@misc{17196, abstract = {This .zip File contains the data for the figures presented in the main text and supplementary material of "A gate tunable transmon qubit in planar Ge" by O.Sagi et al. The measurements were done using Qcodes. The description of the files and the instructions on opening the data can be found in the Readme. An additional Jupyter Notebook is attached that walks through the data analysis.}, author = {Sagi, Oliver}, publisher = {Institute of Science and Technology Austria}, title = {{A gate-tunable transmon in planar Ge}}, doi = {10.15479/AT:ISTA:17196}, year = {2024}, } @unpublished{18144, abstract = {High kinetic inductance superconductors are gaining increasing interest for the realisation of qubits, amplifiers and detectors.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 have integrated a granular aluminium resonator, having a characteristic impedance exceeding the resistance quantum, with a germanium double quantum dot and demonstrate strong charge-photon coupling with a rate of $g_\text{c}/2\pi= (566 \pm 2)$ MHz. This was achieved due to the realisation of a wireless ohmmeter, which allows \emph{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 k$\Omega$ (1 nH per square) is now possible. 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 Manjón, Alba Garzón and Arbiol, Jordi and Chrastina, Daniel and Isella, Giovanni and Pop, Ioan M. and Katsaros, Georgios}, booktitle = {arXiv}, title = {{Strong charge-photon coupling in planar germanium enabled by granular aluminium superinductors}}, doi = {10.48550/arXiv.2407.03079}, 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{14489, abstract = {Microwave-optics entanglement is a vital component for building hybrid quantum networks. Here, a new mechanism for preparing stationary entanglement between microwave and optical cavity fields in a cavity optomagnomechanical system is proposed. It consists of a magnon mode in a ferrimagnetic crystal that couples directly to a microwave cavity mode via the magnetic dipole interaction and indirectly to an optical cavity through the deformation displacement of the crystal. The mechanical displacement is induced by the magnetostrictive force and coupled to the optical cavity via radiation pressure. Both the opto- and magnomechanical couplings are dispersive. Magnon–phonon entanglement is created via magnomechanical parametric down-conversion, which is further distributed to optical and microwave photons via simultaneous optomechanical beamsplitter interaction and electromagnonic state-swap interaction, yielding stationary microwave-optics entanglement. The microwave-optics entanglement is robust against thermal noise, which will find broad potential applications in quantum networks and quantum information processing with hybrid quantum systems.}, author = {Fan, Zhi Yuan and Qiu, Liu and Gröblacher, Simon and Li, Jie}, issn = {1863-8899}, journal = {Laser and Photonics Reviews}, number = {12}, publisher = {Wiley}, title = {{Microwave-optics entanglement via cavity optomagnomechanics}}, doi = {10.1002/lpor.202200866}, volume = {17}, year = {2023}, } @article{14553, abstract = {Quantum state tomography is an essential component of modern quantum technology. In application to continuous-variable harmonic-oscillator systems, such as the electromagnetic field, existing tomography methods typically reconstruct the state in discrete bases, and are hence limited to states with relatively low amplitudes and energies. Here, we overcome this limitation by utilizing a feed-forward neural network to obtain the density matrix directly in the continuous position basis. An important benefit of our approach is the ability to choose specific regions in the phase space for detailed reconstruction. This results in a relatively slow scaling of the amount of resources required for the reconstruction with the state amplitude, and hence allows us to dramatically increase the range of amplitudes accessible with our method.}, author = {Fedotova, Ekaterina and Kuznetsov, Nikolai and Tiunov, Egor and Ulanov, A. E. and Lvovsky, A. I.}, issn = {2469-9934}, journal = {Physical Review A}, number = {4}, publisher = {American Physical Society}, title = {{Continuous-variable quantum tomography of high-amplitude states}}, doi = {10.1103/PhysRevA.108.042430}, volume = {108}, year = {2023}, } @article{12819, abstract = {Reaching a high cavity population with a coherent pump in the strong-coupling regime of a single-atom laser is impossible due to the photon blockade effect. In this Letter, we experimentally demonstrate that in a single-atom maser based on a transmon strongly coupled to two resonators, it is possible to pump over a dozen photons into the system. The first high-quality resonator plays the role of a usual lasing cavity, and the second one presents a controlled dissipation channel, bolstering population inversion, and modifies the energy-level structure to lift the blockade. As confirmation of the lasing action, we observe conventional laser features such as a narrowing of the emission linewidth and external signal amplification. Additionally, we report unique single-atom features: self-quenching and several lasing thresholds.