The Race to Build a Fault-Tolerant Superconducting Quantum Computer

Amazon, Google, and IBM are all pursuing different strategies to reduce error rates

6 min read
A rendering showing different layers of silver circuitry. The top layer is labeled "IBM Quantum" and "Eagle."

The 127-qubit Eagle quantum processor by IBM.

IBM Research

Quantum computers are theoretically far more powerful than classical computers on important tasks such as investigating novel battery designs or discovering new drugs, but they are currently too error prone for practical use. Now Amazon, Google, IBM, and others are pursuing a bevy of innovative strategies to develop fault-tolerant quantum computers based on superconducting circuits.

Whereas classical computers switch transistors either on or off to symbolize data as ones or zeroes, quantum computers use quantum bits, or "qubits," which because of the surreal nature of quantum physics can exist in a state of superposition where they are both 1 and 0 at the same time. This essentially lets each qubit perform two calculations at once. The more qubits that are quantum-mechanically linked, or entangled, the more calculations they can perform simultaneously.

Present-day state-of-the-art quantum computers typically suffer roughly one error every 1,000 operations. Many practical applications demand error rates lower by a billionfold or more, says Oskar Painter, head of Amazon Web Services' (AWS) quantum-hardware program and an experimental physicist at the California Institute of Technology.

In addition to building qubits that are physically less prone to mistakes, scientists often hope to compensate for high error rates by spreading quantum information across many redundant qubits. This would help quantum computers detect and correct errors, so that a cluster of a thousand or so "physical qubits," the kinds that researchers have developed to date, can make up one useful "logical qubit." Many tech giants aim to develop fault-tolerant quantum computers using superconducting circuits as qubits because such hardware is scalable to thousands of physical qubits in the near future, Painter says.

Amazon's Schrödinger's Cats

One strategy Amazon is exploring to build a fault-tolerant quantum computer involves machines that are inherently stable against error. The key is a hardware version of Schrödinger's cat—the thought experiment in which a cat in a box is suspended in a fuzzy state between life and death—until someone looks in the box and the cat is then either living or dead.

Scientists can theoretically encode data in a qubit using virtually any pair of states of a quantum system—for instance, two of a molecule's potentially many different energy levels. Amazon researchers are investigating using so-called cat states, a pair of states as opposed to one another as the states of life and death experienced by Schrödinger's hypothetical cat.

Specifically, Amazon's design relies on an oscillator whose fluctuations move in phase. Its cat states rely on pairs of fluctuations with opposite phases in the same oscillator. Painter and his colleagues found they could make cat qubits that are highly resistant to bit flip, where a qubit flips from one state to another, one of two main sources of error a superconducting qubit can have.

A photo of a golden disc with a silver plate labeled "AWS" bolted on top.A microwave package encloses the quantum processor. The packaging is designed to shield the qubits from environmental noise while enabling communication with the control system.AWS Quantum Computing

"One of the biggest challenges today when it comes to superconducting quantum computing is reducing bit flip," Painter says. Bit flips result from environmental noise, such as spikes of heat or electricity, so using cat qubits can help protect a quantum computer against outside disruption, he notes.

This strategy does make cat qubits more vulnerable to the other common source of error a superconducting qubit can have, known as phase flip, where it switches between one of two opposite phases. However, Amazon says it can then use quantum error-correction schemes to compensate for phase flips. Focusing on just one kind of error instead of two "reduces the overhead resources you need for quantum computing by a factor of a square root," Painter says. This can readily amount to orders of magnitude fewer physical qubits needed.

At the AWS re:Invent conference in December, Painter and his colleagues described research on cat qubits that were roughly 20,000 times as biased toward bit flip than phase flip. "That's a big, big step forward," he notes. Within the next year, they hope to reveal a logical qubit based on cat qubits, he adds.

Google's Codes

In theory, using more physical qubits can result in an exponential suppression of quantum-computing error rates. Google recently showed this was possible using its 54-qubit Sycamore quantum computer, which in 2019 carried out a calculation in 200 seconds that the company estimated would take Summit, the world's most powerful supercomputer at that time, 10,000 years.

