Ask HN: How are quantum circuits constructed on cloud-based quantum computers?

[ColinEberhardt]

I've been trying to wrap my head around quantum computing, with mixed success. I have a reasonable understanding of how quantum circuits work and the basic principles. However, there is one big missing piece in my understanding, which I've not been able to fill be googling ...

You can use a tool like qiskit to create a quantum circuit, then send it to a cloud quantum computer. How does this circuit get constructed?

With a digital computer, it would either have to be assembled manually, or simulated. Neither seem practical.

Are quantum circuits assembled by tiny robots? (please say they are!)

[ThePhysicist]

Here's some code I wrote 12 years ago, which implements Grover's algorithm on a superconducting quantum processor for two qubits [1]. This is not using a quantum algorithm compiler (which didn't exist at the time), all pulses are handcrafted. In principle it's rather simple, for each operation you want to perform you add the necessary pulse sequence, which can be either microwave pulses on each of the qubits (which drive the qubits along the x- or y-axis of the Bloch sphere), or voltage pulses in the qubit fluxlines (which drive the qubits along the z-axis of the Bloch sphere and/or put them in resonance with each other so they can interact). Finally, you would add readout pulses to read the qubit state at the end of the algorithm. In the case above I added some more pulses to perform quantum state tomography, i.e. repeat the sequence many times and rotate the final qubit states around different axes before measuring them, which allows to statistically reconstruct the quantum state of the qubits at the end of the algorithm. For a real algorithm run you wouldn't do that but instead directly measure the qubits (which I also did), which gives you a two-bit output value (00, 01, 10, 11).

The pulse sequences get loaded into arbitrary waveform generators and many other parameters are set like microwave generator frequencies. Typically you also perform a tune-up of the system to determine the correct timings and qubit frequencies (as all microwave components and the qubits themselves tend to slightly drift over time).

Today it's much more complicated as you can optimize each gate sequence individually and in conjunction to get higher fidelity, but in principle you can build up any algorithm uses these basic blocks above.

1: https://github.com/adewes/python-qubit-setup/blob/master/scr...