American Physical Society}, title = {{Quantum delocalization of a levitated nanoparticle}}, doi = {10.1103/2yzc-fsm3}, volume = {135}, year = {2025}, } @phdthesis{20371, abstract = {Quantum mechanics reveals a world that defies classical determinism, where uncertainty, superposition, and fluctuations are fundamental aspects. Engineering devices that harness these quantum features requires not only precision, but also a deep understanding of how they interact with their surrounding environment. Superconducting circuits, which exploit macroscopic quantum coherence in low-loss superconducting materials, provide a scalable platform for implementing such systems. Among the critical elements in these circuits, superinductors—high-impedance, dissipation-free inductive components—play a central role by suppressing charge fluctuations. They allow quantum states to be delocalized in phase space, protect qubits from environmental noise, and facilitate access to phenomena such as dual Josephson physics and ultra-strong coupling regimes. This thesis explores two complementary implementations of high-impedance circuits: geometric superinductors, demonstrating that high impedance can be achieved beyond kinetic inductance, and Josephson junction chains, used to investigate both microwave mode properties and DC transport across the superconductor-to-insulator transition. Part I addresses geometric superinductors. Contrary to the common belief that high-impedance superconducting circuits require kinetic inductance, we demonstrate that purely geometric designs can achieve characteristic impedance exceeding the resistance quantum. By exploiting mutual coupling between adjacent turns, coil-based inductors achieve enhanced self-inductance, creating a reliable platform for qubits and resonators. Modeling, simulation, fabrication, and characterization confirm that these elements behave as superinductor. With low loss, high linearity, and minimal stray capacitance, these elements are reproducible, free of uncontrolled tunneling events, and capable of strong magnetic coupling. This establishes geometric superinductors as robust, single-wave-function superconducting devices suitable for hardware protected qubits and hybrid systems. Part II presents classical numerical simulations of a Quantum Phase Slip circuit to study dual Shapiro steps. The circuit consists of an ideal Quantum Phase Slip element embedded in a resistive-inductive environment with a parasitic capacitance. Part III extends the investigation of high characteristic-impedance circuit elements to one-dimensional Josephson junction chains, which act as a quantum simulator for many-body physics and the superconductor–insulator transition. Different devices are realized on both sides of the DC phase transition, showing either a supercurrent branch or Coulomb blockade at zero bias. The effect of the crossover on microwave modes, however, remains insufficiently investigated. Studying these modes provides insight into the interplay between disorder and phase-slip events. Small differences in circuit component sizes determine which side of the transition a device falls on, making these results relevant not only for fundamental understanding but also for the design of quantum devices, emphasizing the crucial role of the electromagnetic environment in stabilizing and controlling fragile quantum states. Together, these results illustrate how carefully engineered high characteristic-impedance elements provide a link between macroscopic circuits and the inherently uncertain quantum world, enabling experiments that probe, control, and ultimately exploit quantum fluctuations for applications in quantum information, metrology, solid state physics and beyond. }, author = {Trioni, Andrea}, isbn = {978-3-99078-067-1}, issn = {2663-337X}, pages = {202}, publisher = {Institute of Science and Technology Austria}, title = {{High-impedance quantum circuits for mesoscopic physics : Geometric superinductors and insulating Josephson Chains}}, doi = {10.15479/AT-ISTA-20371}, year = {2025}, } @article{19401, abstract = {High kinetic inductance superconductors are gaining increasing interest for the realisation of qubits, amplifiers and detectors. Moreover, thanks to their high impedance, quantum buses made of such materials enable large zero-point fluctuations of the voltage, boosting the coupling rates to spin and charge qubits. However, fully exploiting the potential of disordered or granular superconductors is challenging, as their inductance and, therefore, impedance at high values are difficult to control. Here, we report a reproducible fabrication of granular aluminium resonators by developing a wireless ohmmeter, which allows in situ measurements during film deposition and, therefore, control of the kinetic inductance of granular aluminium films. Reproducible fabrication of circuits with impedances (inductances) exceeding 13 kΩ (1 nH per square) is now possible. By integrating a 7.9 kΩ resonator with a germanium double quantum dot, we demonstrate strong charge-photon coupling with a rate of gc/2π = 566 ± 2 MHz. This broadly applicable method opens the path for novel qubits and high-fidelity, long-distance two-qubit gates.