TY - JOUR AB - 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. AU - Fedotova, Ekaterina AU - Kuznetsov, Nikolai AU - Tiunov, Egor AU - Ulanov, A. E. AU - Lvovsky, A. I. ID - 14553 IS - 4 JF - Physical Review A SN - 2469-9926 TI - Continuous-variable quantum tomography of high-amplitude states VL - 108 ER - TY - JOUR AB - 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. AU - Hassani, Farid AU - Peruzzo, Matilda AU - Kapoor, Lucky AU - Trioni, Andrea AU - Zemlicka, Martin AU - Fink, Johannes M ID - 13227 JF - Nature Communications TI - Inductively shunted transmons exhibit noise insensitive plasmon states and a fluxon decay exceeding 3 hours VL - 14 ER - TY - CONF AB - 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. AU - Sahu, Rishabh AU - Qiu, Liu AU - Hease, William J AU - Arnold, Georg M AU - Minoguchi, Yuri AU - Rabl, Peter AU - Fink, Johannes M ID - 14872 SN - 9781957171296 T2 - Frontiers in Optics + Laser Science 2023 TI - Entangling microwaves and telecom wavelength light ER - TY - JOUR AB - 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. AU - Mukhopadhyay, Soham AU - Senior, Jorden L AU - Saez Mollejo, Jaime AU - Puglia, Denise AU - Zemlicka, Martin AU - Fink, Johannes M AU - Higginbotham, Andrew P ID - 14032 JF - Nature Physics KW - General Physics and Astronomy SN - 1745-2473 TI - Superconductivity from a melted insulator in Josephson junction arrays VL - 19 ER - TY - JOUR AB - 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. AU - Fan, Zhi Yuan AU - Qiu, Liu AU - Gröblacher, Simon AU - Li, Jie ID - 14489 IS - 12 JF - Laser and Photonics Reviews SN - 1863-8880 TI - Microwave-optics entanglement via cavity optomagnomechanics VL - 17 ER - TY - CONF AB - We present a quantum-enabled microwave-telecom interface with bidirectional conversion efficiencies up to 15% and added input noise quanta as low as 0.16. Moreover, we observe evidence for electro-optic laser cooling and vacuum amplification. AU - Sahu, Rishabh AU - Hease, William J AU - Rueda Sanchez, Alfredo R AU - Arnold, Georg M AU - Qiu, Liu AU - Fink, Johannes M ID - 12088 SN - 9781557528209 T2 - Conference on Lasers and Electro-Optics TI - Realizing a quantum-enabled interconnect between microwave and telecom light ER - TY - JOUR AB - Solid-state microwave systems offer strong interactions for fast quantum logic and sensing but photons at telecom wavelength are the ideal choice for high-density low-loss quantum interconnects. A general-purpose interface that can make use of single photon effects requires < 1 input noise quanta, which has remained elusive due to either low efficiency or pump induced heating. Here we demonstrate coherent electro-optic modulation on nanosecond-timescales with only 0.16+0.02−0.01 microwave input noise photons with a total bidirectional transduction efficiency of 8.7% (or up to 15% with 0.41+0.02−0.02), as required for near-term heralded quantum network protocols. The use of short and high-power optical pump pulses also enables near-unity cooperativity of the electro-optic interaction leading to an internal pure conversion efficiency of up to 99.5%. Together with the low mode occupancy this provides evidence for electro-optic laser cooling and vacuum amplification as predicted a decade ago. AU - Sahu, Rishabh AU - Hease, William J AU - Rueda Sanchez, Alfredo R AU - Arnold, Georg M AU - Qiu, Liu AU - Fink, Johannes M ID - 10924 JF - Nature Communications TI - Quantum-enabled operation of a microwave-optical interface VL - 13 ER - TY - JOUR AB - Magnetic-field-resilient superconducting circuits enable sensing applications and hybrid quantum computing architectures involving spin or topological qubits and electromechanical elements, as well as studying flux noise and quasiparticle loss. We investigate the effect of in-plane magnetic fields up to 1 T on the spectrum and coherence times of thin-film three-dimensional aluminum transmons. Using a copper cavity, unaffected by strong magnetic fields, we can probe solely the effect of magnetic fields on the transmons. We present data on a single-junction and a superconducting-quantum-interference-device (SQUID) transmon that are cooled down in the same cavity. As expected, the transmon frequencies decrease with increasing field, due to suppression of the superconducting gap and a geometric Fraunhofer-like contribution. Nevertheless, the thin-film transmons show strong magnetic field resilience: both transmons display microsecond coherence up to at least 0.65 T, and T1 remains above 1μs over the entire measurable range. SQUID spectroscopy is feasible up to 1 T, the limit of our magnet. We conclude that thin-film aluminum Josephson junctions are suitable hardware for superconducting