Session T47: High Quality Superconducting Cavities

A superconducting cavity qubit with tens of milliseconds single-photon coherence time

Long-lived memory qubits can significantly reduce error correction overheads in future quantum processors. Superconducting cavities, capable of reaching quality factors well beyond one billion, are promising candidates for such quantum memories. However, these high-Q cavities are yet to be leveraged to achieve extended coherence times due to their coupling to noisy ancilla qubits.

We introduce a quantum memory using a novel niobium cavity controlled by a weakly-coupled transmon ancilla. We show that a single-photon qubit encoded in the cavity achieves lifetimes an order of magnitude beyond the current state of the art [1]. Furthermore, we present a protocol that effectively suppresses ancilla errors propagating to the cavity, and demonstrate the effect of this procedure on the coherence time of bosonic qubits.

[1] O. Milul, B. Guttel et al., PRX Quantum 4, 030336 (2023).   

Strategies and trade-offs for controllability and memory time of microwave cavities in circuit QED: part I

Three-dimensional microwave cavity resonators have been shown to reach lifetimes of the order of a second by careful design and fabrication. Such cavities represent an ideal platform for quantum computing with bosonic qubits, but their efficient control remains an outstanding problem since the large mode volume results in inefficient coupling to nonlinear elements used for their control. Moreover, this coupling induces additional cavity dissipation which can easily destroy the advantage of long intrinsic lifetime. Here, we discuss conditions on, and protocols for, efficient control of these ultra-high-quality microwave cavities using conventional nonlinear circuits. We show that, surprisingly, controlling ultra-high-Q cavities does not require similar quality factors for the auxiliary qubits used to control them. Our work explores a potentially viable roadmap towards using ultra-high-quality microwave cavity resonators for storing and processing information encoded in bosonic qubits. In the first part of this presentation, I will discuss the details of this qubit-induced cavity decoherence and outline the general trade-offs between fast control and long storage time.   

Strategies and trade-offs for controllability and memory time of microwave cavities in circuit QED: part II

Three-dimensional microwave cavity resonators have been shown to reach lifetimes of the order of a second by careful design and fabrication. Such cavities represent an ideal platform for quantum computing with bosonic qubits, but their efficient control remains an outstanding problem since the large mode volume results in inefficient coupling to nonlinear elements used for their control. Moreover, this coupling induces additional cavity dissipation which can easily destroy the advantage of long intrinsic lifetime. Here, we discuss conditions on, and protocols for, efficient control of these ultra-high-quality microwave cavities using conventional nonlinear circuits. We show that, surprisingly, controlling ultra-high-Q cavities does not require similar quality factors for the auxiliary qubits used to control them. Our work explores a potentially viable roadmap towards using ultra-high-quality microwave cavity resonators for storing and processing information encoded in bosonic qubits. In part II, I will talk about our numerical simulations, which demonstrate that efficient control and measurements of cavities with second-scale lifetime is possible with state-of-the-art transmon devices and outline a route towards controlling cavities with even higher quality factors.   

Building a 3D quantum processing unit using a high-coherence SRF cavity

TESLA-shaped superconducting RF (SRF) cavities have been demonstrated to offer photon relaxation times of the order of seconds in the quantum regime [1]. As a result, TESLA cavities coupled to transmon ancilla qubits are promising candidates for 3D quantum processing unit (QPU) and quantum memory. In this talk, we introduce a novel QPU design based on TESLA cavity aimed at maintaining a large quality factor. We achieve this by careful engineering of the package geometry that minimizes the participation of the fundamental mode in different loss channels. This new design allows for the separate control of the high-Q modes and the over-coupled low-Q modes without compromising the functionality of each mode. We further utilize a multimode stripline resonator [2] to characterize the loss channels of the system for predicting and optimizing the design. We report on the long coherence of the storage mode and show how it facilitates high-fidelity operations.   

Progress on 3D qudit architecture with high-coherence SRF cavities

Superconducting radio frequency (SRF) cavities are excellent choices for storing quantum information as quantum d-level systems (qudits) due to their exceptionally long lifetimes and large accessible Hilbert spaces. A common strategy to manipulate the states is to use a nonlinear element like a transmon. There are however several challenges to build a 3D SRF architecture while maintaining a seconds long cavity lifetime. In this presentation, we demonstrate successful integration of superconducting qubits with a single-cell Nb SRF cavities. We discuss the experimental results with different approaches like photon-blockade and selective number dependent arbitrary phase gates to prepare non-classical states. Finally, we discuss our strategies to improve the coherence times and extend the system for building a multi-qudit quantum processor.   

Enhancing photon lifetime in coupled superconducting cavities through phase control

Three-dimensional superconducting cavities with high quality factors have found extensive applications in high-energy physics, quantum information science, sensing, and so on. Here we present our study of the photon lifetime of two co-resonant superconducting cavities coupled by a coaxial cable with a tunable phase parameter. We find that both the phase accumulation on the cable and the phases of initial states affect the lifetime of photons in each cavity as a result of the interference between cavity fields. Our experimental study via ring down measurement at a cryogenic condition observed a factor of two enhancement of cavity photon lifetime by optimizing the resonance frequency matching and the phase parameter. We envision that the coupled superconducting cavities can be exploited for enhancing the performance of three-dimensional quantum memory and quantum information processors.   