}, author = {Janik, Marian and Roux, Kevin Etienne Robert and Borja Espinosa, Carla N and Sagi, Oliver and Baghdadi, Abdulhamid and Adletzberger, Thomas and Calcaterra, Stefano and Botifoll, Marc and Garzón Manjón, Alba and Arbiol, Jordi and Chrastina, Daniel and Isella, Giovanni and Pop, Ioan M. and Katsaros, Georgios}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Strong charge-photon coupling in planar germanium enabled by granular aluminium superinductors}}, doi = {10.1038/s41467-025-57252-4}, volume = {16}, year = {2025}, } @article{20664, abstract = {Conference travel contributes to the climate footprint of academic research. Here, we provide a quantitative estimate of the carbon emissions associated with conference attendance by analyzing travel data from participants of 10 international conferences in the field of magnetic resonance, namely EUROMAR, ENC and ICMRBS. We find that attending a EUROMAR conference produces, on average, more than 1 t CO2 eq.. For the analyzed conferences outside Europe, the corresponding value is about 2–3 times higher, on average, with intercontinental trips amounting to up to 5 t. We compare these conference-related emissions to other activities associated with research and show that conference travel is a substantial portion of the total climate footprint of a researcher in magnetic resonance. We explore several strategies to reduce these emissions, including the impact of selecting conference venues more strategically and the possibility of decentralized conferences. Through a detailed comparison of train versus air travel – accounting for both direct and infrastructure-related emissions – we demonstrate that train travel offers considerable carbon savings. These data may provide a basis for strategic choices of future conferences in the field and for individuals deciding on their conference attendance.}, author = {Kapoor, Lucky and Ruzickova, Natalia and Zivadinovic, Predrag and Leitner, Valentin and Sisak, Maria A and Mweka, Cecelia N and Dobbelaere, Jeroen A and Katsaros, Georgios and Schanda, Paul}, issn = {2699-0016}, journal = {Magnetic Resonance}, number = {2}, pages = {243--256}, publisher = {Copernicus Publications}, title = {{Quantifying the carbon footprint of conference travel: The case of NMR meetings}}, doi = {10.5194/mr-6-243-2025}, volume = {6}, year = {2025}, } @article{19073, abstract = {The rapid development of superconducting quantum hardware is expected to run into substantial restrictions on scalability because error correction in a cryogenic environment has stringent input–output requirements. Classical data centres rely on fibre-optic interconnects to remove similar networking bottlenecks. In the same spirit, ultracold electro-optic links have been proposed and used to generate qubit control signals, or to replace cryogenic readout electronics. So far, these approaches have suffered from either low efficiency, low bandwidth or additional noise. Here we realize radio-over-fibre qubit readout at millikelvin temperatures. We use one device to simultaneously perform upconversion and downconversion between microwave and optical frequencies and so do not require any active or passive cryogenic microwave equipment. We demonstrate all-optical single-shot readout in a circulator-free readout scheme. Importantly, we do not observe any direct radiation impact on the qubit state, despite the absence of shielding elements. This compatibility between superconducting circuits and telecom-wavelength light is not only a prerequisite to establish modular quantum networks, but it is also relevant for multiplexed readout of superconducting photon detectors and classical superconducting logic.}, author = {Arnold, Georg M and Werner, Thomas and Sahu, Rishabh and Kapoor, Lucky and Qiu, Liu and Fink, Johannes M}, issn = {1745-2481}, journal = {Nature Physics}, publisher = {Springer Nature}, title = {{All-optical superconducting qubit readout}}, doi = {10.1038/s41567-024-02741-4}, volume = {21}, year = {2025}, } @phdthesis{18871, abstract = {"Can we do this with a new type of computer - a quantum computer?". This famous quotation of the brilliant Richard Feynman within a conference talk on "Simulating physics with computers.” is often reverently praised as the origin of the field of quantum computing. The idea was to use quantum mechanical systems itself to simulate "Nature", which is inherently quantum mechanical. Now, 43 years later, the theoretical framework of how such a computer can operate has been developed. Two main important concepts for a potential quantum supremacy, superposition and entanglement, have been exploited to design quantum algorithms to significantly speed up certain tasks. Yet, the specific hardware implementation is still far from being certain, in fact the race between the most promising platforms such as superconducting qubits, bosonic codes, cold atoms, trapped ions, optical computing as well as spin qubits has recently intensified. If one also includes the most mature applications of quantum communication technologies, secure quantum key distribution and quantum random number generators, as part of a quantum information technology ecosystem, we are confronted with a plethora of different materials, concepts, and also operation frequencies. While superconducting qubits, bosonic codes and spin qubits work in the regime of approximately 5 GHz and are controlled by electrical fields, trapped ions, cold atoms, and optical quantum computing operate with light in the infrared or visible range. Consequently, a quantum frequency converter or microwave-optic transducer is required to interface the different frequency domains or establish a long-range network connection with suitable telecom fibers. In fact, the combination of different frequency regimes is also an essential part in our classical modern communication network, where computations are performed in electrical circuits and the information exchange over longer distances happens via optical fibers. However, the specific challenges specific to building a quantum computer, also apply to the development of such a quantum frequency transducer: 1) As we deal with single excitations as the carrier of information, i.e. the smallest possible quantity, the signal can easily be corrupted by other noise sources which needs to be avoided by all means. This is also the reason why microwave quantum computers operate at temperature environments close to zero temperature (< 0.1 Kelvin) to avoid corruption by thermal noise. 