Latest Papers

Quantum control of a linear oscillator using a static dispersive coupling to a nonlinear ancilla underpins a wide variety of experiments in circuit QED. Extending this control to more than one oscillator while minimizing the required connectivity to the ancilla would enable hardware-efficient multi-mode entanglement and measurements. We show that the spectrum of an ancilla statically coupled to a single mode can be made to depend on the joint photon number in two modes by applying a strong parametric beamsplitter coupling between them. This `joint-photon number-splitting' regime extends single-oscillator techniques to two-oscillator control, which we use to realize a hardware-efficient erasure check for a dual-rail qubit encoded in two superconducting cavities. By leveraging the beamsplitter coupling already required for single-qubit gates, this scheme permits minimal connectivity between circuit elements. Furthermore, the flexibility to choose the pulse shape allows us to limit the susceptibility to different error channels. We use this scheme to detect leakage errors with a missed erasure fraction of $(9.0 \pm 0.5)\times10^{-4}$, while incurring an erasure rate of $2.92 \pm 0.01\%$ and a Pauli error rate of $0.31 \pm 0.01\%$, both of which are dominated by cavity errors.

The emission of light from an atom represents a fundamental process that provides valuable insights into the atom-light interaction. The Jaynes-Cummings model is one of the simplest fully quantized models to deal with these interactions, allowing for an analytical solution, while exhibiting notable non-trivial effects. We explore new features in the fluorescence emission spectrum for initial "trapping states", which suppress the atomic population inversion. Despite the seemingly dormant activity of the atom, the resulting emission spectra exhibit rich features, and using a dressed-state coordinates formalism, we are able to quantitatively explain the different profiles in the spectrum. We generalize the trapping conditions for non-zero atom-field detuning and also unveil two types of trapping states that lead to spectra with three peaks, in contrast to previously known states: a center peak and one secondary peak on each side. These are a trapping state formed by a Schrödinger cat state with Poissonian statistics (Yurke-Stoler state) and also a different type of "perfect trapping state".

We present spiral resonators of thin film niobium (Nb) that exhibit large geometric inductance, high critical magnetic fields and high single photon quality factors. These low loss geometric inductors can be a compelling alternative to kinetic inductors to create high-impedance superconducting devices for applications that require magnetic fields. By varying the spiral pitch, we realize resonators with characteristic impedances ranging from 3.25-7.09 kΩ. We measure the temperature and magnetic field dependent losses and find that the high-impedance resonators maintain an intrinsic quality factor above {\sim} 10^5 for parallel magnetic fields of up to 1 T. These properties make spiral Nb resonators a promising candidate for quantum devices that require circuit elements with high impedance and magnetic field resilience.

The development of fault-tolerant quantum computers (FTQCs) is receiving increasing attention within the quantum computing community. Like conventional digital computers, FTQCs, which utilize error correction and millions of physical qubits, have the potential to address some of humanity's grand challenges. However, accurate estimates of the tangible scale of future FTQCs, based on transparent assumptions, are uncommon. How many physical qubits are necessary to solve a practical problem intractable for classical hardware? What costs arise from distributing quantum computation across multiple machines? This paper presents an architectural model of a potential FTQC based on superconducting qubits, divided into discrete modules and interconnected via coherent links. We employ a resource estimation framework and software tool to assess the physical resources required to execute specific quantum algorithms compiled into their graph-state form and arranged onto a modular superconducting hardware architecture. Our tool can predict the size, power consumption, and execution time of these algorithms based on explicit assumptions about the system's physical layout, thermal load, and modular connectivity. We assess the resources needed for quantum computation examples that serve as building blocks of proposed applications, quantifying the architectural bottlenecks and trade-offs that remain to be addressed to deliver utility.

Superconducting qubits are one of the most mature platforms for quantum computing, but significant performance improvements are still needed. To improve the engineering of these systems, 3D full-wave computational electromagnetics analyses are increasingly being used. Unfortunately, existing analysis approaches often rely on full-wave simulations using eigenmode solvers that are typically cumbersome, not robust, and computationally prohibitive if devices with more than a few qubits are to be analyzed. To improve the characterization of superconducting circuits while circumventing these drawbacks, this work begins the development of an alternative framework that we illustrate in the context of evaluating the qubit-qubit exchange coupling rate between transmon qubits. This is a key design parameter that determines the entanglement rate for fast multi-qubit gate performance and also affects decoherence sources like qubit crosstalk. Our modeling framework uses a field-based formalism in the context of macroscopic quantum electrodynamics, which we use to show that the qubit-qubit exchange coupling rate can be related to the electromagnetic dyadic Green's function linking the qubits together. We further show how the quantity involving the dyadic Green's function can be related to the impedance response of the system that can be efficiently computed with classical computational electromagnetics tools. We demonstrate the validity and efficacy of this approach by simulating four practical multi-qubit superconducting circuits and evaluating their qubit-qubit exchange coupling rates. We validate our results against a 3D numerical diagonalization method and against experimental data where available. We also demonstrate the impact of the qubit-qubit exchange coupling rate on qubit crosstalk by simulating a multi-coupler device and identifying operating points where the qubit crosstalk becomes zero.

