The key to building a quantum computer is increasing the number of qubits that can be linked together. Despite the investment of vast resources over the past 20 years, the extreme fragility of existing qubits has so far restricted efforts to network them and has even fueled uncertainty about whether the technology will ever materialize.Good to see someone say the obvious -- 20 years and $100M of research fails, that is reason to doubt whether it is possible.
We have also had 20 years of quantum cryptography hype, and no useful application has come out of it. Another cryptographic technology, ECC pairings, has been hyped by the well-funded Voltage Security, and even proposed as an internet standard back in 2007. It had very little practical utility, and now new attacks have killed it. The company is still in business, fooling its customers.