Quantum computers will usher in a new era of computing where problems intractable on today’s most powerful classical machines can finally be tackled. These problems included the simulation of the FeMoco molecule (a key component of the nitrogen economy), steam-methane reforming (used in hydrogen production) and integer factorisation (determining which prime numbers divide a given integer – one of the main characteristics of classical encryption techniques).

There are many challenges to build a useful quantum computer. The core issue is that the quantum bits – qubits – that run the quantum calculations are incredibly unreliable, breaking down when they encounter the slightest environmental noise and causing errors.

At Riverlane, we’re building the Quantum Error Correction Stack to help correct these errors. This includes building a scalable control and calibration system to reduce errors and create reliable qubits.

How do you control a qubit? It’s a complex, multi-disciplinary challenge and one that’s vital to tackle if we ever want to scale quantum computers to the point where they can do something useful for society. It’s a fascinating problem to work on – and it may also surprise you to know that we rely on many classical engineering skills to build our quantum control systems.

Classical hardware design verification is one of our tools to ensure that we build the system right. Design verification is where you prove or test that the system meets its specifications. In other words: given the input, you get the output you expected.

Needless to say, without design verification, we could not ensure that we’re controlling the qubits in the right way. It’s a necessary tool to ensure that we are building the right Quantum Error Correction Stack for our partner hardware companies, and it’s the tool that will ensure that we are engineering tomorrow’s fault-tolerant systems.

In a paper published at this year’s DVCon Europe in Munich and available on arXiv, Riverlane explains how classical device verification techniques are used to verify our Control System: Deltaflow.Control.

Going back to other industries and to previous endeavours, many of the components used in such control systems have been built before. These include:

  • Radio Frequency (RF) signal generation (currently used across our 5G networks)
  • Distributed computing (used for all large-scale networks such as the internet)
  • Real-time system (a vital component for industrial control, autonomous vehicles and in space/defence applications)

But what we have never done is to build a system where all of these components must work together at the same time and place. This is exactly the challenge that we, at Riverlane, face when building our quantum control system.

The Quantum Error Correction Stack requires an entirely new system architecture to be designed and built – one that is scalable as qubit numbers increase. That’s a huge undertaking – and the new arXiv paper focuses on the classical hardware verification methodologies that we need to verify our Deltaflow.Control system as we move from our current Control System (called DC1, which is capable of controlling tens of qubits) up to the next generation (DC2).

The paper describes how the new DC2 system balances tight power, memory and latency constraints to create control signals that enable high-precision manipulation of the amplitude, frequency and phase of the waveforms. The more accurately we can manipulate these parameters, the better we can control the quantum state. DC2 is a system architecture for a distributed system that offers compute at different levels of the Quantum Error Correction Stack: this enables partners to integrate their systems at the appropriate levels. DC2 is portable across different quantum hardware types.

When it comes to verifying DC2, we use all the classical verification techniques that we have in our armour: UVM (Universal Verification Methodology), SystemC modelling environments, golden model-based verification, formal verification, in-lab testing. We also describe how we use more modern “shift left” agile software approaches such as a continuous integration to do “full stack” testing.

This is a complex, ongoing engineering challenge – our Control System is already in the world’s leading quantum laboratories, and we will continue to work on it as quantum computers scale, and we reach the quantum error correction era.

If you’d like to design and implement the world’s first control system for useful quantum computers, then you can find out more here. At the time of writing, we’re looking for skilled digital design engineer and embedded software engineers to join the team and take on this challenge.