A Shortcut to a Ground State
If you spot a friend at the other end of a bustling cocktail party, you can reach him without bumping into people by slowly weaving your way through your fellow guests. If you’re in a hurry, you can still reach him, provided you anticipate how each of the intervening bodies will move. Likewise, you can shift a quantum system from one of its ground states to another, more interesting one, by driving the system slowly. But if you’re in a hurry—if, for example, you’re stringing together a sequence of qubit manipulations—you need to anticipate and mitigate the ways in which such quick driving might excite the system to an unwanted higher state. Such “counterdiabatic driving” has been demonstrated for specific, modestly sized systems whose parameters are well known. Now two groups—Stewart Morawetz and Anatoli Polkovnikov of Boston University and Jernej Finžgar of the Technical University of Munich and his collaborators—have independently devised a recipe for counterdiabatically driving a generic quantum system while requiring less knowledge about its microscopic details [1, 2].
The confounding excitations that must be mitigated are described by the so-called adiabatic gauge potential (AGP). Exact knowledge of the AGP is often unattainable, but it can be approximated. The new recipe begins with a guess at the AGP. This guess can be expressed as a series of operators that live in a mathematical entity known as a Krylov subspace. In principle, these operators can be realized in the lab. For example, for an atomic system, the operators could be equivalent to sequences of laser pulses applied to the atom. To approximate the AGP, only the first few terms in the series are needed. The two groups realized that finding those terms reduces to the simpler problem of fitting a function with a polynomial. The researchers went on to show that, surprisingly, obtaining the fit is tied to how well the system absorbs energy when being zapped: The less absorbent a system is, the easier it is to control with the new recipe.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- S. Morawetz and A. Polkovnikov, “Universal counterdiabatic driving in Krylov space,” PRX Quantum 6, 040320 (2025).
- J. R. Finžgar et al., “Counterdiabatic driving with performance guarantees,” Phys. Rev. Lett. 135, 180602 (2025).