Science Success Story
Bridges Simulations Improve Quantum Approach to Computing
XSEDEenabled work establishes benchmark for beating classical computers, optimizes quantum performance
By Ken Chiacchia, Pittsburgh Supercomputing Center
Credit: Adobe Stock 
Quantum computers promise vastly expanded computing power. But they've yet to prove beyond a doubt that they're faster than classical computers for specific tasks. It isn't even completely clear how best to compare the two. Scientists from Carnegie Mellon University (CMU) and Intel Labs used the XSEDEallocated Bridges system at the Pittsburgh Supercomputing Center (PSC) to simulate a quantum computer, showing how to optimize a particular approach to quantum computing and benchmarking it successfully against classical computing. The work provides a signpost of both how to compare quantum and classical computers, and how to improve the quantum computer so it can win.
Why it's important
The jury is still out on quantum computing's realworld relevance. But there's a lot of promise.
Classical computers, which deal with data in bits of ones and zeroes, are incredibly powerful but have their limitations. Today, we're bumping up against the end of Moore's Law – not really a law, but an observation that, for a long time, we could double the computing power on a single chip about every two years. That hot streak in computer design gave us smartphones today that are as powerful as the first NSFfunded supercomputers in the 1980s. But there are a lot of signs that the era of Moore's Law is coming to a close. If we want to pack more computing power into the same space, we may need a completely different approach.
Quantum computers take advantage of the weird fuzziness of the quantum realm, where the tiniest objects can have two states at once – be two places at once – to varying degrees. Instead of computing in one or zero bits, they can compute in "qubits," which are both one and two at the same time, in varying degrees. Also, qubits can be entangled. That is, once coupled, two quantum particles will keep communicating with each other no matter how far apart they travel. That potentially makes quantum computers able to communicate and work across distant parts of the Earth or even farther. These features make quantum computers potentially far more powerful and able to tackle vastly more complex problems than classical computers.
"This paper is a study on this particular quantum algorithm – QAOA – trying to ask the question ‘What does this algorithm need in terms of quantum computing resources?' [as well as] design the algorithm such that it could be competitive with classical stateoftheart computing. That's the question that everyone is asking in this space: What are quantum computers good for [and] what are they going to be good for in five years and 10 years?"–Jason Larkin, SEI
The biggest "to do" in quantumcomputing research, though, is to demonstrate that a quantum computer can solve a given problem fundamentally faster than a classical computer. Google has claimed such "overwhelming quantum advantage" in one problem, but scientists at IBM and in China have questioned whether the quantum result was sufficiently better. If achieved, quantum supremacy could offer vast improvements in problems like managing data security; simulating realworld quantum chemistry and physics for designing drugs or advanced reactors; solving logistics problems such as airlineflight routing; and applications that scientists haven't even thought of yet.
Jason Larkin of CMU's Software Engineering Institute (SEI) and colleagues at CMU and Intel Labs wanted to better define the comparison of quantum and classical computers, as well as to build a simulated quantum computing framework that both allows testing on as many different types of problems as possible and maximally improves an eventual quantum computer's performance. To do this, they had to "build" a simulated quantum computer. They needed this simulation because current quantum computers are neither large enough nor is their error correction good enough yet to fully test a quantumversusclassical matchup. For this daunting task, they chose the XSEDEallocated Bridges advanced research computer at PSC.
How XSEDE helped
One reason why scientists are keen on developing practical quantum computers is that some important problems grow exponentially with the data – if you double the data, the problem takes four times as many calculations; triple the data, eight times; quadruple, 16 times. In theory, a quantum computer's complex handling of data and computations – which also grows exponentially with the number of qubits in the machine – could solve that kind of problem easily. (Readers can find a catalog of current quantum algorithms and their speedup over classical computers here.)
Simulating a quantum computer, though, also takes exponentially ballooning data as the number of simulated qubits grows. Bridges, with dozens of large and extremememory computing nodes that offered either three or 12 terabytes (3,000 or 12,000 gigabytes) of memory (RAM) each, was the perfect fit for simulating a small quantum computer. (By comparison, a typical laptop computer has 16 or 32 gigabytes of RAM.)
"Simulating quantum computers, at least the way we're doing it, you have to store this data structure. That's two to the number of qubits in size, and by the time you get to around 40 qubits, you're well into the terabyte range … Whatever is the max memory on a node, that kind of defines the biggest qubitsize job we can run. So [XSEDE] had these very nice nodes that have lots of memory and lots of RAM per node, and so we took advantage of that."–Jason Larkin, SEI
Larkin and the team used Bridges to test the quantum approximate optimization algorithm (QAOA), a hybrid quantumclassical set of instructions for running the computer that combines strong error correction – a must, with the fuzzy math of quantum computers – with the ability to run on both classical computers and, at larger scales, on future quantum computers. QAOA gave a strong performance running on Bridges. In particular, it produced approximate solutions to the maximum cut problem, useful in both designing electronic circuits and software validation and verification. Judging QAOA's performance versus classical computing will require more detailed study. The quantum algorithm's performance relative to the classical approach depended on the size of the problem and number of qubits involved, and the scientists would like to understand that relationship better. But their results tested the QAOA approach successfully, taught the team how to optimize it and set a benchmark for what a real quantum computer will need to do to demonstrate quantum supremacy. The team reported their results in the journal IOPscience in April 2022.
Next, the team would like to expand their understanding of QAOA's performance, giving it more rigorous tests against classical computing. The team is now using Bridges2, Bridges' XSEDEallocated successor system.
This work used XSEDE, which is supported by National Science Foundation grant number ACI 1548562. Specifically, it used the Bridges system, which was supported by NSF award number ACI1445606 at PSC.
At a Glance

Quantum computers promise vastly expanded computing power. But they've yet to prove beyond a doubt that they're faster than classical computers for specific tasks.

Scientists from Carnegie Mellon University (CMU) and Intel Labs used the XSEDEallocated Bridges system at the Pittsburgh Supercomputing Center to simulate a quantum computer, showing how to optimize a particular approach to quantum computing and benchmarking it successfully against classical computing.

The work provides a signpost of both how to compare quantum and classical computers, and how to improve the quantum computer so it can win.