Latest Papers

Quantum processors using superconducting qubits suffer from dielectric loss leading to noise and dissipation. Qubits are usually designed as large capacitor pads connected to a non-linear Josephson junction (or SQUID) by a superconducting thin metal wiring. Here, we report on finite-element simulation and experimental results confirming that more than 50% of surface loss in transmon qubits can originated from Josephson junctions wiring and can limit qubit relaxation time. Extracting dielectric loss tangents capacitor pads and wiring based on their participation ratios, we show dominant surface loss of wiring can occur for real qubits designs. Then, we simulate a qubit coupled to a bath of individual TLS defects and show that only a small fraction (~18%) of coupled defects is located within the wiring interfaces, however, their coupling strength is much higher due to stronger electromagnetic field. Finally, we fabricate six tunable floating transmon qubits and experimentally demonstrate up to 20% improvement in qubit quality factor by wiring design optimization.

Manipulating photon states serves as a primary requirement for various optical devices and is of high relevance for quantum information technology. Nevertheless, the fundamental theoretical framework for tens-of-photon states has not been established. This study successfully establishes the matrix-product-state theory to explore the statistics of the tens-of-photon states scattered by optical cavities (OCs), two-level atoms (TLAs), and Jaynes-Cummings emitters (JCEs) in waveguide-QED systems. Taking 10-photon states as an example, we reveal some novel physical results that differ from those for few-photon cases. We verify that OCs do not change the statistics of the incident photon states, being independent of the photon number. However, for the TLAs and JCEs, the photon number strongly impacts the photon bunching and anti-bunching behaviors. As the photon number increases, there exists a maximum strength for the photon-photon correlation induced by the JCE. Especially, the scattered waves by the TLA (or JCE) exhibit extremely different statistics behaviors for the 10-photon cases from those for the bi-photon. These distinguishable conclusions for the tens-of-photon states and the matrix-product-state theory pave the way for the multi-photon manipulation.

Transmon qubits arise from the quantization of nonlinear resonators, systems that are prone to the buildup of strong, possibly chaotic, fluctuations. Such instabilities will likely affect fast gate operations which involve the transient population of higher excited states outside the computational subspace. Here we show that a statistical analysis of the instantaneous eigenphases of the time evolution operator, in particular of their curvatures, allows for identifying the subspace affected by chaotic fluctuations. Our analysis shows that fast entangling gates, operating at speeds close to the so-called quantum speed limit, contain transient regimes where the dynamics indeed becomes partially chaotic for just two transmons.

Semiconductor-based superconducting qubits offer a versatile platform for studying hybrid quantum devices in circuit quantum electrodynamics (cQED) architecture. Most of these cQED experiments utilize coplanar waveguides, where the incorporation of DC gate lines is straightforward. Here, we present a technique for probing gate-tunable hybrid devices using a three-dimensional (3D) microwave cavity. A recess is machined inside the cavity wall for the placement of devices and gate lines. We validate this design using a hybrid device based on an InAs-Al nanowire Josephson junction. The coupling between the device and the cavity is facilitated by a long superconducting strip, the antenna. The Josephson junction and the antenna together form a gatemon qubit. We further demonstrate the gate-tunable cavity shift and two-tone qubit spectroscopy. This technique could be used to probe various quantum devices and materials in a 3D cQED architecture that requires DC gate voltages.

The utility of a quantum computer depends heavily on the ability to reliably perform accurate quantum logic operations. For finding optimal control solutions, it is of particular interest to explore model-free approaches, since their quality is not constrained by the limited accuracy of theoretical models for the quantum processor - in contrast to many established gate implementation strategies. In this work, we utilize a continuous-control reinforcement learning algorithm to design entangling two-qubit gates for superconducting qubits; specifically, our agent constructs cross-resonance and CNOT gates without any prior information about the physical system. Using a simulated environment of fixed-frequency, fixed-coupling transmon qubits, we demonstrate the capability to generate novel pulse sequences that outperform the standard cross-resonance gates in both fidelity and gate duration, while maintaining a comparable susceptibility to stochastic unitary noise. We further showcase an augmentation in training and input information that allows our agent to adapt its pulse design abilities to drifting hardware characteristics, importantly with little to no additional optimization. Our results exhibit clearly the advantages of unbiased adaptive-feedback learning-based optimization methods for transmon gate design.