circuits in the high-magnetic-field regime. AU - Krause, J. AU - Dickel, C. AU - Vaal, E. AU - Vielmetter, M. AU - Feng, J. AU - Bounds, R. AU - Catelani, G. AU - Fink, Johannes M AU - Ando, Yoichi ID - 10940 IS - 3 JF - Physical Review Applied TI - Magnetic field resilience of three-dimensional transmons with thin-film Al/AlOx/Al Josephson junctions approaching 1 T VL - 17 ER - TY - JOUR AB - Micro- and nanoscale optical or microwave cavities are used in a wide range of classical applications and quantum science experiments, ranging from precision measurements, laser technologies to quantum control of mechanical motion. The dissipative photon loss via absorption, present to some extent in any optical cavity, is known to introduce thermo-optical effects and thereby impose fundamental limits on precision measurements. Here, we theoretically and experimentally reveal that such dissipative photon absorption can result in quantum feedback via in-loop field detection of the absorbed optical field, leading to the intracavity field fluctuations to be squashed or antisquashed. A closed-loop dissipative quantum feedback to the cavity field arises. Strikingly, this modifies the optical cavity susceptibility in coherent response measurements (capable of both increasing or decreasing the bare cavity linewidth) and causes excess noise and correlations in incoherent interferometric optomechanical measurements using a cavity, that is parametrically coupled to a mechanical oscillator. We experimentally observe such unanticipated dissipative dynamics in optomechanical spectroscopy of sideband-cooled optomechanical crystal cavitiess at both cryogenic temperature (approximately 8 K) and ambient conditions. The dissipative feedback introduces effective modifications to the optical cavity linewidth and the optomechanical scattering rate and gives rise to excess imprecision noise in the interferometric quantum measurement of mechanical motion. Such dissipative feedback differs fundamentally from a quantum nondemolition feedback, e.g., optical Kerr squeezing. The dissipative feedback itself always results in an antisqueezed out-of-loop optical field, while it can enhance the coexisting Kerr squeezing under certain conditions. Our result applies to cavity spectroscopy in both optical and superconducting microwave cavities, and equally applies to any dissipative feedback mechanism of different bandwidth inside the cavity. It has wide-ranging implications for future dissipation engineering, such as dissipation enhanced sideband cooling and Kerr squeezing, quantum frequency conversion, and nonreciprocity in photonic systems. AU - Qiu, Liu AU - Huang, Guanhao AU - Shomroni, Itay AU - Pan, Jiahe AU - Seidler, Paul AU - Kippenberg, Tobias J. ID - 11353 IS - 2 JF - PRX Quantum TI - Dissipative quantum feedback in measurements using a parametrically coupled microcavity VL - 3 ER - TY - JOUR AB - Over the past few years, the field of quantum information science has seen tremendous progress toward realizing large-scale quantum computers. With demonstrations of quantum computers outperforming classical computers for a select range of problems,1–3 we have finally entered the noisy, intermediate-scale quantum (NISQ) computing era. While the quantum computers of today are technological marvels, they are not yet error corrected, and it is unclear whether any system will scale beyond a few hundred logical qubits without significant changes to architecture and control schemes. Today's quantum systems are analogous to the ENIAC (Electronic Numerical Integrator And Computer) and EDVAC (Electronic Discrete Variable Automatic Computer) systems of the 1940s, which ran on vacuum tubes. These machines were built on a solid, nominally scalable architecture and when they were developed, nobody could have predicted the development of the transistor and the impact of the resulting semiconductor industry. Simply put, the computers of today are nothing like the early computers of the 1940s. We believe that the qubits of future fault-tolerant quantum systems will look quite different from the qubits of the NISQ machines in operation today. This Special Topic issue is devoted to new and emerging quantum systems with a focus on enabling technologies that can eventually lead to the quantum analog to the transistor. We have solicited both research4–18 and perspective articles19–21 to discuss new and emerging qubit systems with a focus on novel materials, encodings, and architectures. We are proud to present a collection that touches on a wide range of technologies including superconductors,7–13,21 semiconductors,15–17,19 and individual atomic qubits.18 AU - Sigillito, Anthony J. AU - Covey, Jacob