Leakage from the computational subspace is a damaging source of noise that degrades the performance of most qubit types. Unlike other types of noise, leakage cannot be overcome by standard quantum error correction techniques and requires dedicated leakage reduction units. In this work, we study the effects of leakage mobility between superconducting qubits on the performance of a quantum stability experiment, which is a benchmark for fault-tolerant logical computation. Using the Fujitsu Quantum Simulator, we perform full density-matrix simulations of stability experiments implemented on the surface code. We observe improved performance with increased mobility, suggesting leakage mobility can itself act as a leakage reduction unit by naturally moving leakage from data to auxiliary qubits, where it is removed upon reset. We compare the performance of standard error-correction circuits with "patch wiggling", a specific leakage reduction technique where data and auxiliary qubits alternate their roles in each round of error correction. We observe that patch wiggling becomes inefficient with increased leakage mobility, in contrast to the improved performance of standard circuits. These observations suggest that the damage of leakage can be overcome by stimulating leakage mobility between qubits without the need for a dedicated leakage reduction unit.

Superconducting qubits can be sensitive to abrupt energy deposits caused by cosmic rays and ambient radioactivity. While previous studies have explored correlated effects in time and space due to cosmic ray interactions, we present the first direct comparison of a transmon qubit's performance measured at two distinct sites: the above-ground SQMS facility (Fermilab, US) and the deep-underground Gran Sasso Laboratory (Italy). Despite the stark difference in radiation levels, we observe a similar average qubit relaxation time of approximately 80 microseconds at both locations. To further investigate potential radiation-induced events, we employ a fast decay detection protocol, comparing the relative rates of triggered events between the two environments. Although intrinsic noise remains the dominant source of single errors in superconducting qubits, our analysis revealed a significant excess of radiation-induced events for high-coherence transmon qubits operated above-ground. Finally, using $γ$-ray sources with increasing activity levels, we evaluate the qubit response in a controlled low-background environment.

We introduce a modified Jaynes-Cummings model with single-photon cavity radiation field but with the atomic system instead of exchanging a single photon as in the Jaynes-Cummings model, it exchanges instead a squeezed photon with squeezing parameter r. This allows us to interpolate between the Rabi model, r = infinity, and the Jaynes-Cummings model, r = 0, by varying r. The model exhibits a quantum phase transition. Accordingly, the quantum phase transition realized in the Rabi model, giving rise to superradiance, also occurs in the Jaynes-Cummings model

Qudits hold great promise for efficient quantum computation and the simulation of high-dimensional quantum systems. Utilizing a local Hilbert space of dimension d > 2 is known to speed up certain quantum algorithms relative to their qubit counterparts given efficient local qudit control and measurement. However, the direct realization of high-dimensional rotations and projectors has proved challenging, with most experiments relying on decompositions of SU(d) operations into series of rotations between two-level subspaces of adjacent states and projective readout of a small number of states. Here we employ simultaneous multi-frequency drives to generate rotations and projections in an effective spin-7/2 system by mapping it onto the energy eigenstates of a superconducting circuit. We implement single-shot readout of the 8 states using a multi-tone dispersive readout (F_assignment = 88.3%) and exploit the strong nonlinearity in a high EJ/EC transmon to simultaneously address each transition and realize a spin displacement operator. By combining the displacement operator with a virtual SNAP gate, we realize arbitrary single-qudit unitary operations in O(d) physical pulses and extract spin displacement gate fidelities ranging from 0.997 to 0.989 for virtual spins of size j = 1 to j = 7/2. These native qudit operations could be combined with entangling operations to explore qudit-based error correction or simulations of lattice gauge theories with qudits. Our multi-frequency approach to qudit control and measurement can be readily extended to other physical platforms that realize a multi-level system coupled to a cavity and can become a building block for efficient qudit-based quantum computation and simulation.

