Researchers at the University of California, Santa Barbara, have become the first to combine a quantum processor with memory that can be used to store instructions and data. This achievement in quantum computing replicates a similar milestone in conventional computer design from the 1940s.
Although quantum computing is now mostly a research subject, it holds out the promise of computers far more capable than those we use today. The power of quantum computers comes from their version of the most basic unit of computing, the bit. In a conventional computer, a bit can represent either 1 or 0 at any time. Thanks to the quirks of quantum mechanics, the equivalent in a quantum computer, a qubit, can represent both values at once. When qubits in such a “superposition” state work together, they can operate on exponentially more data than the same number of regular bits. As a result, quantum computers should be able to defeat encryption that is unbreakable in practice today and perform highly complex simulations.
Linking a processor and memory elements brings such applications closer, because it should make it more practical to control and program a quantum computer can perform, says Matteo Mariantoni, who led the project, which is part of a wider program at UCSB headed by John Martinis and Andrew Cleland.
The design the researchers adopted is known as the von Neumann architecture—named after John von Neumann, who pioneered the idea of making computers that combine processor and memory. Before the first von Neumann designs were built in the late 1940s, computers could be reprogrammed only by physically reconfiguring them. “Every single computer we use in our everyday lives is based on the von Neumann architecture, and we have created the quantum mechanical equivalent,” says Mariantoni.
The only quantum computing system available to buy—priced at $10 million—lacks memory and works like a pre-von Neumann computer.
Qubits can be made in a variety of ways, such as suspending ions or atoms in magnetic fields. The UCSB group used more conventional electrical circuits, albeit ones that must be cooled almost to absolute zero to make them superconducting and activate their quantum behavior. They can be fabricated by chip-making techniques used for conventional computers. Mariantoni says that using superconducting circuits allowed the team to place the qubits and memory elements close together on a single chip, which made possible the new von Neumann-inspired design.