In an article recently published in the journal Nature Communications, researchers demonstrated a coherent on-chip cryogenic microwave pulse generator for superconducting quantum computing on a large scale.
a The conventional approach to manipulating and readout of a superconducting quantum processor. Room temperature electronics are used as control units to generate analog microwave pulses with a well-defined frequency, amplitude, and phase, which are sent to the cryogenic quantum processing unit (QPU) through coaxial cables with careful attenuation and filtering. The significant hardware overhead limits the scaling of the quantum computer. b A conceptual superconducting quantum computer that integrates the QPU with its control units in the cryogenic temperature. The control units may compose cryogenic microwave pulse generators and their control electronics. Such a monolithic integrated architecture enables large-scale superconducting quantum computers. Image Credit: https://www.nature.com/articles/s41467-024-50333-w
Existing Scaling Challenges
The scale of a quantum computer is crucial for executing fully error-corrected quantum algorithms and advancing noisy intermediate-scale quantum applications. Achieving fault-tolerant superconducting quantum computing requires millions of qubits.
Currently, readout and manipulation of qubits in superconducting quantum computers involve significant costs associated with microwave signal generation and routing. This process relies on transmitting microwave signals from room-temperature electronics to the quantum chips within a millikelvin cryogenic dilution fridge.
In practice, microwave signals are delivered to each qubit via coaxial cables from room temperature to the cryogenic environment. While this method can be extended to around 1,000 qubits, it becomes impractical beyond this point due to the substantial costs of electronics and the heat load imposed by extensive cabling. Consequently, this architecture is not suitable for fault-tolerant quantum computing, which requires millions of qubits.
Importance of Monolithic Integration
Monolithic integration of qubits with control and microwave electronics presents a promising solution to scaling challenges in quantum computing. By utilizing tightly integrated chip stackings and circuit blocks instead of macroscale wiring harnesses, this approach significantly reduces both the passive heat load and the system's footprint.
Moreover, this method offers systematic advantages, including enhanced signal fan-out and fan-in capabilities, as well as reduced communication latency. However, a major challenge remains: developing a coherent cryogenic microwave pulse generator with an ultra-small heat load that is compatible with superconducting quantum circuits. This component is crucial for the successful implementation of monolithic integration in quantum computing systems.
The Proposed Approach
In this study, the researchers focused on developing an on-chip coherent cryogenic microwave pulse generator for large-scale superconducting quantum computing. They proposed generating coherent microwave photons using superconducting circuits within a vacuum process. This method allows for convenient generation of microwave photon pulses with precise control over frequency, intensity, and phase. This control is achieved through digital manipulation of magnetic flux across a superconducting quantum interference device (SQUID) embedded in a superconducting resonator.
The cryogenic microwave pulse generator was fabricated using a λ/2 coplanar waveguide resonator with a SQUID embedded in the center conductor, strategically placed at the node of the fundamental mode's electric field. In the experiments, room temperature junction resistances ranged from 50 Ω to 270 Ω, corresponding to inductances of 58 pH to 310 pH at zero flux, accounting for 3.1 % to 11.6 % of the total inductance of the SQUID-embedded resonators.
The SQUID featured two parallel Josephson junctions and, in the case of symmetric junctions, acted as a tunable inductor. The inductance could be adjusted by varying the magnetic flux through the SQUID loop, which was generated by an electrical current in a nearby flux line.
In the SQUID-embedded resonator, the total inductance included contributions from both the flux-dependent SQUID inductance and the coplanar waveguide resonator inductance. For the readout experiment, a three-dimensional (3D) circuit quantum electrodynamics architecture was used.
A transmon superconducting qubit was fabricated on a 7 mm × 7 mm sapphire substrate using standard microfabrication techniques. Its Josephson junctions were created through double-angle evaporation of aluminum and electron beam lithography.
An arbitrary waveform generator with a 1 GHz sampling rate was employed to produce the flux step/overshoot necessary for driving the pulse generator. Flux step/overshoot was delivered to the signal source located at the 10 mK region using superconducting twisted pair wires or coaxial cables.
The output of the cryogenic microwave source was amplified successively using a high electron mobility transistor (HEMT) amplifier and two additional microwave amplifiers at 4 K and room temperature, respectively. For qubit readout, conventional methods were used to prepare and deliver microwave pulses to the qubit sample for control and measurement.
Study Contributions
To summarize, in this study, the researchers successfully developed an on-chip coherent cryogenic microwave pulse generator with simple control and minimal heat load. They achieved coherent microwave photon generation and reported a microwave signal source with exceptional coherence.
The high coherence of the microwave pulses allowed for convenient superposition, enabling the creation of a wide range of microwave signals. This represents a significant improvement over previous microwave photon sources used in cryogenic environments.
Additionally, the digital-like signal driving the microwave source can be transmitted through low-bandwidth channels, such as twisted pairs of wires, which have a significantly lower passive heat load compared to the coaxial cables used in current technology. The researchers also demonstrated the microwave pulse generator's effectiveness in high-fidelity readout of superconducting qubits.
Overall, the device showcased in this work offers operational flexibility, negligible heat load, a small footprint, and high compatibility with existing superconducting quantum circuits. It represents a key technological advancement for enabling large-scale superconducting quantum computers.
Journal Reference
Bao, Z. et al. (2024). A cryogenic on-chip microwave pulse generator for large-scale superconducting quantum computing. Nature Communications, 15(1), 1-9. DOI: 10.1038/s41467-024-50333-w, https://www.nature.com/articles/s41467-024-50333-w
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