Latest Papers

The entanglement of microwave photons and spin qubits in silicon represents a pivotal step forward for quantum information processing utilizing semiconductor quantum dots. Such hybrid spin circuit quantum electrodynamics (cQED) has been achieved by granting a substantial electric dipole moment to a spin by de-localizing it in a double quantum dot under spin-orbit interaction, thereby forming a flopping-mode (FM) spin qubit. Despite its promise, the coherence properties demonstrated to date remain insufficient to envision FM spin qubits as practical single qubits. Here, we present a FM hole spin qubit in a silicon nanowire coupled to a high-impedance niobium nitride microwave resonator for readout. We report Rabi frequencies exceeding 100 MHz and coherence times in the microsecond range, resulting in a high single gate quality factor of 380. This establishes FM spin qubits as fast and reliable qubits. Moreover, using the large frequency tunability of the FM qubit, we reveal for the first time that photonic effects predominantly limit coherence, with radiative decay being the main relaxation channel and photon shot-noise inducing dephasing. These results highlight that optimized microwave engineering can unlock the potential of FM spin qubits in hybrid cQED architectures, offering a scalable and robust platform for fast and coherent spin qubits with strong coupling to microwave photons.

Throughout multiple cooldowns we observe a power-law reduction in time for the rate of multi-qubit correlated poisoning events, while the rate of shifts in qubit offset-charge remains constant; evidence of a non-ionizing source of pair-breaking phonon bursts for superconducting qubits. We investigate different types of sample packaging, some of which are sensitive to mechanical impacts from the cryocooler pulse tube. One possible source of these events comes from relaxation of thermally-induced stresses from differential thermal contraction between the device layer and substrate.

In the pursuit of robust quantum computing, we put forth a platform based on photonic qubits in a circuit-QED environment. Specifically, we propose a versatile two-qubit gate based on two cavities coupled via a transmon, constituting a selective number-dependent phase gate operating on the in-phase eigenmodes of the two cavities, the Eigen-SNAP gate. This gate natively operates in the dispersive coupling regime of the cavities and the transmon, and operates by driving the transmon externally, to imprint desired phases on the number states. As an example for the utility of the Eigen-SNAP gate, we implement a $\sqrt{\text{SWAP}}$ gate on a system of two logical bosonic qubits encoded in the cavities. Further, we use numerical optimization to determine the optimal implementation of the $\sqrt{\text{SWAP}}$. We find that the fidelities of these optimal protocols are only limited by the coherence times of the system's components. These findings pave the way to continuous variable quantum computing in cavity-transmon systems.

When a high-energy particle, such as a $γ$-ray or muon, impacts the substrate of a superconducting qubit chip, large numbers of electron-hole pairs and phonons are created. The ensuing dynamics of the electrons and holes changes the local offset-charge environment for qubits near the impact site. The phonons that are produced have energy above the superconducting gap in the films that compose the qubits, leading to quasiparticle excitations above the superconducting ground state when the phonons impinge on the qubit electrodes. An elevated density of quasiparticles degrades qubit coherence, leading to errors in qubit arrays. Because these pair-breaking phonons spread throughout much of the chip, the errors can be correlated across a large portion of the array, posing a significant challenge for quantum error correction. In order to study the dynamics of $γ$-ray impacts on superconducting qubit arrays, we use a $γ$-ray source outside the dilution refrigerator to controllably irradiate our devices. By using charge-sensitive transmon qubits, we can measure both the offset-charge shifts and quasiparticle poisoning due to the $γ$ irradiation at different doses. We study correlations between offset-charge shifts and quasiparticle poisoning for different qubits in the array and compare this with numerical modeling of charge and phonon dynamics following a $γ$-ray impact. We thus characterize the poisoning footprint of these impacts and quantify the performance of structures for mitigating phonon-mediated quasiparticle poisoning.

Jaynes-Cummings Hamiltonian provides the elemental description of a two-level system interacting with a photonic mode. In this Article, we derive an expression for the transmission response via a photonic signal that describes the hybridized states as separate photonic modes. As a result, we obtain the effective input/output couplings and the internal losses of each mode. These set the decoherence rate of the hybridized states, and provide a simple description of the strength of the response signal, that we call "visibility", and its linewidth. In particular, the result allows us to describe a situation where the coherence increases significantly while the signal remains strongly visible in the response.

Scaling up a superconducting quantum computer will likely require quantum communication between remote chips, which can be implemented using an itinerant microwave photon in a transmission line. To realize high-fidelity communication, it is essential to control the frequency and temporal shape of the microwave photon. In this work, we demonstrate the generation of frequency-tunable shaped microwave photons without resorting to any frequency-tunable circuit element. We develop a framework which treats a microwave resonator as a band-pass filter mediating the interaction between a superconducting qubit and the modes in the transmission line. This interpretation allows us to stimulate the photon emission by an off-resonant drive signal. We characterize how the frequency and temporal shape of the generated photon depends on the frequency and amplitude of the drive signal. By modulating the drive amplitude and frequency, we achieve a frequency tunability of 40 MHz while maintaining the photon mode shape time-symmetric. Through measurements of the quadrature amplitudes of the emitted photons, we demonstrate consistently high state and process fidelities around 95\% across the tunable frequency range. Our hardware-efficient approach eliminates the need for additional biasing lines typically required for frequency tuning, offering a simplified architecture for scalable quantum communication.

