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arXiv:2501.12710 · Physical Review Applied

Cavity-mode initialization via a Rabi-driven qubit

Natan Karaev, Eliya Blumenthal, Guy Moshel, Asaf A. Diringer, Shay Hacohen-Gourgy

Published Jan 21, 2025• 0 citations
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Abstract

Microwave cavity modes with long coherence times are used in many different quantum computing systems. During normal operation of such systems, these modes, called ``memory modes,'' often need to be set to different coherent photonic occupations. In this work we present a technique we call ``Rabi-driven reset'' in which the state of a memory mode is transferred into a decaying mode. This is done through a Rabi-driven qubit that is coupled to both modes via sideband driving tones. The outcome of the method is the initialization of the memory mode in any required coherent state. Simulations are presented to demonstrate the effectiveness of this technique, along with a comparison to an existing coupling method. Our simulations predict a decrease of an order of magnitude in initialization times compared with existing methods.

DOI
10.1103/physrevapplied.23.014043

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Microwave cavity modes with long coherence times are used in many different quantum computing systems. During normal operation of such systems, these modes, called memory modes,'' often need to be set to different coherent photonic occupations. In this work we present a technique we call Rabi-driven reset'' in which the state of a memory mode is transferred into a decaying mode. This is done through a Rabi-driven qubit that is coupled to both modes via sideband driving tones. The outcome of the method is the initialization of the memory mode in any required coherent state. Simulations are presented to demonstrate the effectiveness of this technique, along with a comparison to an existing coupling method. Our simulations predict a decrease of an order of magnitude in initialization times compared with existing methods.
High-Q bosonic memories are central to hardware-efficient quantum error correction, but their isolation makes fast, high-fidelity reset a persistent bottleneck. Existing approaches either rely on weak intermode cross-Kerr conversion or on measurement-based sequences with substantial latency. Here we demonstrate a hardware-efficient Rabi-Driven Reset (RDR) that implements continuous, measurement-free cooling of a superconducting cavity mode. A strong resonant Rabi drive on a transmon, together with sideband drives on the memory and readout modes detuned by the Rabi frequency, converts the dispersive interaction into an effective Jaynes-Cummings coupling between the qubit dressed states and each mode. This realizes a tunable dissipation channel from the memory to the cold readout bath. Crucially, the engineered coupling scales with the qubit-mode dispersive interaction and the drive amplitude, rather than with the intermode cross-Kerr, enabling fast cooling even in very weakly coupled architectures that deliberately suppress direct mode-mode coupling. We demonstrate RDR of a single photon with a decay time of 1.2μs1.2 μs1.2μs, more than two orders of magnitude faster than the intrinsic lifetime. Furthermore, we reset about 30 thermal photons in about 80μs80 μs80μs to a steady-state average photon number of nˉ=0.045±0.025\bar{n} = 0.045 \pm 0.025nˉ=0.045±0.025.
Control and utilization of coherent states of microwave photons is a ubiquitous requirement for the present and near-future implementations of solid-state quantum computers. The rate at which the photon state responds to external driving is limited by the relaxation rate of the storage resonator, which poses a trade-off between fast control and long storage time. Here, we present a control scheme that is designed to drive an unknown photon state to a desired coherent state much faster than the resonator decay rate. Our method utilizes a tunable environment which acts on an ancillary qubit coupled to the resonator. By periodically resetting the qubit and tuning it into resonance with the resonator, possible photon loss and dephasing of the resonator mode are corrected without measurements or active feedback. In general, our method is suitable for accelerating the control of coherent states in high-fidelity resonators.
We present the realization of a cavity quantum electrodynamics setup in which photons of strongly different lifetimes are engineered in different harmonic modes of the same cavity. We achieve this in a superconducting transmission line resonator with superconducting qubits coupled to the different modes. One cavity mode is strongly coupled to a detection line for qubit state readout, while a second long lifetime mode is used for photon storage and coherent quantum operations. We demonstrate sideband-based measurement of photon coherence, generation of n photon Fock states and the scaling of the sideband Rabi frequency with square root of n using a scheme that may be extended to realize sideband-based two-qubit logic gates.
Superconducting 3D microwave cavities offer state-of-the-art coherence times and a well controlled environment for superconducting qubits. In order to realize at the same time fast readout and long-lived quantum information storage, one can couple the qubit both to a low-quality readout and a high-quality storage cavity. However, such systems are bulky compared to their less coherent 2D counterparts. A more compact and scalable approach is achieved by making use of the multimode structure of a 3D cavity. In our work, we investigate such a device where a transmon qubit is capacitively coupled to two modes of a single 3D cavity. The external coupling is engineered so that the memory mode has an about 100 times larger quality factor than the readout mode. Using an all-microwave second-order protocol, we realize a lifetime enhancement of the stored state over the qubit lifetime by a factor of 666 with a ZZZ-fidelity of 82%82\%82%. We also find that this enhancement is not limited by fundamental constraints.
The exploration of highly connected networks of qubits is invaluable for implementing various quantum algorithms and simulations, as it allows for entangling qubits with reduced circuit depth.Here, we demonstrate a multiqubit sideband tone-assisted Rabi-driven gate.Our scheme is inspired by the ion-qubit Mølmer-Sørensen gate and is mediated by a shared photonic mode and Rabi-driven superconducting qubits, which relaxes restrictions on qubit frequencies during fabrication and supports scalability.We achieve a two-qubit gate with a maximum state fidelity of 95% in 310 ns, a three-qubit gate with a state fidelity of 90.5% in 217 ns, and a four-qubit gate with a state fidelity of 66% in 200 ns.Furthermore, we develop a model of the gate that shows that the four-qubit gate is limited by shared resonator losses and the spread of qubit-resonator couplings, which must be addressed to reach high-fidelity operations.
Fast and high-fidelity qubit initialization is crucial for low-frequency qubits such as fluxonium, and in applications of many quantum algorithms and quantum error correction codes. In a circuit quantum electrodynamics system, the initialization is typically achieved by transferring the state between the qubit and a short-lived cavity through microwave driving, also known as the sideband cooling process in atomic system. Constrained by the selection rules from the parity symmetry of the wave functions, the sideband transitions are only enabled by multiphoton processes which require multitone or strong driving. Leveraging the flux tunability of fluxonium, we circumvent this limitation by breaking flux symmetry to enable an interaction between a noncomputational qubit transition and the cavity excitation. With single-tone sideband driving, we realize qubit initialization with a fidelity exceeding 99% within a duration of 300 ns, robust against the variation of control parameters. Furthermore, we show that our initialization scheme has a built-in benefit in simultaneously removing the second-excited state population of the qubit, and can be easily incorporated into a large-scale fluxonium processor.