Qubit by qubit

In theory, quantum computing has the potential to exponentially expand the possibilities of information processing. In practice, will it actually outperform a conventional, classical computer?

There’s only one way to find out: build a quantum computer and test it.

The task of building a quantum computer combines the exploration of fundamental quantum physics with the challenge of engineering new technologies. It’s the perfect scientific mix for Institute for Quantum Computing (IQC) researcher MATTEO MARIANTONI, also a professor in the Department of Physics and Astronomy.

Inside Mariantoni’s Laboratory for Digital Quantum Matter (DQM), superconducting quantum bits (or qubits, the basic unit of quantum information) are carefully studied in an effort to build an extensible quantum processor.

The advantage of superposition

Electronic circuits in classical computers are characterized by two states, 0 and 1. But superconducting qubits can be in a superposition of states, both 0 and 1 at the same time, thanks to quantum mechanics.

This ability gives qubits in superposition robust processing power. Here’s the wrinkle: it takes great care to maintain superposition. Quantum mechanical states are very fragile and interact easily with their environment. Interaction with the environment in the circuit eventually causes the qubit to decay and randomly transition from one state to another. As a result, qubits cannot store information for long. "To implement a universal quantum computer, we need to correct and remove those errors caused by interaction and decay," Mariantoni explained.

A logical qubit

In pursuit of quantum-error correction, Mariantoni and his team engineered the quantum socket. It’s a three-dimensional wiring technique that connects traditional electronics to quantum circuits using spring-loaded pins for individual qubits. The quantum socket can connect 100 up to 1,000 superconducting qubits in a group called a logical qubit. Grouping the qubits together like this reduces the effects of errors for individual qubits and aligns the error rate more closely to that of a classical computer.

Chip-on-chip bonding

Mariantoni’s team also developed a new technology to help qubits maintain a more stable quantum state. Using an etching technique, the team carved a network of tunnels into the surface of a silicon wafer. The tunnels are lined with metal and bonded above the superconducting quantum circuit. The tunnels shield qubits from environmental interferences like electromagnetic fields and increase the length of time data can be stored.

“We believe this approach will significantly improve our ability to control and measure a superconducting qubit,” noted Mariantoni. “Combined with the quantum socket, the chip-on-chip bonding technique builds the base of an extensible quantum computing architecture.” The DQM lab is paving the way towards the realization of a universal quantum computer, qubit by qubit.