P. AU - Fink, Johannes M AU - Petersson, Karl AU - Preble, Stefan ID - 11417 IS - 19 JF - Applied Physics Letters SN - 0003-6951 TI - Emerging qubit systems: Guest editorial VL - 120 ER - TY - JOUR AB - 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. AU - Agustí, J. AU - Minoguchi, Y. AU - Fink, Johannes M AU - Rabl, P. ID - 11591 IS - 6 JF - Physical Review A SN - 2469-9926 TI - Long-distance distribution of qubit-qubit entanglement using Gaussian-correlated photonic beams VL - 105 ER - TY - GEN AB - 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. AU - Zemlicka, Martin AU - Redchenko, Elena AU - Peruzzo, Matilda AU - Hassani, Farid AU - Trioni, Andrea AU - Barzanjeh, Shabir AU - Fink, Johannes M ID - 14520 TI - Compact vacuum gap transmon qubits: Selective and sensitive probes for superconductor surface losses ER - TY - THES AB - Recent substantial advances in the feld of superconducting circuits have shown its potential as a leading platform for future quantum computing. In contrast to classical computers based on bits that are represented by a single binary value, 0 or 1, quantum bits (or qubits) can be in a superposition of both. Thus, quantum computers can store and handle more information at the same time and a quantum advantage has already been demonstrated for two types of computational tasks. Rapid progress in academic and industry labs accelerates the development of superconducting processors which may soon fnd applications in complex computations, chemical simulations, cryptography, and optimization. Now that these machines are scaled up to tackle such problems the questions of qubit interconnects and networks becomes very relevant. How to route signals on-chip between diferent processor components? What is the most efcient way to entangle qubits? And how to then send and process entangled signals between distant cryostats hosting superconducting processors? In this thesis, we are looking for solutions to these problems by studying the collective behavior of superconducting qubit ensembles. We frst demonstrate on-demand tunable directional scattering of microwave photons from a pair of qubits in a waveguide. Such a device can route microwave photons on-chip with a high diode efciency. Then we focus on studying ultra-strong coupling regimes between light (microwave photons) and matter (superconducting qubits), a regime that could be promising for extremely fast multi-qubit entanglement generation. Finally, we show coherent pulse storage and periodic revivals in a fve qubit ensemble strongly coupled to a resonator. Such a reconfgurable storage device could be used as part of a quantum repeater that is needed for longer-distance quantum communication. The achieved high degree of control over multi-qubit ensembles highlights not only the beautiful physics of circuit quantum electrodynamics, it also represents the frst step toward new quantum simulation and communication methods, and certain techniques may also fnd applications in future superconducting quantum computing hardware. AU - Redchenko, Elena ID - 12366 SN - 2663-337X TI - Controllable states of superconducting Qubit ensembles ER - TY - GEN AB - 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. AU - Simbierowicz, Slawomir AU - Shi, Chunyan AU - Collodo, Michele AU - Kirste, Moritz AU - Hassani, Farid AU - Fink, Johannes M AU - Bylander, Jonas AU - Perez Lozano, Daniel AU - Lake, Russell ID - 10645 KW - Application note TI - Qubit energy-relaxation statistics in the Bluefors quantum measurement system ER - TY - GEN AB - 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. AU - Lake, Russell AU - Simbierowicz, Slawomir AU - Krantz, Philip AU - Hassani, Farid AU - Fink, Johannes M ID - 10644 KW - Application note TI - The Bluefors dilution refrigerator as an integrated quantum measurement system ER - TY - JOUR AB - In the recent years important experimental advances in resonant electro-optic modulators as high-efficiency sources for coherent frequency combs and as devices for quantum information transfer have been realized, where strong optical and microwave mode coupling were achieved. These features suggest electro-optic-based devices as candidates for entangled optical frequency comb sources. In the present work, I study the generation of entangled optical frequency combs in millimeter-sized resonant electro-optic modulators. These devices profit from the experimentally proven advantages such as nearly constant optical free spectral ranges over several gigahertz, and high optical and microwave quality factors. The generation of frequency multiplexed quantum channels with spectral bandwidth in the MHz range for conservative parameter values paves the