In an earlier post I discussed how qubits are made using the Superconducting Qubit method. In this post I discuss the Trapped Ion Qubit method.
What Is a Trapped Ion Qubit?
A quantum computer needs a basic unit of information called a qubit (short for "quantum bit"). A classical bit is either 0 or 1. A qubit can also be in a combination of 0 and 1 at the same time, a property called superposition. When multiple qubits interact, they can become entangled, meaning the state of one depends on the state of another, no matter how far apart they are. These two properties, superposition and entanglement, give quantum computers their potential power.
A trapped ion qubit uses a single atom that has been stripped of one electron (making it a positively charged ion) as the qubit. The ion is held in place by electric fields inside a vacuum chamber and controlled with laser beams. Two specific energy levels inside the atom serve as the 0 and 1 states. Companies building quantum computers this way include IonQ, Quantinuum (Honeywell), Alpine Quantum Technologies and Oxford Ionics.
Why Use Atoms?
Every atom of the same type is identical. Every ytterbium-171 ion in the universe has exactly the same internal structure. There is nothing to manufacture; nature provides the qubit. The competing approach, superconducting circuits (used by IBM and Google), builds qubits out of tiny electrical components on a chip. These fabricated qubits always have small manufacturing differences that require individual tuning.
Atoms hold quantum information for a long time. A qubit is useful only as long as it maintains its quantum state, a property measured by "coherence time." Trapped ion qubits hold their state for 10 seconds to several minutes. Superconducting qubits typically last about 100 to 500 microseconds (millionths of a second), roughly 10,000 times shorter. Longer coherence time means more operations can be performed before errors accumulate.
Common ion species. The atoms most often used are ytterbium-171 (171Yb+, used by IonQ), barium-137 (137Ba+, used in newer IonQ systems), and calcium-40 (40Ca+, used by AQT and Oxford Ionics). Each species has different laser wavelength requirements, which affects the engineering complexity of the system.
How the Trap Works
A basic law of physics (Earnshaw's theorem) says you cannot hold a charged particle in place using only constant electric fields. Trapped ion systems get around this with a device called a Paul trap, which uses rapidly oscillating electric fields.
The Paul trap. Electrodes surrounding the ion produce a radiofrequency (RF) electric field that flips direction millions of times per second (typically 10 to 100 million times). The ion cannot keep up with the rapid switching, so it experiences an average force that pushes it toward the center of the trap. Additional constant-voltage electrodes keep the ion from drifting along the length of the trap. The result: the ion floats in empty space, about 50 to 100 micrometers (roughly the width of a human hair) above the electrode surface.
Modern trap chips. Early traps used hand-assembled metal rods. Today's systems use microfabricated chips, similar to computer chips, with all electrodes printed on a flat surface using standard semiconductor manufacturing. This allows many trapping zones on one chip, so ions can be moved around for different computations.
The vacuum. The ion must not collide with air molecules, which would knock it out of its quantum state or eject it from the trap. The chamber is pumped down to a pressure of about 10-11 torr, roughly one hundred-billionth of atmospheric pressure. At this pressure, a stray gas molecule would hit the ion only once every few hours. Achieving this requires baking the chamber at high temperature for days and using specialized pumps.
Controlling the Ion with Lasers
Lasers do all the work: cooling the ion down, setting its initial state, performing computations, and reading the result.
Cooling. After the ion is loaded into the trap, it vibrates too much for precise control. A laser tuned to a specific frequency repeatedly hits the ion with photons (particles of light) in a way that slows it down, similar to how throwing tennis balls at a moving shopping cart from the front would slow it. This "laser cooling" brings the ion nearly to a standstill, at a temperature below one-thousandth of a degree above absolute zero.
Setting the starting state. Before a computation begins, the qubit must be set to a known state (0). A technique called optical pumping uses a laser to drive the ion into the 0 state with over 99.9% reliability.
Performing operations (gates). To change a single qubit's state (a single-qubit gate), a pair of laser beams is aimed at the ion. By adjusting the laser frequency, intensity, and duration, the ion can be rotated between its 0 and 1 states in any desired proportion. These operations succeed better than 99.99% of the time.
To entangle two qubits (a two-qubit gate), laser beams are aimed at both ions simultaneously. The beams couple the ions' internal states through their shared vibration in the trap, temporarily exchanging energy through the motion and then returning the vibrational energy to its starting point while leaving the qubits entangled. This is called the Mølmer-Sørensen gate. It works between any pair of ions in the chain, giving trapped ions "all-to-all" connectivity. Superconducting qubits can only interact directly with their immediate neighbors on the chip. Two-qubit gate success rates are 99.5% to 99.9%.
