Millimeter-Wave Superconducting Qubit (2411.11170v2)

Abstract: Manipulating the electromagnetic spectrum at the single-photon level is fundamental for quantum experiments. In the visible and infrared range, this can be accomplished with atomic quantum emitters, and with superconducting qubits such control is extended to the microwave range (below 10 GHz). Meanwhile, the region between these two energy ranges presents an unexplored opportunity for innovation. We bridge this gap by scaling up a superconducting qubit to the millimeter-wave range (near 100 GHz). Working in this energy range greatly reduces sensitivity to thermal noise compared to microwave devices, enabling operation at significantly higher temperatures, up to 1 K. This has many advantages by removing the dependence on rare $^3$He for refrigeration, simplifying cryogenic systems, and providing orders of magnitude higher cooling power, lending the flexibility needed for novel quantum sensing and hybrid experiments. Using low-loss niobium trilayer junctions, we realize a qubit at 72 GHz cooled to 0.87 K using only $^4$He. We perform Rabi oscillations to establish control over the qubit state, and measure relaxation and dephasing times of 15.8 and 17.4 ns respectively. This demonstration of a millimeter-wave quantum emitter offers exciting prospects for enhanced sensitivity thresholds in high-frequency photon detection, provides new options for quantum transduction and for scaling up and speeding up quantum computing, enables integration of quantum systems where $^3$He refrigeration units are impractical, and importantly paves the way for quantum experiments exploring a novel energy range.

• The paper presents a 72 GHz transmon superconducting qubit enabling operations near 0.87 K, with implications for simpler cryogenics and hybrid quantum systems.
• Experimental results show the 72 GHz qubit has a T1 of 15.8 ns and T2* of 17.4 ns, demonstrating coherence properties competitive with early microwave qubits.
• Operating at millimeter-wave frequencies allows the qubit to function near 1 K using simpler 4He cryogenics, significantly reducing hardware complexity and system costs.

Overview of "A Millimeter-Wave Superconducting Qubit"

The paper presented in the paper "A Millimeter-Wave Superconducting Qubit" explores the implementation of superconducting qubits operating at millimeter-wave frequencies, specifically at 72 GHz, enabling quantum operations at higher temperatures near 0.87 K. This research addresses the underexplored region between microwave and infrared frequencies, offering alternatives for temperature operation extremes that traditionally require complex refrigeration systems reliant on rare helium isotopes. This transition to higher frequencies displays both practical and broad-reaching implications for quantum computing, detections, hybrid systems, and quantum communication infrastructure.

Key Findings

1. Device Implementation: The authors detail the creation of a transmon qubit using low-loss niobium trilayer junctions. This design reduces thermal noise sensitivity and accommodates operational temperature elevation. The qubit geometry incorporates a niobium/aluminum trilayer on sapphire, transitioning from the trillion times lower superconducting energy gap to the millimeter-wave spectrum through enhanced critical current densities.
2. Experimental Results: The designed qubit exhibited quantifiable characteristics, including a transition frequency of 72.137 GHz, a relaxation time $T_1$ of 15.8 ns, and a dephasing time $T_2^*$ of 17.4 ns. The coherence properties of this system are found to be competitive with early generation microwave qubits, highlighting significant room for future improvement as these systems evolve.
3. Thermal and Power Advantages: Operating at higher frequencies allows these qubits to function at temperatures close to 1 K, accessible with simpler $^4$He-based cryogenic systems, forgoing the need for $^3$He. This effectively simplifies the hardware requirements for quantum computing systems, promising augmented cooling power and reduced system costs.
4. Implications for Hybrid and Quantum Communication Systems: The shift to millimeter-wave frequencies opens new avenues for integrating quantum systems that require higher operational coherence and stability in noisy environments. It positions these qubits for potential roles in quantum transduction as well as photon or spin-based hybrid experiments, benefiting both quantum sensing and potential cosmic observation tasks.

Theoretical and Practical Implications

Theoretically, millimeter-wave qubits extend quantum manipulability into new electromagnetic territories, lending themselves to exploring interactions with high-energy photons. Practically, this development has immediate utility in reducing thermal complexity, enhancing quantum sensing capabilities, and possibly facilitating mobile or space-based quantum systems where refrigeration limits are strict and cumbersome to implement.

Speculative Outlook

Future work will likely focus on refining junction quality and coherence times. Better filtering of input signals and optimizing qubit junction processes to remove defects and reduce material loss are essential to fully realizing the potential of these devices. As coherence properties improve, such robust qubits could exponentially augment quantum computation systems' scalability and integration into broader technological implementations, including secure communications over significant distances. Furthermore, advanced cooling methods and noise mitigation will consolidate the gains made by increased frequency applications in superconducting circuits.

In summary, the paper lays foundational work for millimeter-wave qubits' integration into quantum technologies, advocating significant advantages through frequency elevation and the potential disruption of traditional quantum hardware paradigms. The implications provide promising signals for the future of superconducting quantum systems and their application across newer, demanding frontier areas.