Quantum computing breakthrough achieved with on-chip pulse generator

Quantum computing has, for a while now, been heralded as the future of complex problem-solving. It is touted as the harbinger of doom for today’s encryption systems. However, scaling effectively has been an Achilles heel.

The field currently grapples with the challenge of scaling quantum computers to millions of qubits. This scale is essential for executing fully error-corrected quantum algorithms and advancing noisy intermediate-scale quantum applications. Moreover, the existing methods for readout and manipulation of qubits are both cost-intensive and cumbersome.

In present systems, microwave signals are transmitted from room-temperature electronics to quantum chips housed within cryogenic dilution fridges at millikelvin temperatures. This involves routing these signals through coaxial cables, a method that becomes impractical beyond a certain point.

While it is feasible to extend this setup to around 1,000 qubits, scaling beyond incurs costs and heat load significantly, reports AZoQuantum .

The critical bottleneck here is traditional architecture, which cannot handle the extensive wiring and the heat dissipation that comes with scaling to this extent.

A promising solution

Monolithic integration can be a solution to this issue. By tightly integrating qubits with control and microwave electronics and replacing macroscale wiring with chip stackings and circuit blocks, this approach can reduce both passive heat load and system footprint.

Monolithic integration offers systematic advantages such as improved signal fan-out and fan-in capabilities and reduced communication latency. Additionally, it minimizes the reliance on extensive wiring harnesses— a major source of heat load and complexity.

However, it requires a coherent cryogenic microwave pulse generator that is compatible with superconducting quantum circuits. A new study reveals such a signal source driven by digital-like signals, generating pulsed microwave emission 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 within a vacuum process to gain precise control over the frequency, intensity, and phase by digitally manipulating 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 its center conductor. The SQUID, featuring two parallel Josephson junctions, acts as a tunable inductor and allows the resonator’s properties to be adjusted through variation of the magnetic flux.

The total inductance of the SQUID-embedded resonator included both the flux-dependent SQUID inductance and the coplanar waveguide resonator inductance. For the readout, a three-dimensional (3D) circuit quantum electrodynamics architecture was employed.

In their experiments, the researchers used room temperature junction resistances ranging from 50 Ω to 270 Ω, corresponding to inductances of 58 pH to 310 pH at zero flux. These values accounted for 3.1% to 11.6% of the total inductance of the SQUID-embedded resonators.

To drive the pulse generator, the team employed an arbitrary waveform generator with a 1 GHz sampling rate. This generator delivered the necessary flux step/overshoot to the signal source in the cryogenic environment. The output of the cryogenic microwave source was then amplified by using a series of amplifiers at different temperature stages.

Impact and prospects

The team’s on-chip coherent cryogenic microwave pulse generator showcased exceptional coherence in generating microwave photon pulses. This is a significant advancement over previous microwave photon sources used in cryogenic environments.

This high coherence enables convenient superposition, allowing a wide range of microwave signals to be created. This breakthrough 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 .