More than one way to make a qubit


The goal of building a quantum computer is to exploit the quirks of quantum physics to solve certain problems much faster than a traditional computer. And at the heart of a quantum computer is the quantum bit, or qubit, the quantum equivalent of the 1s and 0s that underpin our digital lives.

“A qubit is the cornerstone of quantum information science technology,” says Joseph Heremans, an electrical engineer at the US Department of Energy’s Argonne National Laboratory.

Traditional bits can be any type of switch, anything that can switch from 0 to 1. But building a qubit requires something more.

“A qubit is essentially a quantum state of matter,” Heremans explains. “And it has some weird properties that allow you to store more information and process more information” than a traditional bit.

These strange properties include superposition (the ability to be in a mixed state, a weighted combination of 1s and 0s) and entanglement (in which multiple qubits share a common quantum state). Both might seem hard to find. Fortunately, nature has provided many options and engineers have concocted a few more.

Researchers are exploring more than half a dozen ways to implement qubits, with two promising approaches currently being explored: superconducting circuits and trapped ions.

Before

Ions, atoms that have lost one or more of their electrons, emerged as a promising qubit platform at the dawn of experimental quantum computing in the mid-1990s. In fact, the first qubit ever built was fabricated at from a single beryllium ion.

Ions are natural quantum objects: two of the discrete energy levels of their remaining electrons can represent a 0 or a 1; these energy levels are easily manipulated by lasers; and because the ions are electrically charged, they are easily held in place by electromagnetic fields. Not much new was needed to produce trapped ion qubits. Existing technology could handle it.

Another advantage of trapped ions is that they are staunch defenders against a qubit’s greatest enemy: information loss. Quantum states are fragile and superpositions only exist if the qubits do not interact with anything. A stray atom or an unexpected photon can collapse the quantum state. In physics, the qubit “decoheres”. And decoherence sounds the death knell for all quantum information technology.

“We want a system where we can manipulate it, because we want to do calculations, but the environment doesn’t speak to it too much,” says Kenneth Brown, an electrical engineer at Duke University.

Trapped ions tick both boxes. Held safely in a dark vacuum, they have little interaction with the environment, he says.

Because of this robustness, trapped ions have some of the lowest error rates of any qubit technology. But they struggle to grow beyond small-scale demos. Adding more ions to the mix makes it harder for the lasers controlling them to choose who to talk to. And moving to more qubits means using a lot of auxiliary technologies, such as vacuum systems, lasers and electromagnetic traps.

The largest trapped-ion quantum computer on the market is a 32-qubit machine built by IonQ, headquartered in College Park, Maryland. But quantum engineers want machines with hundreds, if not thousands, of qubits.

Enter the superconducting qubit

Just a few years after the first trapped ion qubit, researchers produced the first qubit implemented in a superconducting circuit, in which an electric current oscillates back and forth around a microscopic circuit etched on a chip.

When cooled to temperatures a few hundredths of a degree above absolute zero, the oscillator circuit can behave like a quantum object: a flash of radio waves tuned to the right frequency can put the circuit into one of two distinct energy levels, corresponding to a quantum 1 or 0. Tracking zaps can direct it to a superposition of these two states.

“They are a very promising way to make quantum computers” because they can be made on microchips, says Paul Welander, physicist at SLAC National Accelerator Laboratory. “And microfabrication is something we’ve been doing for a long time in the semiconductor industry.”

Leveraging techniques used to fabricate computer chips, a fabricator can fabricate superconducting circuits on large wafers.

Another advantage of the superconducting circuit is “the ability to make a device hundreds of micrometers wide and yet it behaves like an atom,” says Welander.

Engineers get all the quantity of an atom but with the ability to design and customize its properties by adjusting circuit parameters.

These circuits are also extremely fast, running through each step of a calculation in nanoseconds. And because they are circuits, they can be designed to meet the needs of engineers.