}, author = {Sokolova, Alesya and Kalacheva, D. A. and Fedorov, G. P. and Astafiev, O. V.}, issn = {2469-9934}, journal = {Physical Review A}, number = {3}, publisher = {American Physical Society}, title = {{Overcoming photon blockade in a circuit-QED single-atom maser with engineered metastability and strong coupling}}, doi = {10.1103/PhysRevA.107.L031701}, volume = {107}, year = {2023}, } @inproceedings{14872, abstract = {We entangled microwave and optical photons for the first time as verified by a measured two-mode vacuum squeezing of 0.7 dB. This electro-optic entanglement is the key resource needed to connect cryogenic quantum circuits.}, author = {Sahu, Rishabh and Qiu, Liu and Hease, William J and Arnold, Georg M and Minoguchi, Yuri and Rabl, Peter and Fink, Johannes M}, booktitle = {Frontiers in Optics + Laser Science 2023}, isbn = {9781957171296}, location = {Tacoma, WA, United States}, publisher = {Optica Publishing Group}, title = {{Entangling microwaves and telecom wavelength light}}, doi = {10.1364/ls.2023.lm1f.3}, 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}, } @article{13106, abstract = {Quantum entanglement is a key resource in currently developed quantum technologies. Sharing this fragile property between superconducting microwave circuits and optical or atomic systems would enable new functionalities, but this has been hindered by an energy scale mismatch of >104 and the resulting mutually imposed loss and noise. In this work, we created and verified entanglement between microwave and optical fields in a millikelvin environment. Using an optically pulsed superconducting electro-optical device, we show entanglement between propagating microwave and optical fields in the continuous variable domain. This achievement not only paves the way for entanglement between superconducting circuits and telecom wavelength light, but also has wide-ranging implications for hybrid quantum networks in the context of modularization, scaling, sensing, and cross-platform verification.}, author = {Sahu, Rishabh and Qiu, Liu and Hease, William J and Arnold, Georg M and Minoguchi, Y. and Rabl, P. and Fink, Johannes M}, issn = {1095-9203}, journal = {Science}, keywords = {Multidisciplinary}, number = {6646}, pages = {718--721}, publisher = {American Association for the Advancement of Science}, title = {{Entangling microwaves with light}}, doi = {10.1126/science.adg3812}, volume = {380}, year = {2023}, } @misc{13122, abstract = {Data for submitted article "Entangling microwaves with light" at arXiv:2301.03315v1}, author = {Sahu, Rishabh}, publisher = {Zenodo}, title = {{Entangling microwaves with light}}, doi = {10.5281/ZENODO.7789417}, year = {2023}, } @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}, } @misc{13124, abstract = {This dataset comprises all data shown in the figures of the submitted article "Tunable directional photon scattering from a pair of superconducting qubits" at arXiv:2205.03293. Additional raw data are available from the corresponding author on reasonable request.}, author = {Redchenko, Elena and Poshakinskiy, Alexander and Sett, Riya and Zemlicka, Martin and Poddubny, Alexander and Fink, Johannes M}, publisher = {Zenodo}, title = {{Tunable directional photon scattering from a pair of superconducting qubits}}, doi = {10.5281/ZENODO.7858567}, year = {2023}, } @phdthesis{13175, abstract = {About a 100 years ago, we discovered that our universe is inherently noisy, that is, measuring any physical quantity with a precision beyond a certain point is not possible because of an omnipresent inherent noise. We call this - the quantum noise. Certain physical processes allow this quantum noise to get correlated in conjugate physical variables. These quantum correlations can be used to go beyond the potential of our inherently noisy universe and obtain a quantum advantage over the classical applications. Quantum noise being inherent also means that, at the fundamental level, the physical quantities are not well defined and therefore, objects can stay in multiple states at the same time. For example, the position of a particle not being well defined means that the particle is in multiple positions at the same time. About 4 decades ago, we started exploring the possibility of using objects which can be in multiple states at the same time to increase the dimensionality in computation. Thus, the field of quantum computing was born. We discovered that using quantum entanglement, a property closely related to quantum correlations, can be used to speed up computation of certain problems, such as factorisation of large numbers, faster than any known classical algorithm. Thus began the pursuit to make quantum computers a reality. Till date, we have explored quantum control over many physical systems including photons, spins, atoms, ions and even simple circuits made up of superconducting material. However, there persists one ubiquitous theme. The more readily a system interacts with an external field or matter, the more easily we can control it. But