A new circuit-wiring scheme developed over the last three years by RIKEN’s Superconducting Quantum Electronics Research Team, in collaboration with other institutes, opens the door to scaling up to 100 or more qubits within the next decade.
It is much harder to create a qubit than a conventional bit, and full control over the quantum-mechanical behavior of a circuit is needed. Scientists have come up with a few ways to do this with some reliability. At RIKEN, a superconducting circuit with an element called a Josephson junction is used to create a useful quantum-mechanical effect. In this way, qubits can now be produced reliably and repeatedly with nanofabrication techniques commonly used in the semiconductor industry.
The challenge of scalability arises from the fact that each qubit then needs wiring and connections that produce controls and readouts with minimal crosstalk. As the team moved past tiny two-by-two or four-by-four arrays of qubits, they have realized just how densely the associated wiring can be packed, and they have had to create better systems and fabrication methods to avoid getting our wires crossed, literally.
At RIKEN, they have built a four-by-four array of qubits using their own wiring scheme, where the connections to each qubit are made vertically from the backside of a chip, rather than a separate ‘flip chip’ interface used by other groups that brings the wiring pads to the edges of a quantum chip. This involves some sophisticated fabrication with a dense array of superconducting vias (electrical connections) through a silicon chip, but it should allow us to scale up to much larger devices. The team is working toward a 64-qubit device, which they hope to have within the next three years. This will be followed by a 100-qubit device in another five years as part of a nationally funded research program. This platform should ultimately allow up to a 1,000 qubits to be integrated on a single chip.
The other major challenge for quantum computers is how to deal with the intrinsic vulnerability of the qubits to fluctuations or noise from outside forces such as temperature, to maintain the quantum coherence. Now, by cooling quantum computers to cryogenic temperatures and creating several other environmental controls, they can maintain coherence for up to 100 microseconds. A few hundred microseconds would allow to perform a few thousand information processing operations, on average, before coherence is lost. (RIKEN)
