Google pulled ahead of the pack this week in the race toward “quantum supremacy,” with the introduction of a new 72-qubit quantum processor called Bristlecone. The gate-based superconducting system will be instrumental in Google’s efforts to demonstrate the first quantum technology to show advantage over today’s leading classical machines.
Bristlecone (so nicknamed due to the qubits being arranged in a pinecone pattern) will be employed as a testbed for researching system error rates and boosting scalability of Google’s qubit technology and for exploring near-term applications in quantum simulation, optimization, and machine learning.
The Mountain View, Calif.-based research team created Bristlecone by scaling up its previous 9-qubit array. Announced by the Google team last year, that (smaller) system “demonstrated low error rates for readout (1%), single-qubit gates (0.1%) and most importantly two-qubit gates (0.6%),” according to Julian Kelly, Research Scientist, Quantum AI Lab, who shared details of the new processor in a blog post.
“This device uses the same scheme for coupling, control, and readout, but is scaled to a square array of 72 qubits,” stated Kelly. “We chose a device of this size to be able to demonstrate quantum supremacy in the future, investigate first and second order error-correction using the surface code, and to facilitate quantum algorithm development on actual hardware.”
Google’s goal is to achieve similar performance to the best error rates of the 9-qubit device, but do so across all 72 qubits of Bristlecone. It’s essentially a recipe for achieving quantum supremacy, which according to the Google team’s calculations “can be comfortably demonstrated with 49 qubits, a circuit depth exceeding 40, and a two-qubit error below 0.5%.”
Circuit depth refers to how many logical operations a quantum processor can carry out before coherence decays. To evaluate its progress, the team has created a benchmarking tool that assigns a single system error by applying random quantum circuits to the device and checks the sampled output distribution against a classical simulation.
“We’re just starting testing,” physicist John Martinis of Google and the University of California, Santa Barbara told Science News. “From what we know so far, we’re very optimistic.” He added that the quantum supremacy demonstration could happen within a few months if everything goes well.
The concept of quantum supremacy is an important milestone, although it says little about the computer’s real-world usefulness. The initial threshold marks the point at which a quantum machine outperforms a classical supercomputer on a well-defined computational task. Building a broadly useful quantum computer is likely to require tens of thousands (or millions) of physical qubits and is posited to be a decade away. The chart above, from Google, depicts the massive scaling challenge to get from quantum supremacy to a useful error corrected quantum computer, a jump of more than four orders of magnitude in qubits and a two order magnitude reduction in error rate.
Google is not the only company working to usher in the quantum era. IBM announced a 50-qubit prototype in November and is also making a 20-qubit commercial system available through its cloud computing platform. Intel, Microsoft and Berkeley, Calif.-based startup Rigetti are also working on quantum technologies, as is D-Wave, which sold the first quantum annealing machine in 2011.