Session G48: Superconducting Qubits: Gates & Readout

Non-standard gates in superconducting parametric gate architectures

One of the main challenges in the realization of superconducting quantum computers is the ability to do high fidelity two-qubit entangling gates. Coupling transmon qubits with a parametrically driven tunable coupler enables fast entangling gates, but strong drives can introduce leakage and add unwanted coherent drive terms to the Hamiltonian. Targeting non-standard gates generates a family of operations that allows flexibility to pick gates with low leakage. Additionaly, extra coherent interactions do not contribute to gate infidelity. We present results on non-standard parametric entangling gates optimized for speed and leakage suppression while maintaining efficient compilation to standard two-qubit gates.   

High-fidelity Parametric Entangling Gates for Superconducting Quantum Processors

Parametrically-activated gates driven by a flux-tunable coupler are a promising approach to engineering fast, high-fidelity entanglement between fixed-frequency transmons. By selecting the proper resonance condition, we can generate both XY-like and ZZ-like interactions, improving the expressiveness of hardware native gatesets in superconducting quantum processors. In this work, we realize and benchmark high-fidelity iSWAP and CZ gates between qubits. We then demonstrate how these techniques can be extended to 3-level qutrit systems to engineer a two-qutrit parametric CZ gate.   

Demonstration of two-qubit gates under different bias conditions

One common approach to realize fast two-qubit gates in superconducting qubits is using baseband flux pulses to tune the two qubit states in resonance with each other. High-fidelity two-qubit gates in flux-tunable superconducting qubits have been demonstrated with a variety of flux pulse envelopes, including unipolar, bipolar, and parametric modulation. Each envelope has its own advantage under specific bias settings of the modulated qubit, allowing to refocus the system or remove the need for long-time corrections. Here we present our work on achieving high-fidelity randomized benchmarked two-qubit gate under different idling conditions of the qubit bias.   

The effect of spectator qubit on CR gate performance

Superconducting qubits have emerged to become a promising platform for quantum computing [1]. In a superconducting circuit, many qubits are coupled electronically to one another [2]. A cross-resonance (CR) gate is a particularly important two-qubit gate, which enables operation of CNOT in these superconducting circuits. In CR gate, two qubits, the so-called control and target, are coupled and the control qubit is driven at frequency of the target qubit [3,4]. In this process, the control and target qubits are open to other qubits on the chip, the so-called spectator qubits . We study the entropy flow from one spectator qubit to another spectator qubit via the control and target qubits.

[1] J. Clarke, F. Wilhelm, Superconducting quantum bits, Nature 453, 1031–1042 (2008)

[2] Xuexin Xu, M.H. Ansari, ZZ freedom in two qubit gates, Phys. Rev. Applied 15, 064074 (2021), arXiv:2009.00485

[3] Jaseung Ku, Xuexin Xu, Markus Brink, David C. McKay, Jared B. Hertzberg, Mohammad H. Ansari, B.L.T. Plourde, Suppression of Unwanted ZZ Interactions in a Hybrid Two-Qubit System, Phys. Rev. Lett. 125, 200504 (2020), arXiv:2003.02775

[4] Xuexin Xu, M. Ansari, Parasitic-Free Gate: An Error-Protected Cross-Resonance Switch in Weakly Tunable Architectures, Phys. Rev. Applied 19, 024057, arXiv:2212.05519     

Stray multiqubit interactions in quantum processors

The advancement of quantum computing encounters a significant hurdle in the form of undesirable stray couplings. These couplings have the potential to impede gate operations and lead to fidelity loss. In this talk, we delve into an investigation of the influence of stray couplings within a multiqubit circuit. We introduce analytical expressions to quantitatively measure these effects, highlighting the essential parameters to mitigate their parasitic impact. We also reveal the feasibility of completely suppressing all stray interactions. Furthermore, our research explores the exciting prospect of achieving multiqubit capabilities surpassing those of traditional two-body interactions. This breakthrough holds promise for enhancing the optimization and overall performance of quantum algorithms.   

Modelling two-qubit gates of superconducting transmon processors

Two-qubit gate errors remain one of the biggest obstacles on the road towards scalable quantum processors and circuits. In fixed-coupling superconducting transmon qubits, crosstalk effects can cause a significant degradation of two-qubit gate fidelities [1]. A precise understanding of these errors and their origin is an important step for error mitigation and thus crucial for the implementation of deeper circuits. We develop a noise model based on Hamiltonian simulation of coupled three-level systems, taking into account higher transmon levels and their anharmonicities. By comparing simulation results with experiments on IBM hardware, we identify important error mechanisms such as frequency collisions and investigate their impact on gate fidelities.

