Illustration: Bryan Christie Design

Click Here to learn the basics of a Qubit and Quantum Entanglement

Since then, physicists have come up with at least half a dozen potential ways to do quantum computation—including using the atomic nuclei in dissolved organic compounds as qubits and manipulating electrons within superconducting loops. With few exceptions, though, these schemes will never lead to a quantum computer that can solve a useful problem, because they simply can't handle more than a dozen or so qubits, and what's needed are hundreds—if not thousands.

We can't construct a full-scale ion trap big enough to house that many qubits. So the only way we can see to build a practical quantum computer is to borrow a page from the electronics industry and build the equivalent of quantum integrated circuits. The analogy here is to transistors—traps work pretty much the same way if you shrink them down enough and put many of them on the same piece of semiconductor. That was demonstrated just last year when our research group at the University of Michigan and Wineland's group at NIST independently produced the first ion-trap microchips built with the same techniques that microprocessor and MEMS makers employ. These chips are far from being useful computers themselves, but they are the first step in a path that could take us beyond the limits of computing as we know it.

The heart of any quantum computer, whether it's built on a sliver of semiconductor or not, is the qubit. A word about the qubit: it's odd.

In an ordinary computer, information is stored as bits, usually a minuscule reservoir of charge or the charge's absence in a memory cell's capacitor. At any given instant, an ordinary binary digit can be in one and only one of two different states. But the value of a qubit is determined by the quantum states of individual particles. So, like those quantum states, a qubit can have the value 1, or 0, or it can be—in the paradoxical world of the quantum—both values at the same time. This versatility is central to the power of quantum computers. In an ordinary computer you can represent a number between 0 and 31 using five binary digits. But using the same number of qubits you could represent all 32 numbers at once and perform the same calculation on them simultaneously. And that's not even the end of the weirdness: two or more qubits can be linked together in ways no two transistors could ever be, influencing each other instantaneously—even if they are separated by a distance of light-years.

The specific quantum state of a particle that is generally exploited to determine a qubit's value is called spin. In an ion-trap computer as well as several other schemes, the value of a qubit is determined by the direction of a particle's spin state.

Spin is a measure of a particle's angular momentum. Angular momentum is easy to understand for large spinning objects like a basketball, but photons, electrons, and other fundamental particles that make good qubits are as close as you can get to being dimension less points in space. The question is, How can they spin?

They don't. Like many aspects of quantum mechanics, spin makes no intuitive sense—even to physicists. But it's real, and it's something measurable. For a particle, spin is an intrinsic property like charge, not something that comes about because of physical rotation.

Spin has direction—up or down, in quantum computing's shorthand—and it's the direction we use to represent the value of the qubit. The qubits used in ion-trap quantum computers rely on the spin state of an ion's outermost electron and that of its nucleus. If the electron's spin is aligned with that of the nucleus it orbits, we say the qubit is in the 1 state. If the two quantum states are pointing in opposite directions, we say the qubit is in the 0state. And the qubit ion can be put in a combination of 1 and 0 if the electron's spin is itself a combination of up and down.

This ability of a qubit to have two values simultaneously is called the principle of superposition, and it allows a register of qubits to hold exponentially more information than the register with the same number of classical bits. For two ordinary bits, for example, the possible combinations are 00, 01, 10, or 11. But for qubits in a state of superposition, their values could be all four of those numbers at the same time.