Usually, I reflexively delete press releases. This one was no different, but as the message vanished, the subject line registered—“IonQ… quantum computing.” It took a second, but I realized that the name might mean something I had never expected: a commercial, ion-based, quantum computer. A quick visit to the trash confirmed my suspicion.
After some negotiation, I was in receipt of a super-secret paper demonstrating the capabilities of IonQ’s new computer.
Engineering ions is not simple
Making an ion-based quantum computer seems like a bad idea. Think about the engineering required to commercialize the computer. You need to have qubits (quantum bits) that are interconnected so that they can perform logic operations, and those qubits need to preserve their quantum-ness.
Pretty much all commercial quantum computing efforts focus on using superconducting rings of current as the qubit. These circuits can take advantage of all the engineering tools available for printed circuit board technology. The control circuits and readouts are all electronic—they send in and receive microwave signals via lines on an integrated circuit. The qubits are interconnected by lines that couple them together. In other words, the engineering is comparatively easy.
In research labs all around the world, however, there are small-scale quantum computers based on strings of ions (an ion is an atom with an electron removed). The ions float in a near-perfect vacuum, trapped by electric fields. Each ion needs to be addressed by two laser beams. The interconnection between qubits occurs via the natural motion of the qubits: they all vibrate together.
Ion qubits and their logic operations outperform their superconducting brethren by a huge margin. But engineering this type of computer at a commercial scale has been an entirely new challenge.
There’s water in your computer
A paper released by researchers uses the IonQ computer to calculate the ground state energy of a water molecule. The calculation itself is something almost any modern computer can do. What makes the calculation stand out is the number of operations required to complete the operation. In choosing water, the researchers have shown ion-based quantum computing at its best.
Let’s put this in perspective. To model a water molecule, the researchers use a standard approach, where it’s assumed that the electrons in a water molecule take on a bit of the character of electrons that are in oxygen and a bit of the character of electrons that are in hydrogen. The trick to the calculation is to determine the balance of the admixtures and their energies.
The calculation that does this is a repeated approximation, where additional correction terms make the result (hopefully) more accurate as the calculation proceeds. This allows you to, in a sense, choose your accuracy based on when you stop doing math. As you might expect, each correction term requires more computational resources—for a quantum computer with only a few qubits, that’s a challenge.
But if you have nice, stable, long-lived qubits that all talk to each other with a high degree of reliability, then you have a bit of wiggle room. That’s one of the key points of this paper: the ion computer allows you to play some clever tricks that reduce the total number of qubits needed in exchange for increasing the number of operations required. That only works if you have time to perform all the operations, and time is something that quantum systems don’t always provide in abundance.