EEWeb 27 Nov 2023
Qubits are the foundation of every quantum computer. They’re also incredibly difficult to control because the slightest amount of noise causes them to collapse. This is where quantum-error correction (QEC) comes in. This is, essentially, a set of tools that oversees and interacts with the qubits to make sure they don’t break down.
“Quantum-error correction is about taking unreliable qubits and using them to build reliable quantum computers that always give you the correct output to your computation,” said Earl Campbell, VP of quantum science at Riverlane, which is developing these tools in its Quantum Error Correction Stack.
Quantum-error–corrected (sometimes called “fault tolerant”) quantum computers are the next phase of quantum computing. Today’s quantum computers are noisy and often called noisy intermediate-scale quantum (NISQ) devices. This means that approximately every 1 in 100 or 1 in 1,000 quantum operations ends in an error. So you cannot trust the output of your computation.
“Quantum-error correction is all about getting around that problem and making devices work, even in the presence of noise,” Campbell said.
Once achieved, QEC will allow us to reach the so-called TeraQuop threshold, where a trillion (Tera) reliable quantum operations (Quops) can run before an error appears. This will allow scientists to solve certain problems that are intractable on classical supercomputers.
But to reach the TeraQuop regime requires many millions of qubits. Today’s devices have a few dozen.
Why do we need so many qubits? Again, this is due to noise and the ways in which QEC works.
A quantum-error–correcting code is used to embed a logical qubit into many physical qubits. In other words, a group of physical qubits protects the logical qubit as it runs the calculation. Part of this process means that you must continually measure all the physical qubits to look for signposts that something has gone wrong nearby.
This information, which is called the syndrome data, is then passed onto a key component in the Quantum Error Correction Stack called the quantum decoder. “This is one of the things that Riverlane builds,” Campbell said. “It takes this information and infers what corrective steps need to be taken in response to the evidence of errors that you’ve observed.”
Another key component of the Quantum Error Correction Stack is the quantum control system. This generates high-accuracy, high-speed pulse sequences to control the qubits using affordable off-the-shelf hardware. Riverlane’s Quantum Error Correction Stack is comprised of both the control and decoder tools to achieve scalable QEC. It is also customizable and works across multiple qubit types, making it a versatile solution for different quantum architectures.
But building the quantum hardware and the Quantum Error Correction Stack to turn unreliable qubits into reliable quantum computers is too much work for one company. That’s why it’s vital for QEC companies like Riverlane to work with leading quantum hardware companies like Infleqtion.
“Building better quantum computers is a very challenging task,” said Mark Saffman, chief scientist at Infleqtion and professor of physics at the University of Wisconsin–Madison. “It’s perhaps the most demanding technological challenge that humankind has faced in the last century and requires the expertise and capabilities of different groups of people with different backgrounds.”
Infleqtion is a multi-platform quantum technology company, developing a range of cutting-edge quantum products. In his work, Saffman is focused on “building better qubits and better quantum computing systems to control those qubits and do useful things with them.”
This work focuses on neutral atom quantum hardware, which is a naturally scalable qubit type with strong connectivity, fidelity and qubit lifetimes achieved at room temperature (where many other qubit types require advanced cryogenic solutions to reach their operation temperatures in the millikelvin region).
“The field of quantum computing is at a very interesting point,” Saffman said. “People have recently demonstrated that you can do things with a quantum computer that you could not do on any classical computer, even a supercomputer, in a reasonable amount of time.
“But they haven’t yet done useful things with these quantum computers,” he added. “They’ve just shown they’re faster for certain, let me say, toy problems. So the path forward is, on the one hand, to get quantum-error correction working with big systems that manage a very large amount of data, but also, on the other hand, to look for possibilities of finding useful applications of quantum computers even before we get to full error correction.”
For the last 12 months, Infleqtion has been using Riverlane’s control hardware to help improve its qubits. “We have a couple of Riverlane control systems that have FPGAs inside and can be programmed to generate pulses that control laser beams and perform operations on our qubits,” Saffman said.
“It’s already been a very valuable collaboration because there’s complementary expertise between Infleqtion and Riverlane,” he added. “Working together, we’re able to tackle some challenges faster than we would separately, and this also feeds into the notion of codesign.”
Codesign is about building highly complex systems, but not designing the software or electronics or physical qubit platforms in isolation. Instead, it’s about making the whole system work together.
“It’s bridging these expertise areas together that really informs how to design and implement large systems; also, it’s been a lot of fun to work together,” Saffman said.
Campbell agreed that the collaboration is also vital for Riverlane’s work: “It’s incredibly valuable for Riverlane to work with a great partner like Mark and Infleqtion, especially since Riverlane is building a Quantum Error Correction Stack but we don’t have any qubits. So we love to partner with companies that build really great qubits.”
Simply put, you cannot scale qubit numbers without QEC, but you cannot develop QEC without high-quality qubits.
By working together, Infleqtion and Riverlane can test and optimize the designs and execution of their qubits and Quantum Error Correction Stack, respectively, as quantum computers continue to scale and move away from the NISQ era.
This shift away from NISQ and into QEC will be incremental, though, with early use cases expected before we reach the TeraQuop threshold. “Quantum-error correction will open a path toward utility on some limited set of problems, even before we get all the way to full fault-tolerant error correction, so for me, quantum-error correction is an inspiration to continue improving the systems we have,” Saffman said.
“What I’m finding particularly interesting at the current time is that the paths forward, both toward quantum-error correction [and] also toward the possibility of utility pre-error correction, are actually quite well-aligned,” he added. “It was often said that the specific capabilities we need to implement error correction are not the same as we need for just doing better with our non-error-corrected machines. And I’m thinking more these days that that’s not so accurate.”
But what do we need to reach QEC? “We need more qubits,” Saffman said. “We need better qubits with better coherence, we need better control, higher-fidelity gate operations, better measurements. Everything has to be better. And that will lead us toward error correction.”
To achieve this, the collaboration between Riverlane and Infleqtion will continue to grow as quantum computers continue to scale toward utility, with both companies already preparing for a fault-tolerant future.
“We know what we need; Riverlane knows how to make it available for us using their quantum-error–correction expertise,” Saffman said. “This shared development accelerates the process of getting to where we need to be to make these systems work better and make quantum computers useful, sooner.”
“Quantum Conversation with Riverlane’s Earl Campbell and Infleqtion’s Mark Saffman on the importance of error detection”