A switch for qubits handles 100 picowatts of readout power

Ziyi Zhao of JILA, National Institute of Standards and Technology and the University of Colorado, Boulder, and colleagues have designed a new superconducting switch capable of handling more than 100 pW of readout power, a crucial step toward scaling quantum processors. The switch utilizes a stable persistent current associated with tens of flux quanta, minimizing the need for constant recalibration and reducing static power consumption. This design addresses a key limitation in current quantum systems by employing direct current actuation, which reduces potential crosstalk between densely integrated qubits and facilitates more modular cryogenic measurements. The work demonstrates a switch with over 20 dB isolation, comparable to commercial ferrite isolators, and a modulation bandwidth broader than 600 MHz.

Persistent Current Bias Enables Long-lived Switch States Researchers have designed a new microwave switch that minimizes energy consumption and signal interference, addressing critical limitations in current quantum systems. The design centers around a persistent current bias and direct current actuation, a departure from conventional methods that depend on continuous magnetic flux biasing and dynamic flux actuation. These traditional approaches often struggle with isolating control signals, creating crosstalk that limits the density of integrated switches and other sensitive components. The new switch circumvents this issue by trapping a current within the superconducting loop, maintaining a stable state for extended periods. Measurements reveal the persistent current remains consistent for less than 1% decay per day, a characteristic crucial for reliable operation in complex quantum circuits. This stability is achieved through a carefully engineered inductive Wheatstone bridge, incorporating 20 tunable inductors, each an antisymmetric rf-SQUID, to implement the necessary inductance. Beyond stability, the switch demonstrates performance metrics suitable for advanced quantum information processing. Transmission measurements show greater than 20 dB of isolation in the off state, a level comparable to commercially available ferrite isolators. This high isolation minimizes unwanted signal leakage, ensuring the integrity of quantum states. The switch’s modulation bandwidth is broader than 600 MHz, enabling multiplexing schemes that allow for efficient routing of signals to multiple qubits or detectors. The ability to handle power levels exceeding 100 pW is sufficient for both resonator readout tones and amplifier pumps, essential components in many quantum systems. The researchers report, “We first show that the persistent current, corresponding to up to hundreds of flux quanta, can be reliably and precisely trapped within 200 μs,” highlighting the speed and precision of the current stabilization process. The switch’s control scheme relies on a heat-activated persistent current source to initially trap the current within the superconducting bridge. Once established, the current is maintained with minimal energy expenditure. Actuation, or switching between states, is then achieved using direct current, eliminating the need for dynamic magnetic flux control. This direct current actuation is a key feature because it significantly reduces potential crosstalk with neighboring devices. The researchers demonstrated that the switch can be actuated within a nanosecond, allowing for rapid reconfiguration of circuit connectivity. Detailed analysis of the switch’s behavior reveals a periodic relationship between the trapped flux and the current required to increment it, a characteristic of the bridge’s current-dependent differential inductance. To validate the design, the team performed cryogenic microwave measurements, assessing the switch’s performance under realistic operating conditions. These measurements confirmed the switch’s broadband characteristics, with over 2 GHz of bandwidth in the off state. The researchers also demonstrated the ability to precisely control the trapped flux, achieving quantized steps corresponding to integer numbers of flux quanta. They explain this illustrates the discrete nature of the current control. Repeated measurements over time showed the trapped flux remained consistent, with less than 1% decay per day, further demonstrating the long-term stability of the design. The implications of this work extend beyond quantum computing; the switch’s low-loss and broadband characteristics make it suitable for a range of cryogenic applications, including large-format detectors and modular quantum networks. In these systems, switches are used to dynamically re-allocate resources, route signals, and facilitate in-situ calibration. The ability to integrate these switches densely, without the limitations imposed by crosstalk and power consumption, is crucial for building complex and scalable systems.

