Connecting a quantum processor with quantum memory could make it possible to perform complex calculations that are far beyond the power of conventional computers. 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.
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.
When chilled almost to absolute zero, this chip becomes a quantum computer that includes both a processor (the two black squares) and memory (the snaking lines on either side). Credit: Erik Lucero
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.
Mark 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.”