We'll update this story with links to pre-print when it's posted on arXiv.org later today.
In general, I reflexively suppress press releases. This one was no different, but when the message was gone, the subject line was saved: "IonQ … quantum computing". It took a second, but I realized that the name could mean something that I would never have imagined: a commercial image, based on ions, quantum computer. A quick visit to the trash confirmed my suspicions.
After some negotiations, I received a very secret document demonstrating the capabilities of the new IonQ computer.
Engineering is not simple
Making a quantum computer based on ions seems like a bad idea. Think about the engineering required to market the computer. You must have interconnected qubits (quantic bits) in order to perform logical operations, and these qubits must preserve their quantum-ness.
Virtually all commercial efforts in quantum computing focus on the use of superconducting current rings such as qubit. These circuits can take advantage of all the engineering tools available for printed circuit board technology. The control circuits and displays are all electronic: they send and receive microwave signals via lines on a printed circuit board. The qubits are interconnected by lines that connect them. In other words, engineering is relatively easy.
However, in research laboratories around the world, there are small-scale quantum computers based on ion chains (an ion is an atom with an electron removed). The ions float in an almost perfect vacuum, trapped by electric fields. Each ion must be addressed by two laser beams. The interconnection between qubits is via the natural movement of qubits: they vibrate together.
Ionic qubits and their logical operations far surpass their superconducting brethren. But engineering this type of computer on a commercial scale represents a whole new challenge.
There is water in your computer
An article published by researchers uses the computer IonQ to calculate the energy of the ground state of a water molecule. The calculation itself is something that almost any modern computer can do. What distinguishes the calculation, it is the number of operations necessary to complete the operation. By choosing water, the researchers showed the best in quantum computer based on ions.
Let's put this in perspective. To model a water molecule, researchers use a standard approach, in which one assumes that the electrons of a water molecule take on a little character from the electrons that are in the oxygen and a little character of the electrons that are in hydrogen. The trick for the calculation is to determine the balance of the adjuvants and their energies.
The resulting calculation is a repeated approximation, where additional correction terms make the result (hopefully) more accurate as the calculation progresses. This allows you, in a sense, to choose your accuracy depending on when you stop doing mathematics. As you can imagine, each correction term requires more computing resources – for a quantum computer with only a few qubits, it's a challenge.
But if you have beautiful stable qubits, who live a long time and who all speak with a high degree of reliability, you have a little room for maneuver. This is one of the key points of this document: the ion computer allows you to play some clever tricks to reduce the total number of qubits needed in exchange for an increase in the number of operations required. This only works if you have time to perform all the operations, and time is something that quantum systems do not always provide in abundance.
According to the press release, IonQ's computer has 160 or 79 qubits (depending on whether the computer stores or exploits quantum information). Looking at the circuit diagram (the program, essentially), I estimate that the calculation required about 30 qubits. This in itself is quite a leap from the typical ionic quantum computer, which has about ten ions.
This is the least. The calculation requires many sequential gate operations and, unlike digital logic operations, quantum logic operations are not accurate. The error in each operation accumulates on several operations, leaving a ragged calculation. Ionic computers, however, have a very high precision in their operations. The researchers said they were able to perform 50 consecutive operations while keeping the qubit in good condition about three quarters of the time.
The interconnection of the ionic computer has also played an important role in the calculation. The researchers were able to directly entangle arbitrary pairs of bits during the computation. In other quantum computers, geometry does not allow all qubits to be interconnected. As a result, any calculation requires the information to be exchanged between qubits. Since the information disappears after a certain number of operations, each additional step of moving the information reduces the amount of useful calculations that can be made.
The end result is a very close response to the results obtained from standard calculations performed on conventional computers. And this is surely not the end. Hopefully IonQ will soon release more details on the computer – you can be sure we will cover it when they do. Even in the absence of technical details, I am confident that users' results will be consistent.