We explore the statistical behavior of the light emanating from a coherently driven Jaynes-Cummings (JC) oscillator operating in the regime of multiphoton blockade with two monitored output channels causing the loss of coherence at equal rates. We do so by adopting an operational approach which draws the particle and wave aspects of the forwards scattered radiation together, building upon the relationship between quantum optical correlation functions and conditional measurements. We first derive an analytical expression of the intensity cross-correlation function at the peak of the two-photon JC resonance to demonstrate the breakdown of detailed balance. The application of quantum trajectory theory in parallel with the quantum regression formula subsequently uncovers various aspects of temporal asymmetry in the quantum fluctuations characterizing the cascaded process through which a multiphoton resonance is established and read out. We find that monitoring different quadratures of the cavity field in conditional homodyne detection affects the times waited between successive photon counter ``clicks'', which in turn trigger the sampling of the homodyne current. Despite the fact that the steady-state cavity occupation is of the order of a photon, monitoring of the developing bimodality also impacts on the ratio between the emissions directed along the two decoherence channels.

We investigate the occurrence of direct and dual Shapiro steps for a Josephson junction coupled to a finite-size transmission line resonator. We treat both problems through a circuit QED approach with a large, but finite number of photon modes. For the dual case, we do not assume the (approximate) charge-phase duality, but include the full multi-band dynamics for the Josephson junction. Mean-field equations within such Hamiltonian approach reproduce the result obtained through a dissipative classical equation when the number of transmission line modes is large enough. To account for quantum and thermal fluctuations, we go beyond the mean-field treatment within a truncated Wigner approach. The fluctuations are shown to modify both the direct and the dual steps. We show how the dual steps are very sensitive to these fluctuations and identify the key physical parameters for the junction and the transmission line controlling their robustness, which is essential for applications to close the quantum metrological triangle.

Light-matter interactions provide an ideal testground for interplay of critical phenomena, topological transitions, quantum metrology and non-Hermitian physics. We consider two fundamental non-Hermitian Jaynes-Cummings models which possess real energy spectra in parity-time (PT) symmetry and anti-PT symmetry. We show that the quantum Fisher information is critical around the transitions at the exceptional points and exhibits a super universality with respect to different parameters, all energy levels, both models, symmetric phases and symmetry-broken phases. The transitions are found to be both symmetry-breaking Landau-class transitions (LCTs) and symmetry-protected topological-class of transitions (TCTs), thus realizing a simultaneous occurrence of critical LCTs and TCTs which are conventionally incompatible due to contrary symmetry requirements.

The precise engineering of electromagnetic couplings is paramount for constructing scalable and highfidelity superconducting quantum processors. While essential for orchestrating qubit operations, these couplings also present significant design challenges, including the mitigation of crosstalk and the management of environmental decoherence. A clear and unified theoretical framework is therefore crucial for the design, simulation, and analysis of these complex quantum circuits. This paper presents a comprehensive theoretical treatment of the fundamental electromagnetic coupling mechanisms in superconducting devices. Starting from first principles, we formulate the equations of motion and derive the input-output relations for canonical systems, including a single resonator coupled to a multi-port microwave network, interacting resonators, and coupled transmission lines. We review rigorous definitions for key parameters such as the energy decay rate (\k{appa}) and the dimensionless coupling coefficient (ζ) and connect these formalisms to practical methods of parameter extraction from electromagnetic simulations. This work provides a rigorous and pedagogical foundation for understanding and modeling linear electromagnetic interactions, serving as a vital resource for the development of advanced superconducting quantum hardware.

In this paper, the entanglement dynamics of different subsystems such as atom-atom, atom-field and field-field with radiation field in squeezed coherent thermal states for the intensity-dependent double Jaynes-Cummings model (IDDJCM) and double Jaynes-Cummings model (DJCM) are investigated. The effects of squeezed and thermal photons on entanglement are examined, revealing their complementary roles in shaping the entanglement behavior in both models. One of the main features of the double Jaynes-Cummings model is the observation of entanglement sudden death for every subsystem. The effects of various interactions such as Ising interaction, single photon exchange interaction and dipole-dipole interaction on entanglement dynamics are studied. The effects of detuning, Kerr-nonlinearity on the entanglement dynamics are investigated for every subsystem. It is noticed that proper choice of the interactions parameters, detuning and Kerr-nonlinearity effectively removes entanglement deaths from the dynamics.

In superconducting qubits, suppression of spontaneous emission is essential to achieve fast dispersive measurement and reset without sacrificing qubit lifetime. We show that resonator-mediated decay of the qubit mode to the feedline can be suppressed using destructive interference, where the readout resonator is coupled to the feedline at two points. This "interferometric Purcell filter" does not require dedicated filter components or impedance mismatch in the feedline, making it suitable for applications such as all-pass readout. We design and fabricate a device with the proposed scheme and demonstrate suppression of resonator-mediated decay that exceeds 2 orders of magnitude over a bandwidth of 400 MHz for a resonator linewidth of 13.8 MHz.

