Microwave drives are essential for implementing control and readout operations in superconducting quantum circuits. However, increasing the drive strength eventually leads to unwantedstate transitions which limit the speed and fidelity of such operations. In this work, we systematically investigate such transitions in a fixed-frequency qubit subjected to microwave drives spanning a 9 GHz frequency range. We identify the physical origins of these transitions and classify them into three categories. (1) Resonant energy exchange with parasitic two-level systems, activated by drive-induced ac-Stark shifts, (2) multi-photon transitions to non-computational states, intrinsic to the circuit Hamiltonian, and (3) inelastic scattering processes in which the drive causes a state transition in the superconducting circuit, while transferring excess energy to a spurious electromagnetic mode or two-level system (TLS) material defect. We show that the Floquet steady-state simulation, complemented by an electromagnetic simulation of the physical device, accurately predicts the observed transitions that do not involve TLS. Our results provide a comprehensive classification of these transitions and offer mitigation strategies through informed choices of drive frequency as well as improved circuit design.
We present a SNAIL-based parametric amplifier that integrates a lumped-element impedance matching network for increased bandwidth and an on-chip pump-port filter for efficient pumpdelivery. The amplifier is fabricated using a single-layer optical lithography step, followed by a single-layer electron beam lithography step. We measure a flat 20 dB gain profile with less than 1 dB ripple across a bandwidth of up to 250 MHz on multiple devices, demonstrating robust performance against variations arising from fabrication and packaging. We characterize the amplifier’s linearity by analyzing gain compression and intermodulation distortion under simultaneous multi-tone excitation. We show that the intermodulation products remain suppressed by more than 23 dB relative to the signal tones, even at the 1 dB gain compression point. We further validate its utility by performing simultaneous high-fidelity readout of two transmon qubits, achieving state assignment fidelities of 99.51% and 98.55%. The combination of compact design, fabrication simplicity, and performance robustness makes this amplifier a practical device for quantum experiments with superconducting circuits.
Readout of superconducting qubits faces a trade-off between measurement speed and unwanted back-action on the qubit caused by the readout drive, such as T1 degradation and leakage outof the computational subspace. The readout is typically benchmarked by integrating the readout signal and choosing a binary threshold to extract the „readout fidelity“. We show that such a characterization may significantly overlook readout-induced leakage errors. We introduce a method to quantitatively assess this error by repeatedly executing a composite operation — a readout preceded by a randomized qubit-flip. We apply this technique to characterize the dispersive readout of an intrinsically Purcell-protected qubit. We report a binary readout fidelity of 99.63% and quantum non-demolition (QND) fidelity exceeding 99.00% which takes into account a leakage error rate of 0.12±0.03%, under a repetition rate of (380ns)−1 for the composite operation.
Encoding a qubit in a high quality superconducting microwave cavity offers the opportunity to perform the first layer of error correction in a single device, but presents a challenge:how can quantum oscillators be controlled while introducing a minimal number of additional error channels? We focus on the two-qubit portion of this control problem by using a 3-wave mixing coupling element to engineer a programmable beamsplitter interaction between two bosonic modes separated by more than an octave in frequency, without introducing major additional sources of decoherence. Combining this with single-oscillator control provided by a dispersively coupled transmon provides a framework for quantum control of multiple encoded qubits. The beamsplitter interaction gbs is fast relative to the timescale of oscillator decoherence, enabling over 103 beamsplitter operations per coherence time, and approaching the typical rate of the dispersive coupling χ used for individual oscillator control. Further, the programmable coupling is engineered without adding unwanted interactions between the oscillators, as evidenced by the high on-off ratio of the operations, which can exceed 105. We then introduce a new protocol to realize a hybrid controlled-SWAP operation in the regime gbs≈χ, in which a transmon provides the control bit for the SWAP of two bosonic modes. Finally, we use this gate in a SWAP test to project a pair of bosonic qubits into a Bell state with measurement-corrected fidelity of 95.5%±0.2%.
