A new five-qubit array from UCSB’s Martinis Group is on the threshold of making a quantum computer technologically feasible to build
“Even the best state-of-the-art [quantum computer] 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.”
a, Optical image of the integrated Josephson quantum processor, consisting of aluminium (dark) on sapphire (light). The five cross-shaped devices (Q0–Q4) are the Xmon variant of the transmon qubits30, placed in a linear array. To the left of the qubits are five meandering coplanar waveguide resonators used for individual state readout. Control wiring is brought in from the contact pads at the edge of the chip, ending at the right of the qubits. b, Circuit diagram. Our architecture uses direct, nearest-neighbour coupling of the qubits (red/orange), made possible by the nodal connectivity of the Xmon qubit. Using a single readout line, each qubit can be measured using frequency-domain multiplexing (blue). Individual qubits are driven through capacitively coupled microwave control lines (XY), and frequency control is achieved through inductively coupled d.c. lines (Z) (violet). c, Schematic representation of an entangling operation using a controlled-phase gate with unitary representation UCZ; (I) qubits at rest, at distinct frequencies with minimal interaction; (II) when brought near resonance, the state-dependent frequency shift brings about a rotation conditional on the qubit states; (III) qubits are returned to their rest frequency.
Nature – Superconducting quantum circuits at the surface code threshold for fault tolerance
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.”
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 correction6 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 state8, 9 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.
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