These larger quantum superpositions are usually entangled, meaning that the measurements of the individual qubits will be correlated. Quantum entanglement can be thought of as an invisible wiring between particles that cannot be replicated in classical physics, a wiring that Einstein called “spooky action at a distance.” In our trapped-ion experiments, for example, each electrically levitated ion behaves like a microscopic bar magnet; the qubit states of 1 and 0 can correspond to two possible orientations of each atomic magnet (say, up and down). Laser cooling, which drains kinetic energy from atoms by scattering photons, brings the ions almost to rest within the trap. Because the ions reside in a vacuum chamber, they are isolated from the environment, yet the electric repulsion among them provides a strong interaction for producing entanglement. And laser beams thinner than a human hair can be targeted on individual atoms to manipulate and measure the data stored in the qubits.
Over the past few years scientists have performed many of the proof-of-principle experiments in quantum computing with trapped ions. Researchers have produced entangled states of up to eight qubits and have shown that these rudimentary computers can run simple algorithms. It appears straightforward (though technically very challenging) to scale up the trapped-ion approach to much larger numbers of qubits. Taking the lead from classical computers, this effort would involve sequencing a few types of quantum logic gates, each made up of only a few trapped ions. Scientists could adapt conventional error-correction techniques to the quantum world by using multiple ions to encode each qubit. Here the redundant encoding of information allows the system to tolerate errors, as long as they occur at a sufficiently low rate. In the end, a useful trapped-ion quantum computer would most likely entail the storage and manipulation of at least thousands of ions, trapped in complex arrays of electrodes on microscopic chips.
The first requirement for making a “universal” quantum computer—one that can perform all possible computations—is reliable memory. If we put a qubit in a superposition state of 0 and 1, with the ion’s magnetic orientation pointing up and down at the same time, it must remain in that state until the data are processed or measured. Researchers have long known that ions held in electromagnetic traps can act as very good qubit memory registers, with superposition lifetimes (also known as coherence times) exceeding 10 minutes. These relatively long lifetimes result from the extremely weak interaction between an ion and its surroundings.
The second essential ingredient for quantum computing is the ability to manipulate a single qubit. If the qubits are based on the magnetic orientation of a trapped ion, researchers can use oscillating magnetic fields, applied for a specified duration, to flip a qubit (changing it from 0 to 1, and vice versa) or to put it in a superposition state. Given the small distances between the trapped ions—typically a few millionths of a meter—it is difficult to localize the oscillating fields to an individual ion, which is important because we will often want to change one qubit’s orientation without changing that of its neighbors. We can solve this problem, however, by using laser beams that are focused on the particular qubit (or qubits) of interest.
The third basic requirement is the ability to devise at least one type of logic gate between qubits. It can take the same form as classical logic gates—the AND and OR gates that are the building blocks of conventional processors—but it must also act on the superposition states unique to qubits. A popular choice for a two-qubit logic gate is called a controlled not (CNOT) gate. Let us call the qubit inputs A and B. A is the control bit. If the value of A is 0, the CNOT gate leaves B unchanged; if A is 1, the gate flips B, changing its value from 0 to 1, and vice versa. This gate is also called a conditional logic gate, because the action taken on qubit input B (whether the bit is flipped or not) depends on the condition of qubit input A.