Exploring the noise spectrum impacting a qubit and extending its coherence duration are fundamental components of quantum technologies. In this study, we introduce parametric spectroscopy, a method that merges parametric modulation of a qubit's energy gap with dynamical decoupling sequences. The parametric modulation provides high sensitivity to extensive regions of the noise spectrum, while dynamical decoupling reduces the effect of driving noise. Our theoretical study shows that parametric spectroscopy enables access to the difficult high-frequency domain of the flux spectrum in transmons.

State-of-the-art superconducting quantum processors containing tens to hundreds of qubits have demonstrated the building blocks for realizing fault-tolerant quantum computation. Nonetheless, a fundamental barrier to scaling further is the prevalence of fluctuating quantum two-level system (TLS) defects that can couple resonantly to qubits, causing excess decoherence and enhanced gate errors. Here we introduce a scalable architecture for site-specific and in-situ manipulation of TLS frequencies out of the spectral vicinity of our qubits. Our method is resource efficient, combining TLS frequency tuning and universal single qubit control into a single on-chip control line per qubit. We independently control each qubit's dissipative environment to dynamically improve both qubit coherence times and single qubit gate fidelities -- with a constant time overhead that does not scale with the device size. Over a period of 40 hours across 6 qubits, we demonstrate a $36\%$ improvement in average single qubit error rates and a $17\%$ improvement in average energy relaxation times. Critically, we realize a 4-fold suppression in the occurrence of TLS-induced performance outliers, and a complete reduction of simultaneous outlier events. These results mark a significant step toward overcoming the challenges that TLS defects pose to scaling superconducting quantum processors.

The implementation of a quantum router capable of performing both quantum signal routing and quantum addressing (a Q2-router) represents a key step toward building quantum networks and quantum random access memories. We realize a Q2-router that uses fixed-frequency transmon qubits to implement a routing protocol based on two native controlled-iSWAP gates. These gates leverage a large ZZ interaction to selectively route information according to a quantum address. We find an estimated average routing fidelity of 95.3%, with errors arising primarily from decoherence or state preparation and measurement. We present a comprehensive calibration and characterization of both the c-iSWAP gates and the overall routing protocol through randomized benchmarking techniques and state tomography.

A high-fidelity CZ gate utilizing a double-transmon coupler (DTC) has recently been demonstrated as a building block for superconducting quantum processors. Like many other kinds of tunable couplers, however, the DTC requires a finite DC current for flux-biasing the coupler at the idling point to turn off the coupling, necessitating extra care for wiring and heat-load management. To address this issue, we theoretically propose and experimentally realize a novel coupling scheme by introducing a shunt capacitance between the two transmons of the DTC at zero-flux bias, which demonstrates high-fidelity CZ-gate performance comparable to the previous DTC. Through a comprehensive error budget analysis using multiple randomized benchmarking methods, we also identify that the current fidelity is limited by the decoherence through the coupler. Moreover, we experimentally demonstrate the wide operational flux range of the capacitively shunted DTC, which solves the challenging issue of remnant flux existing even with careful magnetic shielding.

The quantum Rabi model (QRM) is a cornerstone in the study of light-matter interactions within cavity and circuit quantum electrodynamics (QED). It effectively captures the dynamics of a two-level system coupled to a single-mode resonator, serving as a foundation for understanding quantum optical phenomena in a great variety of systems. However, this model may produce inaccurate results for large coupling strengths, even in systems with high anharmonicity. Moreover, issues of gauge invariance further undermine its reliability. In this work, we introduce a renormalized QRM that incorporates the effective influence of higher atomic energy levels, providing a significantly more accurate representation of the system while still maintaining a two-level description. To demonstrate the versatility of this approach, we present two different examples: an atom in a double-well potential and a superconducting artificial atom (fluxonium qubit). This procedure opens new possibilities for precisely engineering and understanding cavity and circuit QED systems, which are highly sought-after, especially for quantum information processing.

We present a flip-chip architecture for an array of coupled superconducting qubits, in which circuit components reside inside individual microwave enclosures. In contrast to other flip-chip approaches, the qubit chips in our architecture are electrically floating, which guarantees a simple, fully modular assembly of capacitively coupled circuit components such as qubit, control, and coupling structures, as well as reduced crosstalk between the components. We validate the concept with a chain of three nearest neighbor coupled generalized flux qubits in which the center qubit acts as a frequency-tunable coupler. Using this coupler, we demonstrate a transverse coupling on/off ratio $\approx$ 50, zz-crosstalk $\approx$ 0.7 kHz between resonant qubits and isolation between the qubit enclosures > 60 dB.

The article addresses the important and relevant task of remote induction of quantum dynamic scenarios. This involves transferring such scenarios from donor atoms to a target atom. This induction is based on the enhancement of quantum transitions in the presence of multiple photons of the same transition. We use the quantum master equation for the Tavis-Cummings-Hubbard (TCH) model with multiple cavities connected to the target cavity via waveguides. The dependence of the efficiency and transfer of the scenario on the number of donor cavities, the number of atoms in them, and the bandwidth of the waveguides is investigated.

