I am curious about what are quantum computers actually doing today. I seem to only be able to find information online about what they could be used for in the future (i.e. breaking encryption, quantum annealing, simulations, etc.). However, it seems that these use cases are all theoretical and far off in the future. What are quantum computers actually being used for today? Is there any advantage to using a quantum computer over a classical computer with current quantum technology?
You are correct that the use cases are theoretical and possibly far in the future; today's quantum computers don't beat classical ones for useful tasks yet, and, for all intents and purposes, aren't being used for anything other than quantum computing research. They are both too noisy and too small to execute flagship algorithms like Shor's for factoring, and although there is research into NISQ (noisy intermediate-scale quantum) algorithms that make use of current or upcoming devices, whether they'll turn out to be useful still remains to be seen.
For the first question:
What are quantum computers actually being used for today?
- They have been used in proof-of-principle experiments of various kind. I can mention for instance for applications in chemistry with NISQ VQE and QAOAs.
- They are being used for simulating experiments that address the quantum-to-classical transition, for instance trying to prove quantum Darwinism.
- Time crystals are an example of present application of quantum computers where we have new physics being probed in these devices.
- At the level of hardware the mere workforce for constructing and understanding fault-tolerant quantum computation has lead to large number of hardware developments, so we can think as quantum computation also probing a variety of different physics from superconducting to linear-optics.
- They have also been used in studying how quantum memories affect metrology.
Now the second question regarding advantages for state-of-the-art quantum computers.
Is there any advantage to using a quantum computer over a classical computer with current quantum technology?
- There is computational advantage in simulating permanents and other figures of merit of such. This is an advantage that is grounded by complex theoretic results that are rigorous. If this can be made useful is still to be shown. So far, such a problem has no utility and serves only to show existence of high gaps in the computational power between classical and quantum systems.
- There are discoveries or proof-of-principle experiments that have been done and would have been much more challenging without having access to the noisy computers we have so far. Think that quantum computing is a different paradigm, that is not only about computation, but also about a new experimental paradigm for testing physics (opinion based last paragraph).
- We are very far from applications that use algorithms such as Shor's algorithms or, in general, provably useful life changing applications. But I would also not claim that quantum computers haven't done anything useful either. Advantages in metrology and interesting physics had been successfully tested in these devices and may have important impact.
- We are not capable of implementing most useful algorithms because they demand faul-tolerant quantum computers to work in its full glory.
Right now Google and IBM are using them to find solutions to logistical problems and knapsack-type problems. VQE or variational quantum eigensolvers are showing promise in making them more efficient than their classical counterparts. The VQEs normally have a classical part that is updated by the quantum part. It is considered a hybrid algorithm and blends quantum computing with classical computing. QAOAs and other hybrid near-term algorithms are showing promise while we work on getting cleaner noise-less qubits and more qubits in a quantum computer. Pennylane is also used for machine learning in chemical simulations. There is plenty of work being done with these computers. There will be so much more to do in the future too.
Other use cases:
This is an organization I built for some quantum code sharing. Contributions are appreciated.