[1] A. Ketterer and T. Wellens, Phys. Rev. Applied 20, 034065 (2023)   

Bichromatic Driving in Floquet Qubits

We study bichromatically driven single and coupled qubit systems for their application in single and two-qubit Floquet gates. Using the Floquet theory framework, we compare the full numerical calculation of quasienergies with the ones obtained analytically through rotating wave approximation and generalized Van-Vleck perturbation theory. Floquet qubits have been proposed to have dynamically induced high coherence lifetime sweet spots. We extend that argument to the case of bichromatic driving and observe dynamical sweet regions. We apply the bichromatic Floquet theory to superconducting fluxonium architecture to study single and two qubit gates.   

Investigating Single-Qubit Gate Speed and Fidelity

For superconducting qubits, fast gates are necessary to increase the circuit depth or the total number of operations before decoherence decreases the state fidelity. This is done by applying short, high-power microwave pulses resonant with the qubit. However, as gate time decreases, shorter pulses encompass a larger frequency bandwidth. In transmon qubits, this increases leakage out of the computation subspace because of the qubit's relatively small anharmonicity. This unwanted leakage can be mitigated using well-known techniques such as pulse-shaping and derivative removal by adiabatic gates (DRAG). Modern hardware allows for fast, arbitrary control over pulse envelopes enabling the use of new gate-error mitigation strategies. To explore improvements to single qubit gates, we investigate qubit leakage and gate fidelities in planar transmon devices. We compare measured qubit gate speed and fidelity while using these common leakage-mitigation schemes and compare our results to those obtained using new mitigation techniques.   

Fast, high fidelity, quantum non-demolition readout with an intrinsically Purcell protected qubit

Fast, high fidelity, quantum-non demolition readout is a key resource for a wide range of applications in quantum processing units, including qubit initialization, quantum sensing, measurement-based entanglement generation and quantum error correction. This is achieved in superconducting circuits by an approximate dispersive Hamiltonian between the qubit defined in the two lowest levels of a transmon and its readout resonator. The speed and fidelity of such readout techniques depends on the linewidth of the readout resonator, the strength of the dispersive interaction and the power of the readout drive. At the same time, it is important to suppress the radiative Purcell decay of the qubit via the resonator. Typically, dedicated Purcell filters are coupled to readout resonators to suppress such unwanted effects. 

Recently, it has been shown [1] that reducing the qubit-readout detuning is largely beneficial to circumvent the detrimental nonlinear state transitions caused by the readout tone. However, designing effective Purcell filters for such small detuning following the conventional approach is particularly challenging. We demonstrate an intrinsically Purcell protected qubit realized in a multimodal circuit [2], while retaining strong measurability through a mediated dispersive interaction. We explain the properties of such a circuit and demonstrate a state-of-the-art fast, high fidelity, quantum non-demolition qubit readout in our experimental realization of the device.

[1] Xu Xiao et al. arXiv:2304.13656 (2023)

[2] Gambetta et al., PRL 106, 030502 (2011)    

Readout of a Transmon Qubit using an All-Pass Readout Resonator with Interference Purcell Suppression

Impedance mismatch in the readout bus is a leading cause of high variance in measurement rate κ in superconducting quantum processors. Moreover, the addition of bulky and high-magnetic field circulators and isolators is often needed for impedance matching. In this work, we demonstrate transmission-based readout of a transmon qubit using a directional readout resonator. Whereas a typical readout resonator would have a sharp dip in |S21| on resonance, our directional resonator demonstrates a dip of less than 1dB on resonance, thus closely preserving the 50-ohm readout bus. This both maximizes measurement efficiency and avoids needing a weakly-coupled port, a major source of impedance mismatch in many standard qubit readout schemes. To enable fast readout and reset, we propose a novel interference Purcell filter compatible with directional readout and demonstrate Purcell suppression by 2 orders of magnitude over a bandwidth of more than 600 MHz. This architecture is expected to facilitate more scalable and modular design of quantum processors.   