way towards novel uses in long-distance hybrid quantum networks, quantum key distribution, enhanced optical metrology, and quantum computing. AU - Rueda Sanchez, Alfredo R ID - 9242 IS - 2 JF - Physical Review A SN - 2469-9926 TI - Frequency-multiplexed hybrid optical entangled source based on the Pockels effect VL - 103 ER - TY - GEN AB - 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. AU - Peruzzo, Matilda AU - Hassani, Farid AU - Szep, Grisha AU - Trioni, Andrea AU - Redchenko, Elena AU - Zemlicka, Martin AU - Fink, Johannes M ID - 13057 TI - Geometric superinductance qubits: Controlling phase delocalization across a single Josephson junction ER - TY - JOUR AB - 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. AU - Peruzzo, Matilda AU - Hassani, Farid AU - Szep, Gregory AU - Trioni, Andrea AU - Redchenko, Elena AU - Zemlicka, Martin AU - Fink, Johannes M ID - 9928 IS - 4 JF - PRX Quantum KW - quantum physics KW - mesoscale and nanoscale physics TI - Geometric superinductance qubits: Controlling phase delocalization across a single Josephson junction VL - 2 ER - TY - THES AB - This work is concerned with two fascinating circuit quantum electrodynamics components, the Josephson junction and the geometric superinductor, and the interesting experiments that can be done by combining the two. The Josephson junction has revolutionized the field of superconducting circuits as a non-linear dissipation-less circuit element and is used in almost all superconducting qubit implementations since the 90s. On the other hand, the superinductor is a relatively new circuit element introduced as a key component of the fluxonium qubit in 2009. This is an inductor with characteristic impedance larger than the resistance quantum and self-resonance frequency in the GHz regime. The combination of these two elements can occur in two fundamental ways: in parallel and in series. When connected in parallel the two create the fluxonium qubit, a loop with large inductance and a rich energy spectrum reliant on quantum tunneling. On the other hand placing the two elements in series aids with the measurement of the IV curve of a single Josephson junction in a high impedance environment. In this limit theory predicts that the junction will behave as its dual element: the phase-slip junction. While the Josephson junction acts as a non-linear inductor the phase-slip junction has the behavior of a non-linear capacitance and can be used to measure new Josephson junction phenomena, namely Coulomb blockade of Cooper pairs and phase-locked Bloch oscillations. The latter experiment allows for a direct link between frequency and current which is an elusive connection in quantum metrology. This work introduces the geometric superinductor, a superconducting circuit element where the high inductance is due to the geometry rather than the material properties of the superconductor, realized from a highly miniaturized superconducting planar coil. These structures will be described and characterized as resonators and qubit inductors and progress towards the measurement of phase-locked Bloch oscillations will be presented. AU - Peruzzo, Matilda ID - 9920 KW - quantum computing KW - superinductor KW - quantum metrology SN - 2663-337X TI - Geometric superinductors and their applications in circuit quantum electrodynamics ER - TY - JOUR AB - The quantum bits (qubits) on which superconducting quantum computers are based have energy scales corresponding to photons with GHz frequencies. The energy of photons in the gigahertz domain is too low to allow transmission through the noisy room-temperature environment, where the signal would be lost in thermal noise. Optical photons, on the other hand, have much higher energies, and signals can be detected using highly efficient single-photon detectors. Transduction from microwave to optical frequencies is therefore a potential enabling technology for quantum devices. However, in such a device the optical pump can be a source of thermal noise and thus degrade the fidelity; the similarity of input microwave state to the output optical state. In order to investigate the magnitude of this effect we model the sub-Kelvin thermal behavior of an electro-optic transducer based on a lithium niobate whispering gallery mode resonator. We find that there is an optimum power level for a continuous pump, whilst pulsed operation of the pump increases the fidelity of the conversion. AU - Mobassem, Sonia AU - Lambert, Nicholas J. AU - Rueda Sanchez, Alfredo R AU - Fink, Johannes M AU - Leuchs, Gerd AU - Schwefel, Harald G.L. ID - 9815 IS - 4 JF - Quantum Science and Technology TI - Thermal noise in electro-optic devices at cryogenic temperatures VL - 6 ER -