2) The frequency interface generally needs to preserve the phase of the signal as an essential part of the quantum state. And 3) Quantum signals cannot be copied which would be a typical strategy to account for errors in classical computers. And finally, there is a challenge specific to microwave-optic transducers: While quantum computers are operating in one specific frequency domain, microwave-optic transducers combine microwave and optical fields in one device. This results in the particular challenge that high-energy optical radiation, which is usually well-shielded from superconducting microwave quantum processors, are now an essential part of the device. The concomitant optical radiation in the operating transducer will inevitably have a detrimental effect on the superconducting microwave components. Together with the requirement of minimal background noise for quantum-limited operation as described above, v heating from the absorption of optical photons within the same device where single microwave excitations are processed forms a formidable challenge. This thesis aims to address this challenge by developing microwave-optic transducers where the impact of optical absorption on superconducting circuits in general and superconducting qubits specifically can be mitigated. In our first approach, we developed a compact device with optimized interaction strengths between the different frequency domains. This minimizes the optical powers used for transducer operation and thus the optical absorption heating. This work was - to the best of our knowledge - the first comprehensive noise study, in an integrated microwave-optic transducer. Unfortunately, we saw that the optical absorption heating added noise way above a single excitation. Consequently, a potential quantum signal would have been buried in the noise, added by the transduction. Building on this insight, we utilized a three-dimensional microwave-optic transducer instead of an integrated device. The larger heat capacity of the macroscopic device with a size of a few millimeters can absorb a larger fraction of the optical heating before it increases the temperature of the device. This allowed us to interface the transducer directly with a superconducting qubit to readout the qubit state in a novel all-optical manner. We showed that the microwave-optic transducer can be operated in a regime in which optical fields don’t harm the sensitive qubit. This is an important prerequisite for the operation of microwave-optic transducers in conjunction with microwave quantum processors and brings the integration and seamless orchestration of different frequency components in a quantum network a step closer. }, author = {Arnold, Georg M}, issn = {2663-337X}, pages = {135}, publisher = {Institute of Science and Technology Austria}, title = {{Microwave-optic interconnects for superconducting circuits}}, doi = {10.15479/at:ista:18871}, year = {2025}, } @phdthesis{19533, abstract = {This thesis explores advancements in quantum remote sensing and non-equilibrium phase transitions in the microwave regime, with a focus on dissipative phase transitions and quantumenhanced sensing. In the first project, I experimentally studied photon blockade breakdown as a dissipative phase transition in a zero-dimensional cavity-qubit system. By defining an appropriate thermodynamic limit, we demonstrated that the observed bistability is a genuine signature of a first-order phase transition in this system. This work provides insight into non-equilibrium quantum dynamics and phase transitions in driven-dissipative open quantum systems. The second project focuses on the experimental realization of a phase-conjugate receiver for quantum illumination (QI), a quantum sensing protocol that enhances target detection in noisy environments using entangled light. While an ideal spontaneous parametric down-conversion (SPDC) source and receiver could, in theory, provide up to a 6 dB advantage over classical illumination, no such ideal receiver exists. Instead, we explore an experimental realization of a phase-conjugate receiver for QI in the microwave regime at millikelvin temperatures using a Josephson parametric converter (JPC) as a source of continuous-variable Gaussian entangled signal-idler pairs, where a maximum 3 dB advantage is theoretically achievable. We investigate key experimental limitations that constrain practical QI performance, contributing to the development of quantum-enhanced sensing. Additionally, this thesis presents efficient digital signal processing (DSP) techniques implemented in C++ and Python in collaboration with Przemysław Zieliński and Luka Drmić. These methods, optimized using the Intel Integrated Performance Primitives (IPP) library, have been essential in data acquisition, noise filtering, and correlation analysis across multiple research projects. Although not real-time, these DSP techniques significantly enhance the accuracy of quantum measurements. Overall, this thesis advances quantum-enhanced sensing by establishing the thermodynamic limit in a single transmon-cavity system and experimentally exploring a phase-conjugate receiver for QI. These