this also means that such a system can easily interact with a noisy environment and quickly lose its coherence. Consequently, such systems like electron spins need to be protected from the environment to ensure the longevity of their coherence. Other systems like nuclear spins are naturally protected as they do not interact easily with the environment. But, due to the same reason, it is harder to interact with such systems. After decades of experimentation with various systems, we are convinced that no one type of quantum system would be the best for all the quantum applications. We would need hybrid systems which are all interconnected - much like the current internet where all sorts of devices can all talk to each other - but now for quantum devices. A quantum internet. Optical photons are the best contenders to carry information for the quantum internet. They can carry quantum information cheaply and without much loss - the same reasons which has made them the backbone of our current internet. Following this direction, many systems, like trapped ions, have already demonstrated successful quantum links over a large distances using optical photons. However, some of the most promising contenders for quantum computing which are based on microwave frequencies have been left behind. This is because high energy optical photons can adversely affect fragile low-energy microwave systems. In this thesis, we present substantial progress on this missing quantum link between microwave and optics using electrooptical nonlinearities in lithium niobate. The nonlinearities are enhanced by using resonant cavities for all the involved modes leading to observation of strong direct coupling between optical and microwave frequencies. With this strong coupling we are not only able to achieve almost 100\% internal conversion efficiency with low added noise, thus presenting a quantum-enabled transducer, but also we are able to observe novel effects such as cooling of a microwave mode using optics. The strong coupling regime also leads to direct observation of dynamical backaction effect between microwave and optical frequencies which are studied in detail here. Finally, we also report first observation of microwave-optics entanglement in form of two-mode squeezed vacuum squeezed 0.7dB below vacuum level. With this new bridge between microwave and optics, the microwave-based quantum technologies can finally be a part of a quantum network which is based on optical photons - putting us one step closer to a future with quantum internet. }, author = {Sahu, Rishabh}, isbn = {978-3-99078-030-5}, issn = {2663-337X}, keywords = {quantum optics, electrooptics, quantum networks, quantum communication, transduction}, pages = {202}, publisher = {Institute of Science and Technology Austria}, title = {{Cavity quantum electrooptics}}, doi = {10.15479/at:ista:13175}, year = {2023}, } @phdthesis{12900, abstract = {About a 100 years ago, we discovered that our universe is inherently noisy, that is, measuring any physical quantity with a precision beyond a certain point is not possible because of an omnipresent inherent noise. We call this - the quantum noise. Certain physical processes allow this quantum noise to get correlated in conjugate physical variables. These quantum correlations can be used to go beyond the potential of our inherently noisy universe and obtain a quantum advantage over the classical applications. Quantum noise being inherent also means that, at the fundamental level, the physical quantities are not well defined and therefore, objects can stay in multiple states at the same time. For example, the position of a particle not being well defined means that the particle is in multiple positions at the same time. About 4 decades ago, we started exploring the possibility of using objects which can be in multiple states at the same time to increase the dimensionality in computation. Thus, the field of quantum computing was born. We discovered that using quantum entanglement, a property closely related to quantum correlations, can be used to speed up computation of certain problems, such as factorisation of large numbers, faster than any known classical algorithm. Thus began the pursuit to make quantum computers a reality. Till date, we have explored quantum control over many physical systems including photons, spins, atoms, ions and even simple circuits made up of superconducting material. However, there persists one ubiquitous theme. The more readily a system interacts with an external field or matter, the more easily we can control it. But this also means that such a system can easily interact with a noisy environment and quickly lose its coherence. Consequently, such systems like electron spins need to be protected from the environment to ensure the longevity of their coherence. Other systems like nuclear spins are naturally protected as they do not interact easily with the environment. But, due to the same reason, it is harder to interact with such systems. After decades of experimentation with various systems, we are convinced that no one type of quantum system