Engineering high-fidelity two-qubit gates is an indispensable step toward practical quantum computing. For superconducting quantum platforms, one important setback is the stray interaction between qubits, which causes significant coherent errors. For transmon qubits, protocols for mitigating such errors usually involve fine-tuning the hardware parameters or introducing usually noisy flux-tunable couplers. In this work, we propose a simple scheme to cancel these stray interactions. The coupler used for such cancellation is a driven high-coherence resonator, where the amplitude and frequency of the drive serve as control knobs. Through the resonator-induced-phase (RIP) interaction, the static ZZ coupling can be entirely neutralized. We numerically show that such a scheme can enable short and high-fidelity entangling gates, including cross-resonance CNOT gates within 40 ns and adiabatic CZ gates within 140 ns. Our architecture is not only ZZ free but also contains no extra noisy components, such that it preserves the coherence times of fixed-frequency transmon qubits. With the state-of-the-art coherence times, the error of our cross-resonance CNOT gate can be reduced to below 1e-4.

The Holstein model, which describes purely local coupling of an itinerant excitation (electron, hole, exciton) with zero-dimensional (dispersionless) phonons, represents the paradigm for short-range excitation-phonon interactions. It is demonstrated here how spectral properties of small Holstein polarons -- heavily phonon-dressed quasiparticles, formed in the strong-coupling regime of the Holstein model -- can be extracted from an analog quantum simulator of this model. This simulator, which is meant to operate in the dispersive regime of circuit quantum electrodynamics, has the form of an array of capacitively coupled superconducting transmon qubits and microwave resonators, the latter being subject to a weak external driving. The magnitude of $XY$-type coupling between adjacent qubits in this system can be tuned through an external flux threading the SQUID loops between those qubits; this translates into an {\em in-situ} flux-tunable hopping amplitude of a fictitious itinerant spinless-fermion excitation, allowing one to access all the relevant physical regimes of the Holstein model. By employing the kernel-polynomial method, based on expanding dynamical response functions in Chebyshev polynomials of the first kind and their recurrence relation, the relevant single-particle momentum-frequency resolved spectral function of this system is computed here for a broad range of parameter values. To complement the evaluation of the spectral function, it is also explained how -- by making use of the many-body version of the Ramsey interference protocol -- this dynamical-response function can be measured in the envisioned analog simulator.

This article proposes a new method to increase the efficiency of stimulated Raman adiabatic passage (STIRAP) in superconducting circuits using a shortcut to the adiabaticity (STA) method. The STA speeds up the adiabatic process before decoherence has a significant effect, thus leading to increased efficiency. This method achieves fast, high-fidelity coherent population transfer, known as super-adiabatic STIRAP (saSTIRAP), in a dressed state-engineered $Λ$ system with polariton states in circuit QED.

Kerr parametric oscillators are potential building blocks for fault-tolerant quantum computers. They can stabilize Kerr-cat qubits, which offer advantages toward the encoding and manipulation of error-protected quantum information. The recent realization of Kerr-cat qubits made use of the nonlinearity of the SNAIL transmon superconducting circuit and a squeezing drive. Increasing nonlinearities can enable faster gate times, but, as shown here, can also induce chaos and melt the qubit away. We determine the region of validity of the Kerr-cat qubit and discuss how its disintegration could be experimentally detected. The danger zone for parametric quantum computation is also a potential playground for investigating quantum chaos with driven superconducting circuits.

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.

