In search of the quantum computer

A sidebar to the Bell Labs special report, Reviving an icon

Quantum mechanics is the study of the very small -- how matter and light behave on the atomic and subatomic scale. The science, however, contains a paradox. At the minutest level the behavior of matter can be described, but it cannot be measured. It’s not that scientists have not yet developed an instrument precise enough to measure phenomena on the subatomic scale -- behavior on the quantum level simply cannot be measured. At the quantum level, matter exists in an infinite number of states. Trying to measure any object’s current quantum state will reveal only one of millions of possible states, not the actual state in which it exists. The concept is almost impossible for the layman to imagine because it bears no resemblance to how we perceive the physical world. But scientists believe that this philosophical paradox can be applied to real-world applications.

Bell Labs’ director of theoretical physics research Steve Simon is one such researcher that believes quantum mechanics can be applied to building a quantum computer, a device that could theoretically run circles around the binary computers of today -- at least for solving certain types of computing problems. Just as in communications, the fundamental unit of information in computing is the bit. Shannon’s bit can only be assigned one of two values, a zero or one -- like a light switch, it’s either on or it’s off. But the basic unit of quantum information, the qubit, can have not only a value of one or zero but also what quantum physicists call a “superposition” of both values.

Simon said to imagine stopping one of today’s computers in mid-calculation: all of the bits would be in an “on” state or an “off” state. But if you were to stop a quantum computer in mid-calculation, the bits could be on, off or any state in between. Considering the much greater amount of possible information that could be carried in a qubit, a quantum computer could potentially have much greater power. Simon, however, stresses the word “potentially.”

Theoretical physics and computer science are only just beginning to understand the possible applications of a quantum computer, or even if one can actually be built. Experiments have demonstrated basic computational operations on a relatively small number of qubits, but a large-scale quantum computer is still elusive. If a quantum computer can be built, however, Simon and many of his peers believe it could perform some functions exponentially faster than a classic computer. A quantum computer, for instance, could sort through immense amounts of data very quickly allowing for the creation of gigantic databases.

Perhaps the quantum computer’s most significant use could be in the fields of security and cryptography. The encryption protecting most of the world’s data is difficult -- if not impossible -- to crack because they rely on long numerical keys. Decrypting the data is a matter of determining the factors of that numerical key. Unlike classical computers, quantum computers would excel at factoring, allowing them to render most traditional keys useless. But the inverse is also true. A quantum computer could be used to create codes that theoretically would be impossible to break.