Google had physical qubits serve either as "data qubits" that encode the logical qubit or "measure qubits" tasked with repeatedly detecting errors in their fellow qubits. When the qubits were arranged in a one-dimensional chain, with each qubit having two neighbors at most, increasing the number of qubits led to an exponential suppression of the rate of bit or phase-flip errors, reducing the amount of errors per round of detections and corrections up to more than a hundredfold when they scaled up the number of qubits from 5 to 21.

However, quantum errors are not limited to just one direction. But when Google researchers used a "surface code," in which they arranged the qubits in a two-dimensional checkerboard pattern, they found that such a design using data and measure qubits performed as expected from computer simulations.

In subsequent work, Google tested how a different two-dimensional qubit grid could help develop lower-error qubits. They employed 31 data qubits on Sycamore to run a "toric code"—so named because it mimics a lattice placed on the surface of a torus—to simulate exotic two-dimensional quasiparticles known as anyons. Anyons are collective excitations not associated with any one qubit, "sort of like how a crowd can do 'the wave' and the wave isn't associated with any particular person," says Kevin Satzinger, a research scientist at Google.

Pairs of anyons can "braid," or swap places. Braiding means that anyons can in a sense remember how they behaved with respect to one another even after they get separated. A "topological qubit" created from pairs of anyons could benefit from a kind of protection conveyed by its topology, defending it from disruptions and reducing error rates.

IBM's Hexagons

Instead of arranging its qubits in square grids, IBM arrays its qubits in hexagonal lattices, with a qubit on each point and another on each flat edge. This pattern, now used in all the company's quantum computers—including its 127-qubit Eagle quantum processor—can reduce potential errors caused by accidental interactions between neighboring qubits. "Each qubit has only two or at most three neighboring qubits," says Sarah Sheldon, a quantum physicist at IBM. (In comparison, in square grids, a qubit may have up to four neighbors.)

IBM believes this new layout will help it scale up faster to much larger practical quantum computers. The company aims to debut its 1,121-qubit Condor processor in 2023.

This architecture does require new quantum error-correction codes adapted to hexagons instead of squares, which IBM is actively researching. Recent work from IBM suggests quantum error-correction codes adapted to hexagons "can lead to a significant reduction in error rates," says Sheldon.

In addition to actively correcting errors, IBM is exploring error-mitigation schemes to avoid them in the first place. One strategy, zero-noise extrapolation, confronts how quantum computers are typically very sensitive to disruptions from noise in their environment. By repeating quantum computations at varying levels of noise, the researchers can extrapolate what the quantum computer would calculate in the absence of noise.

"We've scaled this method to quantum circuits using 26 qubits and have shown it has promise to improve quantum simulations in the near future," Sheldon says. "Ultimately, building a fault-tolerant quantum computer is going to require implementing error correction, but as we build towards that, error mitigation can prove of substantial aid in the near term."

Ancillas and Magic States

Even if quantum computers do scale up to many thousands of physical qubits to potentially support enough logical qubits for practical applications, they will have to deal with another major challenge—the need for extra "ancilla qubits," Painter says.

When two logical qubits are connected in a quantum logic gate—an elementary operation that quantum computers use to perform a computation—scientists want to make sure that each physical qubit in a logical qubit interacts with only one physical qubit in the other logical qubit. This limits the disruption that could happen if a physical qubit experienced an error. Doing so requires ancilla qubits to monitor potential errors, "which counts as additional overhead," Painter says.

The special quantum states in which qubits can serve as ancillas are known as "magic states," and preparing them requires particular hardware dubbed magic-state factories. "No one is even close to making a magic-state factory that works efficiently," Painter says. "It's a grand challenge for the entire industry."

In a 2020 study, Amazon did suggest a way to reduce the number of ancilla qubits needed by at least an order of magnitude. This strategy involves using ancilla qubits not just to detect errors but also events where minor errors grow to major uncorrectable errors. This also reduces the amount of resources needed in terms of magic-state factories.

Amazon, IBM, and others continue to work on magic-state factories. "Once we demonstrate error correction on a logical qubit, then we will have all the building blocks we need to demonstrate magic states," Painter says.

Correction (4 Feb. 2022): An earlier version of this story attributed a quote to Jimmy Chen at Google, when in fact the quote was from Kevin Satzinger, also a research scientist on the Google AI Quantum hardware team.

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The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
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A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay
Blue

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.

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