Off-resonant interaction of fluctuating photons in a resonator with a qubit increases the qubit dephasing rate. We use this effect to measure a small average number of intracavity photons that are coherently or thermally driven. For spectral resolution, we do this by subjecting the qubit to a Carr-Purcell-Meiboom-Gill (CPMG) sequence and record the qubit dephasing rate for various periods between qubit $π$-pulses. The recorded data is then analyzed with formulas for the photon-induced dephasing rate that we have derived for the non-Gaussian noise regime with an arbitrary ratio $2χ/κ$, where $2χ$ is the qubit frequency shift due to a single photon and $κ$ is the resonator decay rate. We show that the presented CPMG dephasing rate formulas agree well with experimental results and demonstrate measurement of thermal and coherent photon populations at the level of a few $10^{-4}$.

Quantum communications technologies require a network of quantum processors connected with low loss and low noise communication channels capable of distributing entangled states. Superconducting microwave qubits operating in cryogenic environments have emerged as promising candidates for quantum processor nodes. However, scaling these systems is challenging because they require bulky microwave components with high thermal loads that can quickly overwhelm the cooling power of a dilution refrigerator. Telecommunication frequency optical signals, meanwhile, can be fabricated in significantly smaller form factors while avoiding challenges due to high signal loss, noise sensitivity, and thermal loads due to their high carrier frequency and propagation in silica optical fibers. Transduction of information via coherent links between optical and microwave frequencies is therefore critical to leverage the advantages of optics for superconducting microwave qubits, while also enabling superconducting processors to be linked with low-loss optical interconnects. Here, we demonstrate coherent optical control of a superconducting qubit. We achieve this by developing a microwave-optical quantum transducer that operates with up to 1.18% conversion efficiency with low added microwave noise, and demonstrate optically-driven Rabi oscillations in a superconducting qubit.

Topological bandstructures interfering with moiré superstructures give rise to a plethora of emergent phenomena, which are pivotal for correlated insulating and superconducting states of twisttronics materials. While quasiperiodicity was up to now a notion mostly reserved for solid-state materials and cold atoms, we here demonstrate the capacity of conventional superconducting circuits to emulate moiré physics in charge space. With two examples, we show that Hofstadter's butterfly and the magic-angle effect, are directly visible in spectroscopic transport measurements. Importantly, these features survive in the presence of harmonic trapping potentials due to parasitic linear capacitances. Our proposed platform benefits from unprecedented tuning capabilities, and opens the door to probe incommensurate physics in virtually any spatial dimension.

The effects of squeezed photons and thermal photons on the entanglement dynamics of atom-atom, atom-field and field-field subsystems are studied for the double Jaynes-Cummings model. For this purpose, squeezed coherent states and Glauber-Lachs states of radiation are chosen as field states. For the atomic states, we choose one of the Bell state as pure state and a Werner-type state as mixed state. Werner-type state is used to understand the effects of mixedness on entanglement. To measure the entanglement between the two atoms, Wootters' concurrence is used; whereas for the atom-field and field-field subsystems, negativity is chosen. The squeezed photons and thermal photons create, destroy and transfer entanglement within various subsystems. Also, the addition of squeezed photons and thermal photons either lengthens or shortens the duration of entanglement sudden deaths (ESD) associated with atom-atom, atom-field and field-field entanglement dynamics in a complementary way. The effects of Ising-type interaction, detuning and Kerr-nonlinearity on the entanglement dynamics are studied. Each of these interactions removes the ESDs associated with various subsystems. We show that new entanglements are created in this atom-field system by introducing Ising-type interaction between the two atoms. With proper choice of the parameters corresponding to Ising-type interaction, detuning and Kerr-nonliearity, entanglement can be transferred among various subsystems.