We report a comparative study on microwave control lines for a transmon qubit using: (i) flexible stripline transmission lines, and (ii) semi-rigid coaxial cables. During each experiment we performed repeated measurements of the energy relaxation and coherence times of a transmon qubit using one of the wiring configurations. Each measurement run spanned 70 h to 250 h of measurement time, and four separate cooldowns were performed so that each configuration could be tested twice. From these datasets we observe that changing the microwave control lines from coaxial cables to flexible stripline transmission lines does not have a measurable effect on coherence compared to thermal cycling the system, or random coherence fluctuations. Our results open up the possibility of large scale integration of qubit control lines with integrated component with planar layouts on flexible substrate.

Realizing the full potential of quantum computing requires large-scale quantum computers capable of running quantum error correction (QEC) to mitigate hardware errors and maintain quantum data coherence. While quantum computers operate within a two-level computational subspace, many processor modalities are inherently multi-level systems. This leads to occasional leakage into energy levels outside the computational subspace, complicating error detection and undermining QEC protocols. The problem is particularly severe in engineered qubit devices like superconducting transmons, a leading technology for fault-tolerant quantum computing. Addressing this challenge requires effective multi-level quantum system readout to identify and mitigate leakage errors. We propose a scalable, high-fidelity three-level readout that reduces FPGA resource usage by $60\times$ compared to the baseline while reducing readout time by 20\%, enabling faster leakage detection. By employing matched filters to detect relaxation and excitation error patterns and integrating a modular lightweight neural network to correct crosstalk errors, the protocol significantly reduces hardware complexity, achieving a $100\times$ reduction in neural network size. Our design supports efficient, real-time implementation on off-the-shelf FPGAs, delivering a 6.6\% relative improvement in readout accuracy over the baseline. This innovation enables faster leakage mitigation, enhances QEC reliability, and accelerates the path toward fault-tolerant quantum computing.

Frequency-multiplexing is an effective method to achieve resource-efficient superconducting qubit readout. Allowing multiple resonators to share a common feedline, the number of cables and passive components involved in the readout of a qubit can be drastically reduced. However, this improvement in scalability comes at the price of a crucial non-ideality -- an increased readout crosstalk. Prior works have targeted building better devices and discriminators to reduce its effects, as readout-crosstalk-induced qubit measurement errors are detrimental to the reliability of a quantum computer. However, in this work, we show that beyond the reliability of a system, readout crosstalk can introduce vulnerabilities in a system being shared among multiple users. These vulnerabilities are directly related to correlated errors due to readout crosstalk. These correlated errors can be exploited by nefarious attackers to predict the state of the victim qubits, resulting in information leakage.

We measure space- and time-correlated charge jumps on a four-qubit device, operating 107 meters below the Earth's surface in a low-radiation, cryogenic facility designed for the characterization of low-threshold particle detectors. The rock overburden of this facility reduces the cosmic ray muon flux by over 99% compared to laboratories at sea level. Combined with 4$π$ coverage of a movable lead shield, this facility enables quantifiable control over the flux of ionizing radiation on the qubit device. Long-time-series charge tomography measurements on these weakly charge-sensitive qubits capture discontinuous jumps in the induced charge on the qubit islands, corresponding to the interaction of ionizing radiation with the qubit substrate. The rate of these charge jumps scales with the flux of ionizing radiation on the qubit package, as characterized by a series of independent measurements on another energy-resolving detector operating simultaneously in the same cryostat with the qubits. Using lead shielding, we achieve a minimum charge jump rate of 0.19$^{+0.04}_{-0.03}$ mHz, almost an order of magnitude lower than that measured in surface tests, but a factor of roughly eight higher than expected based on reduction of ambient gammas alone. We operate four qubits for over 22 consecutive hours with zero correlated charge jumps at length scales above three millimeters.

Superconducting quantum processors are a compelling platform for analog quantum simulation due to the precision control, fast operation, and site-resolved readout inherent to the hardware. Arrays of coupled superconducting qubits natively emulate the dynamics of interacting particles according to the Bose-Hubbard model. However, many interesting condensed-matter phenomena emerge only in the presence of electromagnetic fields. Here, we emulate the dynamics of charged particles in an electromagnetic field using a superconducting quantum simulator. We realize a broadly adjustable synthetic magnetic vector potential by applying continuous modulation tones to all qubits. We verify that the synthetic vector potential obeys requisite properties of electromagnetism: a spatially-varying vector potential breaks time-reversal symmetry and generates a gauge-invariant synthetic magnetic field, and a temporally-varying vector potential produces a synthetic electric field. We demonstrate that the Hall effect--the transverse deflection of a charged particle propagating in an electromagnetic field--exists in the presence of the synthetic electromagnetic field.