Carl Byrd wrote:Will quantum computers replace everything, or will they serve a niche market in computing?
Hi Carl! Thanks for this question.
The best way to think about how a QPU (quantum processing unit) fits into the tech ecosystem is to think of how GPUs (graphics processing units) fit in.
GPUs are really good at some tasks, and they're certainly not a niche market (almost every computer, phone, and smartwatch has one). They also haven't replaced CPUs.
QPUs are expected to be much the same; they're great at some things you can't do with a normal computer.
The first dedicated GPU I used was an SGI Skywriter around 1991. It was about the size of a refrigerator, and less powerful than your phone today. I wrote software which ran on the CPU, using normal RAM, and used a graphics library to access the GPU's capabilities and its special display RAM.
A modern QPU is similarly large (larger, including the cooling system), and also accessed through a library or API.
I presume the heat loss, requiring a cooling system larger than we use at beer festivals, is to do with the mechanics of the machine rather than the smaller losses inevitable from execution speed or deletion of information.
We have an interest in information loss round here. Would a quantum device lose information the way a present‑day irreversible system does?
Hi Campbell, these are two good questions which I'll address individually.
Information loss in a modern QPU is primarily due to decoherence, which is where the quantum state representing your qubit (such as the spin of one single electron) cannot be isolated from the environment well enough, and reverts to an unknown state over time. On many present-day QPUs, this time is measured in picoseconds or nanoseconds. On a fault-tolerant QPU (one which uses many, many physical (unstable) qubits as the mathematical equivalent of a single stable qubit), this information loss can be prevented at the macroscopic (logical) level, even while it's present at the microscopic (physical) level.
Cooling: Different types of QPUs have different cooling needs. Quantum devices which use matter-based qubits which require isolation from the environment must be kept at temperatures around 0.02 Kelvin, which is close to absolute zero. To do this they use liquid helium and multiple cooling stages. Devices which use photons for transmission of qubits, or for actual computation, tend to need cooling only for their single-photon detectors. The qubits themselves live in fibers and waveguides, and are happy at room temperature.