The process of measuring a qubit and re-initializing it to the ground state practically lead to long qubit idle times between re-runs of experiments on a superconducting quantum computer. Here, we propose a protocol for a \textit{demolition measurement} of a transmon qubit that integrates qubit readout with the reset process to minimize qubit idle time. We present a three-staged implementation of this protocol, involving a combined qubit readout and resonator reset scheme that unconditionally resets the resonator at the end of the readout; a leakage removal scheme that can be integrated with the measurement stage; and an unconditional qubit reset. We demonstrate that this protocol could be implemented in $1 \, μ$s with greater than $95 \, \%$ reset fidelity and a $99 \, \%$ readout fidelity without any hardware overhead beyond those commonly used. This provides at least a 50x speed up compared to the passive decay of the qubit, thereby significantly increasing the data-acquisition rate.

We propose a superconducting qubit based on engineering the first and second harmonics of the Josephson energy and phase relation $E_{J1}\cos \varphi$ and $E_{J2}\cos 2\varphi$. By constructing a circuit such that $E_{J2}$ is negative and $|E_{J1}| \ll |E_{J2}|$, we create a periodic potential with two non-degenerate minima. The qubit, which we dub "harmonium", is formed from the lowest-energy states of each minimum. Bit-flip protection of the qubit arises due to the localization of each qubit state to their respective minima, while phase-flip protection can be understood by considering the circuit within the Born-Oppenheimer approximation. We demonstrate with time-domain simulations that single- and two-qubit gates can be performed in approximately one hundred nanoseconds. Finally, we compute the qubit coherence times using numerical diagonalization of the complete circuit in conjunction with state-of-the-art noise models. We estimate out-of-manifold heating times on the order of milliseconds, which can be treated as erasure errors using conventional dispersive readout. We estimate pure-dephasing times on the order of many tens of milliseconds, and bit-flip times on the order of seconds.

Leakage out from the qubit subspace compromises standard quantum error correction protocols and is a challenge for practical quantum computing. We propose a passive leakage removal unit based on an array of coupled disordered transmons and last-site reset by feedback-measurement or dissipation. The transmons have parametric disorder both in frequency and anharmonicity such that the qubit subspace is protected by localization through energy level mismatch while the energy levels for leakage excitations are in resonance for maximized leakage mobility. Leakage excitations propagate through the idle transmons until reaching the last site with feedback-measurement or dissipation removing them. For removing leakage excitations, we find two optimal measurement rates, which are comprehensively understood through two distinct timescales between the propagation and disintegration of leakage excitations. Based only on an array of standard transmon devices, our approach is readily compatible with existing superconducting quantum processor designs under realistic conditions.

We revisit the Jaynes-Cummings model as an autonomous thermodynamic machine, where a qubit is driven by a cavity containing initially a large coherent field. Our analysis reveals a transition between the expected behavior of ideal-work source of the cavity at short times, and a long-time dynamics where the cavity autonomously measures the qubit and exerts a result-dependent drive. This autonomous feedback then purifies the qubit irrespective of its initial state. We show that the cavity functions thermodynamically as an autonomous Maxwell demon, trading mutual information for cooling power.

Entangling power is crucial for quantum information processing. This study examines the Anti-Jaynes-Cummings Model (AJCM) in generating quantum correlations between two atoms interacting via the Ising model and its effect on the entangled system. The AJCM is shown to create entanglement suitable as a quantum channel for information encoding. Interaction parameters act as controls to enhance quantum correlations, increase the capacity of the final atomic state, and improve system efficiency. When the atomic system starts in a maximally entangled state, increasing interaction strength and mean photon number further boosts concurrence and channel capacity.

We introduce a general protocol for obtaining the charge basis density matrix of a superconducting quantum circuit. Inspired by cavity state tomography, our protocol combines Josephson-energy pulse sequences and projective charge-basis readout to access the off-diagonal elements of the density matrix, a scheme we thus dub charge basis tomography. We simulate the reconstruction of the ground state of a target transmon using the Aharonov-Casher effect in a probe qubit to realise projective readout and show the Hilbert-Schmidt distance can detect deviations from the correct model Hamiltonian. Unlocking this ability to validate models using the ground state sets the stage for using transmons to detect interacting and topological phases, particularly in materials where time-domain and spectroscopic probes can be limited by intrinsic noise.

Semiconductor-based Josephson junctions embedded within a Cooper-pair-box can host complex many-body states, such as interacting Andreev states and potentially other quasi-particles of topological origin. Here, we study the insights that could be revealed from a tomographic reconstruction of the Cooper-pair charge distribution of the junction prepared in its ground state. We posit that interacting and topological states can be identified from distinct signatures within the probability distribution of the charge states. Furthermore, the comprehensive dataset provides direct access to information theory metrics elucidating the entanglement between the charge sector of the superconductor and the microscopic degrees of freedom in the junction. We demonstrate how these metrics serve to further classify differences between the types of excitations in the junction.