Superconducting qubits have found their way into the largest working general-purpose quantum computers. The largest, unveiled in November 2021 by IBM, contains 127 qubits. This chip is a step towards the company’s goal of creating a 433-qubit processor in 2022, followed by a 1,121-qubit machine by 2023.

But superconducting circuits also struggle with decoherence.

“They are made up of very many atoms,” says Welander.

This presents plenty of opportunities for something to go wrong – materials and manufacturing processes present a particularly thorny challenge when trying to mass-produce millions of qubits at once.

Hardware interfaces are particularly problematic. Metal electrodes, for example, oxidize easily. “Now we have an uncontrolled state on the surface,” says Welander, which can lead to quantum state decoherence and loss of information.

Another downside is that superconducting circuits have to stay frigid, hovering at temperatures just above absolute zero. This requires extreme refrigeration, which presents challenges for scaling superconducting quantum computers to thousands or millions of qubits.

A menu of options

While these two qubit technologies are perhaps the best known, they’re not the only game in town.

Another approach uses flaws in diamonds. These gemstones are made up of carbon atoms arranged in a rigid, repeating lattice. But sometimes another type of atom comes into play. For example, a nitrogen atom or a vacancy – the absence of an atom – can take the place of a carbon atom. These nitrogen and vacancy impurities are “much like a molecule trapped in the diamond crystal,” Heremans explains.

Here, electrons trapped in the crystal defect store information in a quantum property called spin, a type of intrinsic torque. When measured, spin only takes one of two options – perfect for encoding a 1 or a 0. These options can be toggled with laser light, radio waves or even mechanical stress.

Researchers are also exploring making qubits from electrically neutral atoms, trapped using lasers instead of electromagnetic fields. “Neutral atoms are the most natural qubit candidate,” says Mikhail Lukin, a physicist at Harvard University.

Like ions, neutral atoms can be isolated from the environment and remain cohesive for long periods of time. But modern laser technology gives scientists more flexibility with neutral atoms than electromagnetic traps with trapped ions. Neutral atoms can be organized into many different 2D models, providing more ways to connect atoms and entangle them, leading to more efficient algorithms.

Using neutral atoms, Lukin and his colleagues recently unveiled a special-purpose 256-qubit quantum computer known as the Quantum Simulator, the largest of its kind, with plans to build a 1,000-qubit simulator over the next few years. next two years.

The list of possible qubits continues. Photons, semiconductors, molecules – these and other platforms have potential.

But despite all these options, there is no clear winner. It’s not yet clear what can be scaled to 1,000 qubits or beyond. It is not even certain that there is only one best approach.

“We’re still in hunt and find mode,” says Welander. For quantum computing, “it can actually become something hybrid,” using multiple quantum materials and systems.

Perhaps a single processor will use superconducting qubits working alongside diamond-defect qubits, which could talk to other quantum processors using photon-based qubits.

Ultimately, what makes the “best” qubit depends on how the qubit is used: a good qubit for quantum computing may be different from a good qubit for quantum sensing or a good qubit for quantum communication, says Heremans.

Beyond physics

What is clear is that the progression of qubits is not just a problem of physics. “It really requires expertise in a wide range of fields,” from materials science to chemical and electrical engineering, Welander says.

And it’s not just the qubits themselves that need attention. Qubits require many supporting technologies – vacuum systems, cryogenics, lasers, microwave components, wire nests – all working in sync to get the most out of any quantum processor.

In many ways, quantum computers are where digital computers were in the 1950s and 1960s. In addition, researchers were looking for the right technology to represent 1s and 0s and perform the logical operations necessary for any computation. Bulky vacuum tubes gave way to more compact transistors; germanium transistors gave way to more efficient silicon transistors; integrated circuits allow engineers to cram many transistors and support electronics onto single silicon wafers.

For quantum computing to reach its full potential, qubits still need the right technology. “There are many areas where interested people and intrigued people can plug in and have an impact,” says Welander.

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