Here's How Ion Trap Quantum Computers Work

By Petar Jurcevic and Ryan Mandelbaum

It's easy, in principle, to build a quantum computer: just find a system that obeys the laws of quantum mechanics with properties that you can exploit in order to perform your computations. In practice, it's extremely hard to build a quantum computer; you need to be able to control that system.

Several architectures have arisen as viable controllable quantum systems; perhaps the "leaders" are the superconducting qubits that IBM and Google researches, and the trapped ion approach pursued by companies such as Honeywell and IonQ. You can read about how superconducting qubits work here. As for me, I did my Ph.D at the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria, studying quantum simulations of many-body interacting systems using trapped ions under professor Rainer Blatt and Dr. Christian Roos. I didn't join the team because I had a specific preference for ions; it was an opportunity in entering the field of experimental quantum computation at a time when programs were generally pursued by academic groups with a few notable exceptions. I was lucky to get the opportunity to join one of the major trapped-ion research groups in the world and to experience the evolution of quantum computation early on. 

Based on my experience, I can fairly say that neither technology is "better" or "worse" than the other, but both approaches have strengths as well as challenges that their respective engineers must overcome (if they hope for their devices to become useful in in the future.)

A macroscopic ion trap (via Blatt lab)

 

Ion traps are among the more  "natural" quantum computers; we represent the computer's two bit values with two quantum states of an electron around an atom. We begin setting up our device by loading from a neutral atomic source inside the computer's vacuum chamber, and then add energy with heat or lasers in order to create a tiny stream of neutral atoms. There are many choices of species we could use, calcium and ytterbium being some of the more popular, all of which have their own pros and cons— which choice is based on the engineering path you'd like to go down. Once we have our neutral stream of atoms, we use a laser to rip off a single electron, charging the atom and turning it into an ion. This charged ion then falls into the trapping potential, a specially-generated rotating electromagnetic field; which you can think of as a ball remaining at the base of a quickly-spinning saddle. The longer you wait with this process, the more ions you trap, and therefore, the more qubits you have to work with; each ion represents one qubit.

In order to actually compute with these qubits, we begin by Doppler cooling the ions with lasers of a specially tuned frequency slightly below the electron transition frequency, halting their motion. We use optical light to pump the atom into its ground state, and use sideband cooling, another laser cooling technique, to further cool the atom's motional modes into the ground state.

 

Once the system is initialized, we use laser fields to apply single or two-qubit gates. Single-qubit gates "move" the electron from one state to another, either leaving the electron in the ground, or 0 state, exciting it into the 1 state, or generating a superposition of 0 and 1. We also use lasers to couple these internal degrees of freedom, i.e. qubit states,  to the external motional degrees of freedom in order to generate two qubit gates; the common motion of the ions, due to the harmonic trapping potential, can be thought of as a bus system coupling all the qubits together at once. The laser fields are tuned in a particular way such that the ions' motion gets excited and de-excited only if the qubits are in a particular state; this state-dependent force is know as the Mølmer-Sørensen gate, the most commonly used two-qubit gate scheme in trapped ion architectures. 

 

After we run our circuit of one- and two- qubit gates, it's time to make our final measurements in order to get our readout, the bitstring that represents the computation's results. We use fluorescence; we couple one of the two qubit states to a short-lived transition that scatters a lot of light which we can collect with a CCD camera or a photo multiplier tube, so if the qubit is in this state we see light, and if the qubit is not in this state, we don't. The resulting measurements is therefore represented by a series of dark or light spots—the bitstring's zeros and ones.

 

These systems have natural advantages—depending on which ion you use, ion trap computers can have qubit T1s (the measure of how long until the ions return to their ground state) of several minutes or longer, and T2s (the measure of how long until ions in superposition dephase) of several seconds. All ions of the same species of atom are identical, so there's no variation in your qubits introduced by fabrication. Ions have extremely high gate fidelities, and in fact, reported record fidelities for single- and two-qubit gates have so far come from trapped ion systems. Moreover, state preparation/measurement errors can be orders of magnitude smaller compared to superconducting qubits and are rarely a real concern. Finally, ions in the same to trap have all-to-all connectivity, meaning you can drive gates between any pair of ions in the system. This is a direct consequence of using the common-motional modes as a bus system. Sparsely-connected superconducting qubits instead utilize SWAP gates or teleportation schemes to generate quantum entanglement between far-distant qubits which do not have direct connection between each other. While superconducting computers benefit from the ability to perform a variety of circuits, connectivity gives ion-trapped based platforms an edge in the early stages of quantum computing development, for now.
 

A setup in an ion trap lab (via Blatt lab)

But ions have their own challenges. It can take a long time to cool the ions in the trap, and these system require a lot more hands-on work that makes them more difficult to automate. Selectively performing two-qubit gates on specific and/or individual qubit pairs is a harder engineering challenge on ions than it is on superconducting qubits. There are various approaches being pursued to improve these gates, all with their own set of advantages and challenges, but all of them add an additional layer of complexity, with scalability and performance being a focus of current research. Gates can also take a few orders of magnitude more time to run than in superconducting quantum computers, a pain that can be felt especially in cases where we must perform many iterations on a parametrized quantum circuit communicating with classical computational resources. Building devices with a larger number of qubits can prove especially challenging.
 
There are proposals under development in order to surpass these scaling challenges, of course. One is the quantum charge-coupled device architecture proposal, where a micro-fabricated trap contains various separate trapping regions, including a region where loading happens, a region where computing happens, or a storage region, and the system shuffles ions around depending on how they're being used. Another proposal consists of multiple individual ion traps linked together via optical links, where photons mapped to qubit states can exchange information with ions in another trap. Each of these solutions come with their own considerations; quantum CCD requires many control knobs that all need to be stable themselves, and linking ion traps is (for now) a very inefficient process.

Petar in the lab (via Blatt lab)

But during my seven years researching ions (five as a Ph.D student, two as a postdoctoral researcher), I gained a lot of respect for this architecture that I've carried with me through my career. I think that ions made me really aware of the tiny effects that might not be a big deal now, but will be important to any quantum computing architecture as we scale up. After we made major improvements to our trapped ion setup, we suddenly could measure the elevator movements in our building, for example! And though I cherish my time with ions and I love teasing my fellow transmon-qubit researcher about how easy ion-qubits are made, I'm also very happy I don't have to spend time fiddling with lasers and optics anymore.
 
Quantum technology is certainly further along than ever before, and given the attention, both ion traps and superconducting qubits are advancing quickly. And while there's certainly some business competition, many in the fields have worked in both systems—and it's likely that the quantum ecosystem of the future will incorporate knowledge or hardware from both of these architectures.