In order to simulate larger and larger quantum systems and usher in an age of “quantum supremacy,” researchers are stretching the limits of today’s most advanced supercomputers. Just as classical computing systems have been instrumental in advancing their own forward progression, today’s fastest machines are helping pave the way for quantum computing breakthroughs, which will be revolutionary for applications in quantum chemistry, material science, machine learning, and cryptography.
A research team from ETH Zurich in Switzerland recently succeeded in simulating the largest quantum device yet — a 45-qubit circuit — using the Knights Landing-based Cori II machine at Lawrence Berkeley National Laboratory. Cori II has 9,304 compute nodes each outfitted with one 68-core Intel “Knights Landing” Xeon Phi 7250 processor, linked with the Cray Aries interconnect, with a total peak performance of 29.1 petaflops and 1 PB of aggregate memory.
Thomas Häner and Damian S. Steiger of the Institute for Theoretical Physics at ETH Zurich performed simulations of low-depth random quantum circuits, which were proposed by Google to demonstrate quantum supremacy. In the paper describing the work, the authors specify that “the execution of low-depth random quantum circuits is not scientifically useful on its own” but “running such circuits is of great use to calibrate, validate, and benchmark near-term quantum devices.”
“In order to make use of the full potential of systems featuring multi- and many-core processors, we use automatic code generation and optimization of compute kernels, which also enables performance portability,” they write.
To simulate the 45-qubit quantum circuit, Häner and Steiger used 8,192 Cori II nodes and a total of 0.5PB of memory, achieving an average 0.428 petaflops. In explaining the low performance in relation to peak FLOPS, the team refer to 1) heavy communication overhead (75 percent of circuit simulation time spent in communication) and state 2) kernel performance suffers where few kqubit gates are applied before a global-to-local swap needs to be performed (see section 4.1.2 for further analysis and discussion).
The research team says to the best of their knowledge, this 45-qubit quantum circuit simulation is the largest ever conducted. “Our highly-tuned kernels in combination with the reduced communication requirements allow an improvement in time-to-solution over state-of-the-art simulators by more than an order of magnitude at every scale,” they write.
The next step for the ETH Zurich team is to add more qubits to their simulation. Although 49-qubits is widely-held as the point at which quantum devices surpass the most capable traditional supercomputers and thwart larger simulations, the researchers have a plan to reach this threshold.
“While we do not carry out a classical simulation of 49 qubits, we provide numerical evidence that this may be possible,” the research team states. “Our optimizations allow reducing the number of communication steps required to simulate the entire circuit to just two all-to-alls, making it possible to use, e.g., solid-state drives if the available memory is less than the 8 petabytes required.”
Cori is the flagship resource of the DOE National Energy Research Scientific Computing Center (NERSC). The system was named in honor of the American biochemist Gerty Cori, the first American woman to win a Nobel Prize in science. Cori II is currently ranked number five on the (November 2016) Top500 list. The full system is comprised of two partitions: 2,004 Intel Xeon “Haswell” processor nodes and 9,300 Intel Xeon Phi “Knight’s Landing” nodes. According to its Top500 submission, the KNL partition (Cori II) has a peak performance of 27.9 petaflops and a measured Linpack score of 14 petaflops.
The 11-page paper, “0.5 Petabyte Simulation of a 45-Qubit Quantum Circuit,” is published on arXiv.org. There’s also a writeup of the research at MIT Technology Review.