Quantum computing has been hyped as the future of complex problem solving for some time now – it’s been touted as a harbinger of doom for today’s encryption systems – but effective scaling has proven to be its Achilles heel.
The field is currently grappling with the challenge of scaling quantum computers to millions of qubits. This scale is essential to run fully error-corrected quantum algorithms and advance noisy mid-scale quantum applications. Moreover, existing methods of reading and manipulating qubits are costly and cumbersome.
In current systems, microwave signals travel from room-temperature electronics to a quantum chip housed inside a millikelvin-temperature cryogenic dilution refrigerator, which requires routing these signals over coaxial cables, a method that becomes impractical above a certain point.
It’s possible to scale this setup up to around 1,000 qubits, but expanding beyond that would significantly increase costs and heat loads, AZoQuantum reports.
The key bottleneck here is traditional architectures, which cannot handle the extensive wiring and heat dissipation that comes with scaling to this extent.
A promising solution
Monolithic integration could be a solution to this problem: by tightly integrating the qubits with the control and microwave electronics, and replacing macroscale wiring with chip stacking and circuit blocks, this approach could reduce both the passive thermal load and the system footprint.
Monolithic integration provides systemic benefits such as improved signal fan-out and fan-in capabilities and reduced communication latency, while minimizing the reliance on extensive wiring harnesses, which are a major source of thermal load and complexity.
But this requires a coherent cryogenic microwave pulse generator that is compatible with superconducting quantum circuits. New research reveals such a source, driven by a digital-like signal, producing pulsed microwave radiation with well-controlled phase, intensity and frequency directly at millikelvin temperatures.
The team behind this research proposes an on-chip coherent cryogenic microwave pulse generator. They used superconducting circuits in a vacuum process to precisely control the frequency, intensity and phase by digitally manipulating the magnetic flux across a superconducting quantum interference device (SQUID) embedded in a superconducting resonator.
The team’s device consists of a λ/2 coplanar waveguide resonator with a SQUID embedded in the central conductor. The SQUID with two parallel Josephson junctions acts as a tunable inductor, allowing the characteristics of the resonator to be adjusted by changing the magnetic flux.
The total inductance of the SQUID embedded resonator includes both the flux-dependent SQUID inductance and the coplanar waveguide resonator inductance.A three-dimensional (3D) circuit quantum electrodynamics architecture was adopted for readout.
In their experiments, the researchers used room temperature junction resistances ranging from 50 Ω to 270 Ω, which correspond to inductances at zero flux of 58 pH to 310 pH. These values account for 3.1% to 11.6% of the total inductance of the embedded SQUID resonator.
To drive the pulse generator, the team employed an arbitrary waveform generator with a sampling rate of 1 GHz. This generator provided the necessary flux steps/overshoots for the cryogenic environment signal source. The output of the cryogenic microwave source was then amplified using a series of amplifiers at different temperature stages.
Impact and outlook
The team’s on-chip coherent cryogenic microwave pulse generator exhibited extraordinary coherence in generating microwave photon pulses, a major advance over conventional microwave photon sources used in cryogenic environments.
This high coherence allows for convenient superposition to create a wide range of microwave signals — a breakthrough that could potentially lead to superconducting quantum computers implemented on a large scale.
Details of the team’s research were published in the journal Nature Communications.
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Amal Jos Chacko Amal dreams of a typical work day writing code, taking photos of cool buildings and reading a book by the fire. He loves all things technology, electronics, photography, cars, chess, football, F1 and more.