Lumped-Element Filters for Protecting Qubits from Control and Readout Ports

It is essential to isolate qubits from their environment, which includes all control and readout ports. Each port can open up the qubit system to noise and dissipation. "Purcell filters" have typically described filters that isolate the qubit from the cavity readout port, decoupling qubit relaxation from cavity decay. In this talk, we describe our efforts to decouple the qubits from both the readout and the parametric control ports. We have designed and tested custom band-stop and band-pass filters using microstrip, coplanar waveguide, and lumped-element technologies. These modular designs provide flexibility to implement the filtering in an external package or directly on the qubit chip. In this talk, we will discuss the design methodology, fabrication, and cryogenic characterization of these filters.   

Fast, 3D-integrated qubit readout using a combined bandpass and intrinsic notch filter

Fast and accurate qubit state readout is essential for fault-tolerant quantum computation. In dispersive readout in circuit QED, this requires increasing the coupling strength of readout resonators to the detector line to allow for rapid pointer-state separation without compromising the coherence or addressability of qubits. To this end, providing each readout mode with its own dedicated Purcell resonator has emerged as an effective strategy [1-2] – enhancing both Purcell filtering and measurement addressability in parallel.

Here, we present experimental results on a 3D-integrated superconducting circuit device with CPW readout resonators that couple to dedicated Purcell resonators through a section of their transmission line. The interplay between inductive and capacitive coupling that arises can be exploited to produce an additional notch filter, resulting in an extra layer of Purcell filtering that allows readout resonators to be very strongly coupled to the detector line. This approach demonstrates a qubit readout under 100 ns with an assignment fidelity exceeding 99.6%.

[1] J. Heinsoo et al., Rapid high-fidelity multiplexed readout of superconducting qubits. Physical Review Applied, 10, 034040 (2018)

[2] F. Swiadek et al., Enhancing dispersive readout of superconducting qubits through dynamic control of the dispersive shift: Experiment and theory. arXiv:2307.07765 (2023)   

Broadband Purcell filter design for superconducting qubits

Superconducting qubits are typically measured via dispersively-coupled readout resonators. To achieve fast readout without compromising the qubit's relaxation time T₁, Purcell filters have been developed. Commonly used single-pole Purcell filters have limited bandwidth thus constraining the multiplexed readout capacity. In this work, we present a systematic way to design multi-stage broad bandpass Purcell filters by use of a classical filter synthesis technique. We show that broadband filters can be designed with either equally loaded input and output ports (symmetric filters), or weakly coupled input ports and strongly coupled output ports (asymmetric or singly terminated filters). We analyze the performance of these multi-stage filters and numerically verify that a 6-stage symmetric filter and a 4-stage asymmetric filter with 600 MHz bandwidth can enhance the qubit lifetime over 1000-fold compared to the absence of filters. This is a hundredfold improvement over the performance of a single-pole filter of identical bandwidth. We experimentally implement this approach and test a simple, robust design that only uses sections of transmission lines.   

Rapid unconditional active reset of frequency-tunable superconducting qubits via a metamaterial waveguide Purcell filter

Rapid and deterministic reset of qubits to known pure states is one of the most crucial enablers for quantum information processing. In particular, recent advances in unconditional active reset schemes have led to improvements in various tasks such as quantum error correction and resource state preparation, as their speed and fidelity are not constrained by the quality of readout.

 

We introduce a rapid unconditional active reset scheme for frequency-tunable superconducting qubits by coupling qubits directly to a dispersive bath realized by a metamaterial waveguide Purcell filter. The wide passband of the metamaterial waveguide, in concert with  sharp suppression of the photonic density of states outside of it, enables simultaneous multi-level reset and strong coupling. Through a straightforward parametric flux modulation, we demonstrate the ability to rapidly reset the first three lowest-energy states of a transmon qubit to the ground state, while maintaining a high T₁ lifetime that is not limited by Purcell decay.   

Purcell Filtering for Qubit Readout Using Intrinsic Geometric Multi-modes of the Transmon

Over the past two decades, superconducting circuits have become pivotal in quantum computing, achieving quantum logic gates with minimal errors. As we address the challenges of scaling, the role of microwave engineering, particularly in enhancing the lifetime and coherence of transmons at a reduced system complexity, becomes crucial. Here, we report a novel Purcel filtering technique in cQED by ustilising the instristic multiple geometric modes of the quantum circuit. The precise control of the coupling strenght and the relative phase between the transmon and these higher order geometric modes can result in constructive or destructive interference effects at prescribed frequencies, e.g. the qubit frequency. This can impact the effective coupling strength and energy exchange dynamics between the transmon, the readout resonator, and the external driving ports, and it can be utilised to engineer Purcel filtering without introducing additional circuitry to the quantum processor such as a quarter-wavelenght bandstop filter or an always active multiple drives.