The goal of quantum computing is to harness the peculiar behavior of particles at the quantum scale in order to perform calculations that aren’t believed to be feasible with conventional computers. An ordinary computer stores “bits” of information in transistors, which, like switches, can be configured in one of two states, represented by “1” or “0.” A quantum computer stores “qubits” of information in subatomic particles, such as electrons or photons, which can exist in state 1 or 0, or in a superposition of both states, and which can become entangled with one another, so that the state of one qubit decides the state of another.
Superposition and entanglement cause qubits to behave very differently from bits. Whereas a two-bit circuit in a conventional computer can be in only one of four possible states (0 and 0, 0 and 1, 1 and 0, or 1 and 1), a pair of qubits can be in a combination of all four. As the number of qubits in the circuit increases, the number of possible states, and thus the amount of information contained in the system, increases exponentially. A quantum computer with just a few hundred qubits would be able to solve certain problems more quickly than today’s supercomputers.
The only problem is that no one has managed to construct a quantum circuit with more qubits than you can count on both hands. Chris Lirakis, a physicist in the superconducting quantum computation group at IBM Research, explained that in order to keep the delicate entanglement of a system of qubits from collapsing, the system must be isolated and cooled to a temperature near absolute zero. At the same time, the qubits must be spaced about a centimeter apart to prevent an operation performed on one qubit from altering the states of neighboring ones. This challenge would make a thousand-qubit system far too large to fit into the kinds of refrigerators that can achieve such extreme cooling.
“There are a lot of really serious engineering challenges that need to be brought to bear in order to make the system scalable,” Lirakis said. “It’s this tug-of-war between all these different issues.”
Regev, who worked with Peikert in using quantum ideas to prove the security of lattice-based cryptosystems, says he hopes quantum computers will be built in his lifetime so he can see them in action. “But quantum has made such a great impact that even if quantum computers are never built, I wouldn’t care too much,” he said.
As quantum techniques become more popular among computer scientists, they will likely produce more classical results. “It’s these results that convince me that even if the universe had not been quantum mechanical,” Aaronson said, “eventually computer scientists would have invented quantum computing as a proof tool.”
Reprinted with permission from Simons Science News, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the computational, physical and life sciences.