Inductive Wheatstone Bridge Design for Superconducting Circuits Superconducting circuits offer a promising pathway toward scalable quantum computing and highly sensitive detectors, but managing signal routing and power consumption within these systems presents significant challenges. Current designs often rely on magnetic flux control to switch circuit connectivity, a method hampered by the difficulty of isolating these control lines and the need for continuous power input to maintain a bias. A new switch design, detailed in recent work, addresses these limitations through an inductive Wheatstone bridge configuration and a persistent current bias, achieving broadband performance with minimal static power draw. The design demonstrably traps this current reliably and precisely within 200 microseconds, with a decay of less than one percent per day, a level of stability crucial for maintaining qubit coherence and minimizing signal errors. This long-lived persistent current is achieved by carefully engineering the inductive Wheatstone bridge, a circuit topology that balances currents and minimizes sensitivity to external magnetic fields. The bridge is twisted into a figure-eight shape to minimize flux sensitivity to any uniform background magnetic field, a critical feature for dense integration. The switch’s architecture also prioritizes low power consumption during operation. By employing direct current actuation, the team circumvented the need for dynamic flux modulation, a common source of crosstalk and energy dissipation in conventional designs. This approach allows for rapid switching, while minimizing interference with neighboring devices. The resulting circuit handles larger than 100 pW of power, sufficient for resonator readout tones and amplifier pumps, a significant reduction aimed at addressing the power budget constraints hindering the scalability of quantum processors. The ability to operate at this power level is a key advancement, as it allows for denser packing of control circuitry without overheating or compromising performance. Beyond power efficiency, the switch exhibits impressive signal fidelity. Measurements reveal an isolation exceeding 20 dB, comparable to that of commercial ferrite isolators, effectively blocking unwanted signals and preserving the integrity of quantum information. This high isolation is coupled with a modulation bandwidth broader than 600 MHz, enabling multiplexing schemes that can significantly increase the throughput of cryogenic detectors and quantum processors. The broadband characteristics of the switch are particularly valuable for complex systems requiring the simultaneous transmission of multiple signals across a wide frequency range. The researchers constructed the switch using 20 tunable inductors, implementing the tunable inductors of the bridge with 20 rf-SQUIDs. The circuit’s design incorporates a persistent current source that directs current into the bridge, establishing the initial bias.

The team demonstrated that the switch’s behavior can be finely tuned by adjusting the control currents, allowing for precise manipulation of signal routing and connectivity. Trapped within the circuit, as the researchers detail in their work, this level of control and stability is essential for realizing the full potential of these advanced technologies. 100 Picowatt Power Handling for Resonator Readout This achievement tackles the escalating energy demands that currently limit the density and complexity of quantum processors and detectors. Instead, the team implemented a persistent current bias alongside direct current actuation, a combination intended to minimize static power use and crosstalk between closely packed components. This persistent current, sustained by a specially designed circuit, corresponds to tens of flux quanta and remains stable for extended periods, reducing the need for frequent recalibration. The researchers demonstrated this stability by observing less than 1% decay in the trapped current per day, a significant improvement over systems requiring constant adjustment. Central to the switch’s operation is an inductive Wheatstone bridge, meticulously engineered to minimize the flux sensitivity to any uniform background magnetic field. The bridge incorporates 20 rf-SQUIDs that implement the tunable inductors of the bridge. The circuit schematics, as detailed in their work, feature two wiring layers and a unique figure-eight geometry designed to further isolate the switch from external noise.

The team demonstrated the switch’s ability to rapidly modulate signals, supporting a variety of resonator readout and multiplexing schemes. The implications of this technology extend beyond simply reducing power consumption. Cryogenic microwave measurements, vital for operating these advanced systems, demand sophisticated signal routing and multiplexing capabilities. Switches like this one facilitate large-scale and modular designs, allowing for the dynamic reconfiguration of connections between quantum modules. The researchers validated their design through extensive testing, including measurements of the switch’s transmission characteristics and the stability of the trapped persistent current.

The team observed the persistent current decaying by less than 1% per day. To further characterize the switch, the team examined the relationship between the control currents and the trapped flux. This behavior confirms the switch’s ability to respond predictably to changes in control signals. The researchers also investigated the impact of external magnetic fields, finding that the switch’s performance remained stable even in the presence of moderate interference. This robustness is essential for real-world applications where complete shielding from external noise is often impractical.

The team’s work provides a promising pathway towards building more scalable and energy-efficient quantum systems and cryogenic detectors. Persistent-Current-Biased Switch Reduces Power Consumption These devices, detailed in recent work, address a critical limitation in current quantum systems: the substantial energy demands of maintaining qubit connectivity. Traditional approaches rely on constant magnetic flux biasing and dynamic actuation, both of which consume power and create potential interference, hindering the integration of larger numbers of qubits. The design also incorporates a twisted figure-8 shape for the superconducting bridge, minimizing flux sensitivity to any uniform background magnetic field and enhancing robustness in real-world applications. This is achieved through an inductive Wheatstone bridge design, where carefully balanced currents ensure minimal impact on adjacent circuits. Extensive testing confirmed the switch’s predictable and reliable behavior. To further validate the design, the researchers examined the switch’s response to external magnetic fields. 👉 More information🗞 Persistent-current-biased and current-actuated switch for superconducting circuits✍️ Ziyi Zhao, Eva Gurra, Michael R. Vissers and K. W. Lehnert🧠 DOI: http://link.aps.org/doi/10.1103/9r4s-6zj2 Stay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: