Quantum phenomena do not occur in a Hilbert space. They occur in a laboratory.
Being a theorist, it is easy to forget that physics is an empirical science. This is especially true for those of us working on quantum information. Quantum theory has been so thoroughly tested, that we have gotten into the habit of assuming our theoretical predictions must correspond to physical reality. If an experiment deviates from the theory, we look for technical flaws (and usually find them) before seeking an explanation outside the standard theory. Luckily, we have experimentalists who insist on testing our prediction.
Quantum computers are an extreme prediction of quantum theory. Those of us who expect to see working quantum computers at some point in the future, expect the theory to hold for fairly large systems undergoing complex dynamics. This is a reasonable expectation but it is not trivial. Our only way to convince ourselves that quantum theory holds at fairly large scales, is through experiment. Conversely, the most reasonable way to convince ourselves that the theory breaks down at some scale, is through experiment. Either way, the consequences are immense, either we build quantum computers or we make the most significant scientific discovery in decades.
Unfortunately, building quantum computers is very difficult.
There are many different routes towards quantum computers. The long and difficult roads, are those gearing towards universal quantum computers, i.e those that are at least as powerful as any other quantum computer. The (hopefully) shorter and less difficult roads are those aimed at specialized (or semi or sub-universal) quantum computers. These should outperform classical computers for some specialized tasks and allow a demonstration of quantum supremacy; empirical evidence that quantum mechanics does not break down at a fairly high level of complexity.
One of the difficulties in building quantum computers is optimizing the control sequences. In many cases we end up dealing with catch-22. In order to optimize the sequence we need to simulate the system; in order to simulate the system we need a quantum computer; in order to build a quantum computer we need to optimize the control sequence…..
Recently Jun Li and collaborators found a loophole. The optimization algorithm requires a simulation of the quantum system under the imperfect pulses. This type of simulation can be done efficiently on the same quantum processor. We can generate the imperfect pulse `perfectly’, on our processor and it can obviously simulate itself. In-fact, the task of optimizing pulses seems like a perfect candidate for demonstrating quantum supremacy.
I was lucky to be in the right place at the right time and be part of the group that implemented this idea on a 12-qubit processor. We showed that at the 12-qubit level, this method can outperform a fairly standard computer. It is not a demonstration of quantum supremacy yet, but it seems like a promising road towards this task. It is also a promising way to optimize control pulses.
As a theorist, I cannot see a good reason why quantum computers will not be a reality, but it is always nice to know that physical reality matches my expectations at least at the 12-qubit level.
P.S – A similar paper appeared on arXiv a few days after ours.
- Towards quantum supremacy: enhancing quantum control by bootstrapping a quantum processor – arXiv:1701.01198
- In situ upgrade of quantum simulators to universal computers – arXiv:1701.01723
- Realization of a Quantum Simulator Based Oracle Machine for Solving Quantum Optimal Control Problem – arXiv:1608.00677