would be the best for all the quantum applications. We would need hybrid systems which are all interconnected - much like the current internet where all sorts of devices can all talk to each other - but now for quantum devices. A quantum internet. Optical photons are the best contenders to carry information for the quantum internet. They can carry quantum information cheaply and without much loss - the same reasons which has made them the backbone of our current internet. Following this direction, many systems, like trapped ions, have already demonstrated successful quantum links over a large distances using optical photons. However, some of the most promising contenders for quantum computing which are based on microwave frequencies have been left behind. This is because high energy optical photons can adversely affect fragile low-energy microwave systems. In this thesis, we present substantial progress on this missing quantum link between microwave and optics using electrooptical nonlinearities in lithium niobate. The nonlinearities are enhanced by using resonant cavities for all the involved modes leading to observation of strong direct coupling between optical and microwave frequencies. With this strong coupling we are not only able to achieve almost 100\% internal conversion efficiency with low added noise, thus presenting a quantum-enabled transducer, but also we are able to observe novel effects such as cooling of a microwave mode using optics. The strong coupling regime also leads to direct observation of dynamical backaction effect between microwave and optical frequencies which are studied in detail here. Finally, we also report first observation of microwave-optics entanglement in form of two-mode squeezed vacuum squeezed 0.7dB below vacuum level. With this new bridge between microwave and optics, the microwave-based quantum technologies can finally be a part of a quantum network which is based on optical photons - putting us one step closer to a future with quantum internet. }, author = {Sahu, Rishabh}, isbn = {978-3-99078-030-5}, issn = {2663-337X}, keywords = {quantum optics, electrooptics, quantum networks, quantum communication, transduction}, pages = {190}, publisher = {Institute of Science and Technology Austria}, title = {{Cavity quantum electrooptics}}, doi = {10.15479/at:ista:12900}, year = {2023}, } @article{14032, abstract = {Arrays of Josephson junctions are governed by a competition between superconductivity and repulsive Coulomb interactions, and are expected to exhibit diverging low-temperature resistance when interactions exceed a critical level. Here we report a study of the transport and microwave response of Josephson arrays with interactions exceeding this level. Contrary to expectations, we observe that the array resistance drops dramatically as the temperature is decreased—reminiscent of superconducting behaviour—and then saturates at low temperature. Applying a magnetic field, we eventually observe a transition to a highly resistive regime. These observations can be understood within a theoretical picture that accounts for the effect of thermal fluctuations on the insulating phase. On the basis of the agreement between experiment and theory, we suggest that apparent superconductivity in our Josephson arrays arises from melting the zero-temperature insulator.}, author = {Mukhopadhyay, Soham and Senior, Jorden L and Saez Mollejo, Jaime and Puglia, Denise and Zemlicka, Martin and Fink, Johannes M and Higginbotham, Andrew P}, issn = {1745-2481}, journal = {Nature Physics}, keywords = {General Physics and Astronomy}, pages = {1630--1635}, publisher = {Springer Nature}, title = {{Superconductivity from a melted insulator in Josephson junction arrays}}, doi = {10.1038/s41567-023-02161-w}, volume = {19}, year = {2023}, } @article{13200, abstract = {Recent quantum technologies have established precise quantum control of various microscopic systems using electromagnetic waves. Interfaces based on cryogenic cavity electro-optic systems are particularly promising, due to the direct interaction between microwave and optical fields in the quantum regime. Quantum optical control of superconducting microwave circuits has been precluded so far due to the weak electro-optical coupling as well as quasi-particles induced by the pump laser. Here we report the coherent control of a superconducting microwave cavity using laser pulses in a multimode electro-optical device at millikelvin temperature with near-unity cooperativity. Both the stationary and instantaneous responses of the microwave and optical modes comply with the coherent electro-optical interaction, and reveal only minuscule amount of excess back-action with an unanticipated time delay. Our demonstration enables wide ranges of applications beyond quantum transductions, from squeezing and quantum non-demolition measurements of microwave fields, to entanglement generation and hybrid quantum networks.