There are many ways to cook up a quantum computer.

Transistors—the key components that process and store information in modern computers—aren’t suitable for building quantum computers. Quantum devices need their own hardware for processing and storing quantum information.

Qubits are the currency of quantum information. To make any old qubit, you just need a physical system with two energy levels. A recipe for a good qubit includes a bit more. You’ll need:

energy levels that are stable and don’t shift around an unreasonable amount when it’s a little warm outside or a truck drives by;
control over which energy level the qubit inhabits—including placing the qubit into a superposition;
a way for qubits to interact with one another; and
a way to measure a qubit so that you can extract the result of a quantum computation.

Companies and research groups around the world are vying to see who can make the best qubits. Below we’ve chosen to highlight some of the most common approaches. They all have their advantages and disadvantages, and they all make use of very different kinds of quantum effects. We’ve focused on where the qubit “lives” inside of the different systems and how to perform the most basic manipulations.

Ion Trap Qubits

A qubit can be made from two internal energy levels of a charged atom, or ion, depicted here as different atomic orbitals. Metal electrodes, shown here as a gold chip, can trap multiple ions in a chain. Laser beams zap the ions to perform computational operations. In the simplest operation, a laser switches the ion qubit from the lower energy state (sphere) to a higher energy state (four-leaf clover). (Credit: Olena Shmahalo)

An ion qubit starts with a single charged atom trapped by electric fields. To be usable as a qubit, the ion must be cooled down close to zero temperature (and thus a low energy). Each qubit lives inside of a single ion’s internal energy structure. Focused laser beams prepare the qubit and manipulate it to perform computational operations. For example, lasers set the qubit into the lower energy state, higher energy state or any combination of the two. In the simplest case they cause the qubit to hop between states. Reading out the information from these qubits also involves flashing lasers onto the ions and capturing the fluorescent light that emerges with a camera.

Many different elements can serve as ion qubits, but the most popular ones are ytterbium, calcium, barium, and strontium. Depending on the element, the transitions can be between atomic orbitals or the richer set of states that emerge when taking into account the interactions between the atom’s nucleus and its electrons.

Superconductor Qubits

One way to make a qubit out of superconductors is to fabricate a small island of superconducting material and count the extra electric charge sitting on it. There will be a baseline amount of charge on the island that minimizes energy, and adding any extra charge will raise the energy. The low-energy state and the higher-energy state can form a qubit, and the two states can be manipulated with microwaves (shown here as a white pulse). In superconductors, electrons team up into composite particles called Cooper pairs (depicted here as a cloud with two orbs), so the difference between the low-energy state and the higher-energy state is really two extra charges. (Credit: Olena Shmahalo)

Qubits fashioned from superconductors rely on the quantum behavior of electrical circuits, which have quantum energy levels similar to atoms. In the simplest case, a qubit can be stored in the amount of electrical charge that’s sitting on a small island of superconducting material. The low energy state will have some baseline charge sitting on the island and a higher energy state will have a single extra charge. The island connects, via a narrow path, to another block of superconducting material. Pulses of microwave energy (in the form of photons) can coax the single extra charge to hop on and off the island—or leave it in a state of limbo as a superposition.

There are a variety of other ways to use superconductors to build qubits. The modern approaches (transmon qubits or fluxonium qubits) overcome some of the shortcomings of charge qubits, but at their heart they are a mashup of charge qubits and their kin.

Photon Polarization Qubits

A qubit made from photons lives in the polarization property of light. This is depicted here as individual glowing wavelets. Coated pieces of glass, such as the cube shown in the video, can separate out different polarization states of light. Other elements can switch the polarization from one state (vertical) to the other (horizontal). Photon qubits can also live in other properties, such as color. (Credit: Olena Shmahalo)

Photons are individual packets of electromagnetic energy, often dubbed the quantum particles of light. The simplest photon qubits live in the two polarization states of a single photon. Polarization characterizes the way a wave oscillates—e.g., either horizontally or vertically—and single photons, being the components of electromagnetic waves, can carry a polarization. Coated pieces of glass (not too different from prescription eyeglass lenses) can change the polarization, causing it to undergo transitions between the two states. Other kinds of lenses and optics are used to manipulate and measure a photon’s polarization.

The crux of using photons as qubits is to build a source of light that makes single photons. Scientists can configure atoms, different materials and even electrical circuits to emit one photon at a time. Qubits can also be stored in other photon properties, such as color.

Topological Qubits

Topological qubits are stored in collections of many particles, represented in the video by a sheet of small dots. By manipulating a small region of particles, local excitations called anyons can be created. Braiding these anyons around one another and then fusing them back together changes the state of the qubit. (The view on the right shows the full paths that the anyons trace out in time.) (Credit: Olena Shmahalo)

Topological qubits live inside collections of lots of other qubits. They encode quantum information across the entire collection in a way that is inherently resistant to small errors or disruptions. This makes them particularly promising for longer-term projects like building fault-tolerant quantum computers.

The topological qubits can be manipulated by using anyons, which are particles that can be created by adding energy to a handful of bare qubits. By creating anyons, braiding them around each other in space and time, and then fusing them back together, topological qubits can be flipped, rotated and even measured. The full process of creating, braiding and fusing anyons leaves the total collection of particles in a different state than it started, although it still looks exactly the same at any particular spot. This similarity is the reason that topological qubits are robust to local noise in a way that any of the individual constituent qubits could never manage.

Coming soon...

Neutral atoms!