John Martinis and his team at UC Santa Barbara will join Google in this initiative. John and his group have made great strides in building superconducting quantum electronic components of very high fidelity. John was recently awarded the London Prize recognizing him for his pioneering advances in quantum control and quantum information processing.
Superconducting circuits at the surface code threshold for fault tolerance
The superconducting quantum circuit with five Xmon qubits (cross-shaped devices) placed in a linear array. The quantum device shows logic gates with fidelities at the surface code threshold for fault tolerance. (Photo credit: Erik Lucero.)
With an integrated hardware group the Quantum AI team will now be able to implement and test new designs for quantum optimization and inference processors based on recent theoretical insights as well as our learnings from the D-Wave quantum annealing architecture. We will continue to collaborate with D-Wave scientists and to experiment with the “Vesuvius” machine at NASA Ames which will be upgraded to a 1000 qubit “Washington” processor.
Abstract on fault tolerance hardware
A quantum computer can solve hard problems, such as prime factoring, database searching and quantum simulation, at the cost of needing to protect fragile quantum states from error. Quantum error correction provides this protection by distributing a logical state among many physical quantum bits (qubits) by means of quantum entanglement. Superconductivity is a useful phenomenon in this regard, because it allows the construction of large quantum circuits and is compatible with microfabrication. For superconducting qubits, the surface code approach to quantum computing is a natural choice for error correction, because it uses only nearest-neighbour coupling and rapidly cycled entangling gates. The gate fidelity requirements are modest: the per-step fidelity threshold is only about 99 per cent. Here we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92 per cent and a two-qubit gate fidelity of up to 99.4 per cent. This places Josephson quantum computing at the fault-tolerance threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbour coupling. As a further demonstration, we construct a five-qubit Greenberger–Horne–Zeilinger state using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.
Previous announcement from UCSB about their fault tolerance
A fully functional quantum computer is one of the holy grails of physics. Unlike conventional computers, the quantum version uses qubits (quantum bits), which make direct use of the multiple states of quantum phenomena. When realized, a quantum computer will be millions of times more powerful at certain computations than today’s supercomputers.
A group of UC Santa Barbara physicists has moved one step closer to making a quantum computer a reality by demonstrating a new level of reliability in a five-qubit array. Their findings appeared in the journal Nature.
Quantum computing is anything but simple. It relies on aspects of quantum mechanics such as superposition. This notion holds that any physical object, such as an atom or electron — what quantum computers use to store information — can exist in all of its theoretical states simultaneously. This could take parallel computing to new heights.
“Quantum hardware is very, very unreliable compared to classical hardware,” says Austin Fowler, a staff scientist in the physics department, whose theoretical work inspired the experiments of the Martinis Group. “Even the best state-of-the-art hardware is unreliable. Our paper shows that for the first time reliability has been reached.”
While the Martinis Group has shown logic operations at the threshold, the array must operate below the threshold to provide an acceptable margin of error. “Qubits are faulty, so error correction is necessary,” said graduate student and co-lead author Julian Kelly who worked on the five-qubit array.
“We need to improve and we would like to scale up to larger systems,” said lead author Rami Barends, a postdoctoral fellow with the group. “The intrinsic physics of control and coupling won’t have to change but the engineering around it is going to be a big challenge.”
The unique configuration of the group’s array results from the flexibility of geometry at the superconductive level, which allowed the scientists to create cross-shaped qubits they named Xmons. Superconductivity results when certain materials are cooled to a critical level that removes electrical resistance and eliminates magnetic fields. The team chose to place five Xmons in a single row, with each qubit talking to its nearest neighbor, a simple but effective arrangement.
“Motivated by theoretical work, we started really thinking seriously about what we had to do to move forward,” said John Martinis, a professor in UCSB’s Department of Physics. “It took us a while to figure out how simple it was, and simple, in the end, was really the best.”
“If you want to build a quantum computer, you need a two-dimensional array of such qubits, and the error rate should be below 1 percent,” said Fowler. “If we can get one order of magnitude lower — in the area of 10^-3 or 1 in 1,000 for all our gates — our qubits could become commercially viable. But there are more issues that need to be solved. There are more frequencies to worry about and it’s certainly true that it’s more complex. However, the physics is no different.”
According to Martinis, it was Fowler’s surface code that pointed the way, providing an architecture to put the qubits together in a certain way. “All of a sudden, we knew exactly what it was we wanted to build because of the surface code,” Martinis said. “It took a lot of hard work to figure out how to piece the qubits together and control them properly. The amazing thing is that all of our hopes of how well it would work came true.”