Reading the result. A detection laser is aimed at the ion. If the ion is in state 0, it glows brightly (scattering thousands of photons). If it is in state 1, it stays dark. A camera or photon detector records which ions are bright and which are dark. This measurement takes 100 to 500 microseconds and is correct over 99.5% of the time.
The laser challenge. Each laser must be held at precisely the right frequency: a drift of just 10 kHz (ten thousand cycles per second out of trillions) can cause a computation to fail. A system with N qubits needs roughly 3 to 5 laser beams per qubit, each individually controlled. A 32-qubit system requires over 100 beams. Scaling to hundreds of qubits will require miniaturized optics built directly onto the trap chip.
How Trapped Ions Compare to Superconducting Qubits
Feature | Trapped Ion | Superconducting |
What is the qubit? | A single atom | A fabricated electrical circuit |
How long does it hold state? | 10 seconds to minutes | 100 to 500 microseconds |
Gate accuracy (1 qubit) | > 99.99% | > 99.9% |
Gate accuracy (2 qubit) | 99.5 to 99.9% | 99.0 to 99.5% |
How fast are gates? | 1 to 200 microseconds | 10 to 100 nanoseconds (1,000x faster) |
Which qubits can talk? | Any pair (all-to-all) | Only neighbors on the chip |
Biggest challenge | Laser complexity per qubit | Manufacturing consistency |
Scaling Up
Today's trapped ion processors have 20 to 56 qubits in a single chain. Adding more ions to the chain makes the system harder to control because the ions share an increasingly complex set of vibrational patterns. Beyond about 40 ions, gate operations slow down and lose accuracy.
Splitting into zones. The leading solution is the quantum charge-coupled device (QCCD) architecture. Instead of one long chain, the trap chip has multiple small zones (5 to 10 ions each) connected by transport channels. Voltage adjustments on the electrodes shuttle ions from one zone to another in 10 to 100 microseconds, with very high reliability (over 99.99% per shuttle). Quantinuum's H-series processors use this approach.
Connecting multiple chips. To reach thousands of qubits, separate trap chips must be linked together. The current method uses photons (light particles). An ion on one chip emits a photon that carries its quantum state through a fiber optic cable to another chip. When the photons from two chips are compared at a detector, the two distant ions become entangled. The success rate per attempt is low (about 0.01% to 1%), so this process must be repeated many times. Current linking speeds are 1 to 100 connections per second. For practical large-scale computing, this needs to reach thousands per second, a target for 2027 to 2030.
Where Errors Come From
Vibration noise from the trap surface. The electrode surfaces emit small, random electric fields that cause the ion to vibrate. This "motional heating" is the largest error source for two-qubit gates, because those gates work by coupling through the ion's motion. Heating rates depend strongly on how close the ion is to the surface. Cleaning or polishing the electrodes, or cooling the entire trap to very low temperatures (4 to 77 Kelvin), reduces this noise by 10 to 100 times.
Laser light hitting the wrong ion. Ions in a chain are only about 5 micrometers apart. When a laser targets one ion, a small amount of light spills onto its neighbors (called crosstalk). Careful beam focusing and pulse design reduce this to acceptable levels.
Toward error correction. Every physical qubit makes occasional errors. Quantum error correction uses groups of physical qubits to form a single "logical qubit" that can detect and fix its own errors, similar in concept to how classical computers use checksums. This only works if the physical error rate is low enough, typically below about 0.1%. Trapped ions are at or near this threshold. In 2023, Quantinuum demonstrated the first instance where a logical qubit made from trapped ions actually had a lower error rate than any of its physical components, a milestone for the field.
Current Systems (2024-2025)
• Quantinuum H2: 56 qubits, QCCD architecture, 99.8% two-qubit gate accuracy. First real-time error correction demonstrations.
• IonQ Forte Enterprise: 36 qubits, available through cloud services (AWS, Azure, GCP), 99.5% two-qubit gate accuracy.
• Alpine Quantum Technologies: 24 qubits in a compact, rack-mounted system designed for European deployment.
• Oxford Ionics: Developing microwave-based control (instead of lasers) to reduce optical complexity.
What Comes Next
Trapped ions have the highest gate accuracy and longest coherence times of any qubit technology. The tradeoff is slower gate speeds and the difficulty of managing many precision laser beams. The path to larger systems depends on three developments: miniaturized optics built into the trap chip to replace bulky laboratory lasers, faster photonic links to connect multiple trap modules, and better understanding of the surface noise that limits gate accuracy.
If these engineering problems are solved, systems of 1,000 to 10,000 qubits could be built within the next five years. Because trapped ion gates are already so accurate, fewer physical qubits are needed per error-corrected logical qubit compared to lower-fidelity platforms. That efficiency advantage becomes increasingly important as systems grow.
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