Why every qubit is not created equal

In this blog post, our quantum engineer, Luuk Earl, explains the differences between superconducting and trapped-ion qubits and the impact when scaling up quantum computers.

For years, quantum computing inched forward, one qubit at a time. Now, the world is waking up to the reality that we need millions of qubits for quantum computers to achieve something truly useful for society.

IBM hopes to have 1,000-qubit machines by 2023. Rigetti says it’ll do the same in 2024. Google is reportedly targeting a commercial-grade machine by 2029.

Such scalability is no mean feat, though this is still a long way from the millions of qubits required for quantum computing to solve real-world problems. And qubits sit at the heart of the problem.

What is a qubit anyway?

A qubit is a quantum bit. It’s analogous to a traditional computer bit.

But in a classical computer, bits store information as 1s or 0s. In a quantum computer, qubits can exist in multiple states at the same time, a so-called superposition between 1 and 0.

Due to their probabilistic nature, quantum computers can solve problems that are intractable even on today’s best supercomputers.

However, by their very nature, qubits are incredibly fragile. Even the slightest amount of noise can cause the underlying quantum information to break down, rendering the quantum computer useless.

There are many ways to boost qubit coherence times (the amount of time a qubit can maintain its superposition) and get these machines working at scale. At Universal Quantum, we’ve based our work on six core technical pillars, focusing on creating a million-qubit machine from day one.

A key consideration is the type of qubit you use and there are many different types of qubit to choose from, including silicon quantum dots, photonic qubits, topological qubits and diamond vacancies. But those are outside of the scope of this post.

Instead, I’ll focus on two types of qubits that are popular contenders to unlock the potential of quantum computing: superconducting and trapped-ion qubits.

Superconducting vs. trapped-ion qubits

Let’s start by looking at superconducting qubits. Here, superconducting circuits are essentially used to create a qubit. This type of quantum computer generally consists of static qubits arranged on a grid where they can only interact with their nearest neighbour.

Superconducting systems have been in the spotlight for many years. About 10 to 20 years ago when the field was in its infancy, superconducting quantum bits seemed like the best qubit choice. Other technologies were far behind in terms of the number of qubits they could produce.

But things started to change, and the limits of superconducting machines are now being tested. As we start to scale the number of qubits, superconducting machines can struggle in three key areas:

1. Cooling to near absolute zero (which is minus 273 degrees Celsius),
2. Limited gate connectivity (they can only talk to their neighbours)
3. Short qubit lifetime (aka limited computing time).

This is where trapped ion systems are starting to break through, especially as we scale up the number of qubits, with less stringent refrigeration requirements, improved connectivity and longer qubit lifetimes.

Trapped-ion qubits (as the name suggests) use ions trapped in electric fields to create qubits. To control their quantum state, you can either use laser beams or a microwave field.

Trapped ion designs have led to many significant world records in the quantum computing sector, and an exciting blueprint for a commercially viable quantum machine.

But there are still challenges for trapped ion systems as we continue to scale. This is an issue that Universal Quantum has focused on from the start.

Freeing trapped-ion qubits

Recently, there has been rapid progress for microwave control methods. While many trapped-ion systems rely on lasers to control the qubit’s state, we use a global microwave field to control our trapped-ion qubits at Universal Quantum. So, we don’t need to precisely align so many lasers, while also allowing the qubits to easily interact with each other.

Our unique, electronic quantum computing modules are also based on silicon technology where individual modules are connected using ultrafast electric field links. These electric field links allow our modules to be connected orders of magnitude faster and more reliably than any other approach, providing an exciting route to scale up and providing a trapped-ion architecture that truly scales to solve the world’s most complex problems.

It’s an exciting time to work in quantum computing, and there’s a lot of work to do to reach the million-qubit scale. To find out more about our technology and people, see www.universalquantum.com or email us at leap@universalquantum.com.