Someday quantum computing will help to solve industry-critical problems in the design of molecules and materials. How close are we to achieving this milestone and what problems must be solved to reach it? In the past half-decade, substantial work has gone into the development of near-term quantum algorithms for simulating molecules and materials. These include the variational quantum eigen solver (VQE) and many of its variants. However, for these algorithms to be useful, they need to overcome several critical barriers.
Two of these barriers are 1) the impractically large number of quantum measurements demanded by these algorithms and 2) the difficulty in preparing high-quality approximations of the simulated system’s ground state using limited circuit depth. The first challenge has been addressed in recent work at Zapata and ongoing research is exploring even more solutions to it. The second challenge has also been addressed by previous work at Zapata. However, it remains to find a scalable and reliable solution to this challenge.
An alternative approach to the ground state preparation problem is to use more-traditional quantum algorithms, like quantum phase estimation (QPE). Unfortunately, these traditional quantum algorithms require circuits that are too deep for implementation on near-term quantum devices. Is there a more intermediate strategy? What quantum algorithms might be reliable, while still limiting the depth of the quantum circuits that they use? Might such a technique bring quantum advantage closer to the present?
We introduce the method of state preparation boosting, which uses a limited-depth quantum circuit to reliably approximate the ground state. This circuit, which we call a booster, converts circuit depth into a better approximation of the ground state in a controllable manner.
The method may be used as a stand-alone ground state preparation algorithm or it can be used in tandem with other quantum algorithms, like VQE or QPE, to boost their performance. Accordingly, we expect that this technique will continue to be useful into the era of large-scale fault-tolerant quantum computing. If early fault-tolerant quantum computers of the next decade will be used to solve industry critical problems, then the tool of boosters may help to achieve this milestone even sooner.