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Quantum computing winter

The Quantum computing winter was the period from 1995 to approximately October 2031 when experimental progress on the creation of fault tolerant quantum computers stalled despite significant effort at constructing the machines. The era ended with the publication of the Kitaev-Kalai-Alicki-Preskill (KKAP) theorem in early 2030 which purported to show that the construction of fault-tolerant quantum computers was in fact impossible due to fundamental constraints. The theorem was not widely accepted until experiments performed by Mikhail Lukin's group in early 2031 verified the bounds provided in the KKAP theorem.

Early history

Quantum computing technology looked promising in the late 20th and early 21st century due to the celebrated Fault Tolerance theorems, as well as the rapid experimental progress towards satisfying the fault tolerance threshold. The Fault Tolerance theorem, which at the time was thought to be based on reasonable assumptions, guaranteed scalable, fault tolerant quantum computation could be performed--provided an architecture could be built that had an error rate smaller than a known bound.

In the early 2010s, superconducting qubit architectures designed by John Martinis' group at Google, and then HYPER Inc., looked poised to satisfy the threshold theorems, and considerable work was done to build scaled architectures with many millions of physical qubits by the mid 2020s.

However, despite what seemed to be guarantees via threshold theorems for their architectures, the Martinis group was never able to report large concurrences for more than 12 (disputed) logical qubits.

The scalability wall

Parallel to the development of the scalable, silicon architectures, many groups continued work on other traditional schemes like neutral atoms, trapped ions, and Nuclear Magnetic Resonance (NMR) based devices. These devices, in turn, ran into the now named Scalability Wall of 12 (disputed) entangled encoded qubits. For a discussion on the difference between encoded and physical qubits, see the discussion in Quantum error correction.

The Martinis group hoped that polishing their hardware, and scaling the size of their error correction schemes would allow them to surpass the limit, but progress stalled for more than a decade.

Correlated noise catastrophe

Alexei Kitaev, building on earlier work by Gil Kalai, Robert Alicki, and John Preskill published a series of papers in the late 2020s, culminating in the 2030 theorem now known as the KKAP Theorem, or the Noise Catastrophe Theorem. This proof traced how fundamental limits on the noise experienced by quantum mechanical objects irretrievably destroys the controllability of quantum systems beyond only a few qubits. Uncontrollable correlations were shown to arise in any realistic noise model, essentially disproving the possibility of large scale quantum computation.

Aftermath (This section has been marked as controversial, see the talk page for details)

The immediate aftermath of the publication of the proof was disbelief. Almost all indications pointed towards scalable quantum computation being possible, and that only engineering problems stood in the way of truly scalable quantum computation. The Nobel Prize (2061) winning work of Mikhail Lukin's team at Harvard only reinforced the shock felt by the Quantum Information community when the bounds provided in the KKAP Theorem's proof were explicitly saturated via cold atom experiments. Funding in quantum information science rapidly dwindled in the following years, and the field of Quantum Information was nearly abandoned. The field has since been reinvigorated by Kitaev's recent proof of the possibility of Quantum Gravitational computers in 2061.