The ambition of harnessing the quantum for computation is at odds with the fundamental phenomenon of decoherence. The purpose of quantum error correction (QEC) is to counteract thenatural tendency of a complex system to decohere. This cooperative process, which requires participation of multiple quantum and classical components, creates a special type of dissipation that removes the entropy caused by the errors faster than the rate at which these errors corrupt the stored quantum information. Previous experimental attempts to engineer such a process faced an excessive generation of errors that overwhelmed the error-correcting capability of the process itself. Whether it is practically possible to utilize QEC for extending quantum coherence thus remains an open question. We answer it by demonstrating a fully stabilized and error-corrected logical qubit whose quantum coherence is significantly longer than that of all the imperfect quantum components involved in the QEC process, beating the best of them with a coherence gain of G=2.27±0.07. We achieve this performance by combining innovations in several domains including the fabrication of superconducting quantum circuits and model-free reinforcement learning.
Three-wave mixing is a key process in superconducting quantum information processing, being involved in quantum-limited amplification and parametric coupling between superconductingcavities. These operations can be implemented by SNAIL-based devices that present a Kerr-free flux-bias point where unwanted parasitic effects such as Stark shift are suppressed. However, with a single flux-bias parameter, these circuits can only host one Kerr-free point, limiting the range of their applications. In this Letter, we demonstrate how to overcome this constraint with a gradiometric SNAIL, a doubly-flux biased superconducting circuit for which both effective inductance and Kerr coefficient can be independently tuned. Experimental data show the capability of the gradiometric SNAIL to suppress Kerr effect in a three-wave mixing parametric amplifier over a continuum of flux bias points corresponding to a 1.7 GHz range of operating frequencies.
Two promising architectures for solid-state quantum information processing are electron spins in semiconductor quantum dots and the collective electromagnetic modes of superconductingcircuits. In some aspects, these two platforms are dual to one another: superconducting qubits are more easily coupled but are relatively large among quantum devices (∼mm), while electrostatically-confined electron spins are spatially compact (∼μm) but more complex to link. Here we combine beneficial aspects of both platforms in the Andreev spin qubit: the spin degree of freedom of an electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of a Josephson semiconductor nanowire. We demonstrate coherent spin manipulation by combining single-shot circuit-QED readout and spin-flipping Raman transitions, finding a spin-flip time TS=17 μs and a spin coherence time T2E=52 ns. These results herald a new spin qubit with supercurrent-based circuit-QED integration and further our understanding and control of Andreev levels — the parent states of Majorana zero modes — in semiconductor-superconductor heterostructures.
Nanofabrication techniques for superconducting qubits rely on resist-based masks patterned by electron-beam or optical lithography. We have developed an alternative nanofabricationtechnique based on free-standing silicon shadow masks fabricated from silicon-on-insulator wafers. These silicon shadow masks not only eliminate organic residues associated with resist-based lithography, but also provide a pathway to better understand and control surface-dielectric losses in superconducting qubits by decoupling mask fabrication from substrate preparation. We have successfully fabricated aluminum 3D transmon superconducting qubits with these shadow masks, and demonstrated energy relaxation times on par with state-of-the-art values.
The interaction of photons and coherent quantum systems can be employed to detect electromagnetic radiation with remarkable sensitivity. We introduce a quantum radiometer based on thephoton-induced-dephasing process of a superconducting qubit for sensing microwave radiation at the sub-unit-photon level. Using this radiometer, we demonstrated the radiative cooling of a 1-K microwave resonator and measured its mode temperature with an uncertainty ~0.01 K. We have thus developed a precise tool for studying the thermodynamics of quantum microwave circuits, which provides new solutions for calibrating hybrid quantum systems and detecting candidate particles for dark matter.
We introduce a novel near-quantum-limited amplifier with a large tunable bandwidth and high dynamic range – the Josephson Array Mode Parametric Amplifier (JAMPA). The signal andidler modes involved in the amplification process are realized by the array modes of a chain of 1000 flux tunable, Josephson-junction-based, nonlinear elements. The frequency spacing between array modes is comparable to the flux tunability of the modes, ensuring that any desired frequency can be occupied by a resonant mode, which can further be pumped to produce high gain. We experimentally demonstrate that the device can be operated as a nearly quantum-limited parametric amplifier with 20 dB of gain at almost any frequency within (4-12) GHz band. On average, it has a 3 dB bandwidth of 11 MHz and input 1 dB compression power of -108 dBm, which can go as high as -93 dBm. We envision the application of such a device to the time- and frequency-multiplexed readout of multiple qubits, as well as to the generation of continuous-variable cluster states.