Two physicists at Boston University in Massachusetts, one of whom (Lev Levitin) was an early researcher in quantum computing, have established an upper bound on the speed of computing devices. Thanks to reader Paul Adams for the pointer to this article discussing the development
“No system can overcome that limit. It doesn’t depend on the physical nature of the system or how it’s implemented, what algorithm you use for computation … any choice of hardware and software,” Levitin said. “This bound poses an absolute law of nature, just like the speed of light.”
…In the early 1980s, Levitin singled out a quantum elementary operation, the most basic task a quantum computer could carry out. In a paper published today in the journal Physical Review Letters, Levitin and Toffoli present an equation for the minimum sliver of time it takes for this elementary operation to occur. This establishes the speed limit for all possible computers.
You can read the abstract at Phys. Rev. Lett. here (membership required for full download)
How fast a quantum state can evolve has attracted considerable attention in connection with quantum measurement and information processing. A lower bound on the orthogonalization time, based on the energy spread ΔE, was found by Mandelstam and Tamm. Another bound, based on the average energy E, was established by Margolus and Levitin. The bounds coincide and can be attained by certain initial states if ΔE=E. Yet, the problem remained open when ΔE≠E. We consider the unified bound that involves both ΔE and E. We prove that there exist no initial states that saturate the bound if ΔE≠E. However, the bound remains tight: for any values of ΔE and E, there exists a one-parameter family of initial states that can approach the bound arbitrarily close when the parameter approaches its limit. These results establish the fundamental limit of the operation rate of any information processing system.
Emphasis mine. You can get free access to what looks like an earlier version of the letter from Arxiv here.