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.
The processor consists of two qubits linked by a quantum bus that enables them to communicate. Each is also connected to a memory element into which the qubit can save its current value for later use, serving the function of the RAM - for random access memory - of a conventional computer. The links between the qubits and the memory contain devices known as resonators, zigzagging circuits inside which a qubit’s value can live on for a short time.
Mariantoni’s group has used the new system to run an algorithm that is a kind of computational building block, called a Toffoli gate, which can be used to implement any conventional computer program. The team also used its design to perform a mathematical operation that underlies to the algorithm with which a quantum computer might crack complex data encryption.
David Schuster leads a group at the University of Chicago that also works on quantum computing, including superconducting circuits. He says that superconducting circuits have recently proved to be comparatively reliable. “One of the next big frontiers for these techniques now is scale,” he says. By replicating the Von Neumann architecture the UCSB team have expanded that frontier.
That’s not to say that quantum computers must all adopt that design, though, as conventional computers have. “You could make a computer completely out of qubits and it could do every kind of calculation,” says Schuster. However there are advantages to making use of resonators like those that make up the new design’s memory, he says. “Resonators are easier and more reliable to make than qubits and easier to control,” says Schuster.
Mariantoni agrees. “We can easily scale the number of these unit cells,” he says. “I believe that arrays of resonators will represent the future of quantum computing with integrated circuits.”