findings contribute to quantum metrology, particularly for weak signal detection and remote sensing in noisy environments. }, author = {Sett, Riya}, issn = {2663-337X}, keywords = {phase transition, open quantum system, phase diagram, cavity quantum electrodynamics, superconducting qubits, semiclassical physics, quantum optics, josephson junction, parametric converter, phase conjugation, quantum radar, quantum entanglement, correlation, quantum sensing}, pages = {109}, publisher = {Institute of Science and Technology Austria}, title = {{ Quantum remote sensing and non-equilibrium phase transitions in the microwave regime}}, doi = {10.15479/AT-ISTA-19533}, year = {2025}, } @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}, } @article{14846, abstract = {Contraction and flow of the actin cell cortex have emerged as a common principle by which cells reorganize their cytoplasm and take shape. However, how these cortical flows interact with adjacent cytoplasmic components, changing their form and localization, and how this affects cytoplasmic organization and cell shape remains unclear. Here we show that in ascidian oocytes, the cooperative activities of cortical actomyosin flows and deformation of the adjacent mitochondria-rich myoplasm drive oocyte cytoplasmic reorganization and shape changes following fertilization. We show that vegetal-directed cortical actomyosin flows, established upon oocyte fertilization, lead to both the accumulation of cortical actin at the vegetal pole of the zygote and compression and local buckling of the adjacent elastic solid-like myoplasm layer due to friction forces generated at their interface. Once cortical flows have ceased, the multiple myoplasm buckles resolve into one larger buckle, which again drives the formation of the contraction pole—a protuberance of the zygote’s vegetal pole where maternal mRNAs accumulate. Thus, our findings reveal a mechanism where cortical actomyosin network flows determine cytoplasmic reorganization and cell shape by deforming adjacent cytoplasmic components through friction forces.}, author = {Caballero Mancebo, Silvia and Shinde, Rushikesh and Bolger-Munro, Madison and Peruzzo, Matilda and Szep, Gregory and Steccari, Irene and Labrousse Arias, David and Zheden, Vanessa and Merrin, Jack and Callan-Jones, Andrew and Voituriez, Raphaël and Heisenberg, Carl-Philipp J}, issn = {1745-2481}, journal = {Nature Physics}, pages = {310--321}, publisher = {Springer Nature}, title = {{Friction forces determine cytoplasmic reorganization and shape changes of ascidian oocytes upon fertilization}}, doi = {10.1038/s41567-023-02302-1}, volume = {20}, year = {2024}, } @article{17410, abstract = {We present a cavity-electromechanical system comprising a superconducting quantum interference device which is embedded in a microwave resonator and coupled via a pickup loop to a 6-μ⁢g magnetically levitated superconducting sphere. The motion of the sphere in the magnetic trap induces a frequency shift in the SQUID-cavity system. We use microwave spectroscopy to characterize the system, and we demonstrate that the electromechanical interaction is tunable. The measured displacement sensitivity of 10−7m/√Hz defines a path towards ground-state cooling of levitated particles with Planck-scale masses at millikelvin environment temperatures.}, author = {Schmidt, Philip and Claessen, Remi and Higgins, Gerard and Hofer, Joachim and Hansen, Jannek J. and Asenbaum, Peter and Zemlicka, Martin and Uhl, Kevin and Kleiner, Reinhold and Gross, Rudolf and Huebl, Hans and Trupke, Michael and Aspelmeyer, Markus}, issn = {2331-7019}, journal = {Physical Review Applied}, publisher = {American Physical Society}, title = {{Remote sensing of a levitated superconductor with a flux-tunable microwave cavity}}, doi = {10.1103/PhysRevApplied.22.014078}, volume = {22}, year = {2024}, } @article{17477, abstract = {Trapped-ion systems are a leading platform for quantum information processing, but they are currently limited to 1D and 2D arrays, which imposes restrictions on both their scalability and their range of applications. Here, we propose a path to overcome this limitation by demonstrating that Penning traps can be used to realize remarkably clean bilayer crystals, wherein hundreds of ions self-organize into two well-defined layers. These bilayer crystals are made possible by the inclusion of an anharmonic trapping potential, which is readily implementable with current technology. We study the normal modes of this system and discover salient differences compared to the modes of single-plane crystals. The bilayer geometry and the unique properties of the normal modes open new opportunities—in particular, in quantum sensing and quantum simulation—that are not straightforward in single-plane crystals. Furthermore, we illustrate that it may be possible to extend the ideas presented here to realize multilayer crystals with more than two layers. Our work increases the dimensionality of trapped-ion systems by efficiently utilizing all three spatial dimensions, and it lays the foundation for a new generation of quantum information processing experiments with multilayer 3D crystals of trapped ions.}, author = {Hawaldar, Samarth and Shahi, Prakriti and Carter, Allison L. and Rey, Ana Maria and Bollinger, John J. and Shankar, Athreya}, issn = {2160-3308}, journal = {Physical Review X}, number = {3}, publisher = {American Physical Society}, title = {{Bilayer crystals of trapped ions for quantum information processing}}, doi = {10.1103/PhysRevX.14.031030}, volume = {14}, year = {2024}, } @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}, }