}, author = {Qiu, Liu and Sahu, Rishabh and Hease, William J and Arnold, Georg M and Fink, Johannes M}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Nature Research}, title = {{Coherent optical control of a superconducting microwave cavity via electro-optical dynamical back-action}}, doi = {10.1038/s41467-023-39493-3}, volume = {14}, year = {2023}, } @unpublished{18953, abstract = {The rapid development of superconducting quantum hardware is expected to run into significant I/O restrictions due to the need for large-scale error correction in a cryogenic environment. Classical data centers rely on fiber-optic interconnects to remove similar networking bottlenecks and to allow for reconfigurable, software-defined infrastructures. In the same spirit, ultra-cold electro-optic links have been proposed and used to generate qubit control signals, or to replace cryogenic readout electronics. So far, the latter suffered from either low efficiency, low bandwidth and the need for additional microwave drives, or breaking of Cooper pairs and qubit states. In this work we realize electro-optic microwave photonics at millikelvin temperatures to implement a radio-over-fiber qubit readout that does not require any active or passive cryogenic microwave equipment. We demonstrate all-optical single-shot-readout by means of the Jaynes-Cummings nonlinearity in a circulator-free readout scheme. Importantly, we do not observe any direct radiation impact on the qubit state as verified with high-fidelity quantum-non-demolition measurements 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, it is also relevant for multiplexed readout of superconducting photon detectors and classical superconducting logic. Moreover, this experiment showcases the potential of electro-optic radiometry in harsh environments - an electronics-free sensing principle that extends into the THz regime with applications in radio astronomy, planetary missions and earth observation.}, author = {Arnold, Georg M and Werner, Thomas and Sahu, Rishabh and Kapoor, Lucky and Qiu, Liu and Fink, Johannes M}, booktitle = {arXiv}, title = {{All-optical single-shot readout of a superconducting qubit}}, doi = {10.48550/ARXIV.2310.16817}, year = {2023}, } @article{13117, abstract = {The ability to control the direction of scattered light is crucial to provide flexibility and scalability for a wide range of on-chip applications, such as integrated photonics, quantum information processing, and nonlinear optics. Tunable directionality can be achieved by applying external magnetic fields that modify optical selection rules, by using nonlinear effects, or interactions with vibrations. However, these approaches are less suitable to control microwave photon propagation inside integrated superconducting quantum devices. Here, we demonstrate on-demand tunable directional scattering based on two periodically modulated transmon qubits coupled to a transmission line at a fixed distance. By changing the relative phase between the modulation tones, we realize unidirectional forward or backward photon scattering. Such an in-situ switchable mirror represents a versatile tool for intra- and inter-chip microwave photonic processors. In the future, a lattice of qubits can be used to realize topological circuits that exhibit strong nonreciprocity or chirality.}, author = {Redchenko, Elena and Poshakinskiy, Alexander V. and Sett, Riya and Zemlicka, Martin and Poddubny, Alexander N. and Fink, Johannes M}, issn = {2041-1723}, journal = {Nature Communications}, publisher = {Springer Nature}, title = {{Tunable directional photon scattering from a pair of superconducting qubits}}, doi = {10.1038/s41467-023-38761-6}, volume = {14}, year = {2023}, } @article{11591, abstract = {We investigate the deterministic generation and distribution of entanglement in large quantum networks by driving distant qubits with the output fields of a nondegenerate parametric amplifier. In this setting, the amplifier produces a continuous Gaussian two-mode squeezed state, which acts as a quantum-correlated reservoir for the qubits and relaxes them into a highly entangled steady state. Here we are interested in the maximal amount of entanglement and the optimal entanglement generation rates that can be achieved with this scheme under realistic conditions taking, in particular, the finite amplifier bandwidth, waveguide losses, and propagation delays into account. By combining exact numerical simulations of the full network with approximate analytic results, we predict the optimal working point for the amplifier and the corresponding qubit-qubit entanglement under various conditions. Our findings show that this passive conversion of Gaussian into discrete-variable entanglement offers a robust and experimentally very attractive approach for operating large optical, microwave, or hybrid quantum networks, for which efficient parametric amplifiers are currently developed.}, author = {Agustí, J. and Minoguchi, Y. and Fink, Johannes M and Rabl, P.}, issn = {2469-9934}, journal = {Physical Review A}, number = {6}, publisher = {American Physical Society}, title = {{Long-distance distribution of qubit-qubit entanglement using Gaussian-correlated photonic beams}}, doi = {10.1103/PhysRevA.105.062454}, volume = {105}, year = {2022}, }