Engineering the admittance of external environments connected to superconducting qubits is essential, as increasing the measurement speed introduces spontaneous emission loss to superconducting qubits, known as Purcell loss. Here, we report a broad bandwidth Purcell filter design within a small footprint, which effectively suppresses Purcell loss without losing the fast measurement speed. We characterize the filter's frequency response at 4.3 K and also estimate Purcell loss suppression by finite-element-method simulations of superconducting planar circuit layouts with the proposed filter design. The measured bandwidth is over 790 MHz within 0.29 mm$^2$ while the estimated lifetime enhancement can be over 5000 times with multiple Purcell filters. The presented filter design is expected to be easily integrated on existing superconducting quantum circuits for fast and multiplexed readout without occupying large footprint.

New exact formulas are derived for systems involving Landau-Zener transition rates and for absorption spectra in quantum dots. These rectify previous inaccurate approximations utilized in experimental studies. The exact formulas give an explicit expression for the maxima and minima of the transition rate at any oscillating period and reveal a number of striking physical consequences, such as the suppression of oscillations for half-integer values of the detuning parameter and that the periodic dependence on the detuning parameter changes at special values of the driving field amplitude. The fluorescence spectra of quantum dots exhibit similar properties.

Fixed-frequency superconducting qubits demonstrate remarkable success as platforms for stable and scalable quantum computing. Cross-resonance gates have been the workhorse of fixed-coupling, fixed-frequency superconducting processors, leveraging the entanglement generated by driving one qubit resonantly with a neighbor's frequency to achieve high-fidelity, universal CNOTs. Here, we use on-resonant and off-resonant microwave drives to go beyond cross-resonance, realizing natively interesting two-qubit gates that are not equivalent to CNOTs. In particular, we implement and benchmark native ISWAP, SWAP, $\sqrt{\text{ISWAP}}$, and BSWAP gates. Furthermore, we apply these techniques for an efficient construction of the B-gate: a perfect entangler from which any two-qubit gate can be reached in only two applications. We show these native two-qubit gates are better than their counterparts compiled from cross-resonance gates. We elucidate the resonance conditions required to drive each two-qubit gate and provide a novel frame tracking technique to implement them in Qiskit.

We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged ${T_1}$ energy relaxation times of up to ${270\,μs}$, corresponding to Q = 5 million, and a highest observed value of ${501\,μs}$. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.

Superconducting quantum processors have made significant progress in size and computing potential. However, the practical cryogenic limitations of operating large numbers of superconducting qubits are becoming a bottleneck for further scaling. Due to the low thermal conductivity and the dense optical multiplexing capacity of telecommunications fiber, converting qubit signal processing to the optical domain using microwave-to-optics transduction would significantly relax the strain on cryogenic space and thermal budgets. Here, we demonstrate optical readout of a superconducting transmon qubit through an optical fiber connected via a coaxial cable to a fully integrated piezo-optomechanical transducer. Using a demolition readout technique, we achieve a single shot readout fidelity of 81%. Our results illustrate the benefits of piezo-optomechanical transduction for low-dissipation operation of large quantum processors.

Quantum Sensing is a rapidly expanding research field that finds one of its applications in Fundamental Physics, as the search for Dark Matter. Devices based on superconducting qubits have already been successfully applied in detecting few-GHz single photons via Quantum Non-Demolition measurement (QND). This technique allows us to perform repeatable measurements, bringing remarkable sensitivity improvements and dark count rate suppression in experiments based on high-precision microwave photon detection, such as for Axions and Dark Photons search. In this context, the INFN Qub-IT project goal is to realize an itinerant single-photon counter based on superconducting qubits that will exploit QND for enhancing Axion search experiments. In this study, we present Qub-IT's status towards the realization of its first superconducting qubit device, illustrating design and simulation procedures and the characterization of fabricated Coplanar Waveguide Resonators (CPWs) for readout. We match target qubit parameters and assess a few-percent level agreement between lumped and distributed element simulation models. We reach a maximum internal quality factor of 9.2x10^5 for -92 dBm on-chip readout power.