Crosstalk-Robust Quantum Control in Multimode Bosonic Systems

High-coherence superconducting cavity modes offer an efficient platform for quantum information processing. To realize universal control of these bosonic modes, a transmon ancilla is typically coupled to the cavity, thereby introducing required nonlinearities. However, this configuration is susceptible to crosstalk errors in the dispersive regime, where the ancilla frequency is Stark-shifted by the state of each coupled bosonic mode. This results in a frequency mismatch of the ancilla drive, lowering overall gate fidelities. To mitigate such coherent errors, we employ quantum optimal control to engineer ancilla pulses that are robust to the frequency shifts. These optimized pulses are subsequently integrated into a recently developed two-mode echoed conditional displacement (ECD) protocol. Through numerical simulations, we examine two representative scenarios: the preparation of single-mode Fock states in the presence of spectator modes and the generation of two-mode entangled Bell cat states. Our approach markedly suppresses crosstalk errors, outperforming conventional ancilla control methods by orders of magnitude. These results not only provide guidance for experimentally achieving high-fidelity multimode operations, but also substantiate bosonic systems as competitive candidates for high-performance quantum information processors.   

Control of a long-lived multimode bosonic memory with a weakly coupled transmon ancilla

High-Q 3D multimode cavities coupled to a nonlinear ancilla circuit are a promising platform for quantum computing. This architecture has the advantages of long cavity coherence times and the ability to realize multiplexed control of these modes with minimal control lines. Critical challenges for the viability of this architecture include crosstalk errors that emerge from the dispersive interaction and ancilla errors which propagate to the cavity modes and limit cavity coherence via the inverse Purcell effect.

We explore strategies to mitigate these errors by weakening the coupling between the transmon and the cavity modes and employing multimode versions of control protocols that use conditional displacements for fast gate operations [1]. These gates reduce crosstalk errors by using large cavity displacements as an interaction switch which increases the contrast between the rates of gate operations involving the target modes and those of spurious coherent errors from photons in non-target modes. We will also discuss progress toward realizing a new multimode processor architecture which mitigates crosstalk errors at the hardware level using tunable couplers.

[1] Eickbusch, A. et al. Nat. Phys. 18, 1464–1469 (2022).   

Holographic quantum simulations using a 3D circuit-QED system

Simulating quantum systems is a useful application and one of the viable goals of near-term small-scale quantum computers. Practical applications include predicting chemical kinetics, characterizing phase transitions and simulating information scrambling. holoVQE [1] is a holographic extension of the variational quantum eigensolver algorithm that uses matrix product states (MPS) to simulate a given quantum system with reduced hardware requirements. Experimentally this algorithm can be implemented with bosonic modes in circuit-QED systems [2], which have shown high coherence and can encode error-correcting codes to further enhance their lifetime. In this talk we will show, with theoretical simulations and preliminary experiments, how we can use the Hilbert space of a 3D superconducting cavity to build an MPS and show efficient control of the system in order to build more complex algorithms in the future.

[1] M. Foss-Feig et al., Phys. Rev. Research 3, 033002 (2021).

[2] Wen-Long Ma et al., Science Bulletin 66, 1789 (2021).   

Digital homodyne and heterodyne detection for stationary bosonic modes

Homodyne and heterodyne detection techniques are extensively used to characterize and herald dynamics of propagating modes within the field of quantum optics. However, implementing these methods on stationary modes confined within high-quality superconducting cavities is challenging. Here, we show a protocol tailored for stationary cavities that yields measurement statistics identical to those from homo- or heterodyne detectors. This protocol is based on repeated interactions between the cavity and a qubit, followed by qubit measurements and a weighted integration of the binary measurement outcomes. Our approach enables both balanced and unbalanced heterodyne detection on stationary modes, thereby granting access to a comprehensive toolbox, which for example includes quantum verification protocols.   

Implementation of a conditional displacement gate using the second-order nonlinearity of a cubic transmon in a planar superconducting circuit

The conditional displacement gate plays a crucial role in the framework of bosonic codes. Recently, it has been typically implemented by enhancing a weak cross-Kerr interaction through the irradiation of a pump tone into the resonator [1]. This cross-Kerr interaction is a valuable control resource but can also act as a bit-flip error propagation channel during conditional gate operations. To address this issue, we present results on the implementation of the conditional displacement gate by harnessing the second-order nonlinearity of the cubic transmon [2,3], which is integrated into a planar superconducting circuit. In this approach, the resonator state follows the shortest path in its phase space, effectively decoupling the cross-Kerr interaction during conditional gate operations. Using this gate, we also demonstrate the creation of cat states and squeezed vacuum states through modular measurements with post-selection.

[1] P. Campagne-Ibarcq et al., Nature 584, 368–372 (2020)

[2] A. Noguchi et al., Phys. Rev. A 102, 062408 (2019)

[3] A. Bermudez et al., Phys. Rev. A 85, 040302 (2012)