If I were a qubit
A qubit can exist in superposition states, both 0 and 1 simultaneously. If I were a qubit, my current state would be writing this post, eating my favorite snack, or … writing and eating at the same time! In a quantum system, the superposition property allows multiple possibilities until a measurement is made and it collapses into one of the classical states, 0 or 1. In a linear combination of writing and eating states, I would end up in one state when I decide to focus on writing to avoid my keyboard getting greasy with chips.
We often begin by understanding the qubit behavior before we manipulate qubits. Qubit spectroscopy is the experiment that we conduct to study the qubits, measure how qubits react to applied pulses, how the qubits interact with their environment, and more. If I were a qubit, running a qubit spectroscopy experiment would be like taking a personality test. (It’s time to get to know qu-self!) We run qubit spectroscopy in order to get qubit properties such as qubit energy levels, coherence times, coupling strengths, and noise characteristics. This information is not only crucial to ensure we have precise control to design and improve quantum circuits, but also help optimize parameters for the Rabi or Ramsey experiment later (e.g., qubit-drive frequency, qubit coherence time, etc.)
Next, we control the qubits and run some experiments based on the information we get from qubit spectroscopy. In superconducting qubits, we apply a microwave pulse to the transmon qubit (one of the common superconducting charge qubits) to control the quantum states. The microwave pulse is tuned to have a resonant frequency that matches the energy level difference between the ground state and the excited state. The qubit absorbs the energy from the resonant pulse and transitions from |0> to |1>. In this scenario, we run the Rabi experiment to measure that transition rate and how the qubit oscillates between the ground and excited state by applying a π pulse.
Another common experiment to characterize the properties of qubits is the Ramsey experiment. In the Ramsey experiment, we typically apply two π/2 pulses to measure the qubit coherence time. The first π/2 pulse gets the qubit to the superposition state, then we wait for a delay time and apply another π/2 pulse to get back to the ground state. The goal of this experiment is to learn how long the qubit can maintain in the superposition state. The longer the qubit decoherence time, the better the qubit can maintain its quantum state for a time, and it gives a larger window to perform quantum computations.
If I were a qubit, I would run Rabi and Ramsey experiments repetitively in order to get an accurate measurement of the qubit’s properties (The more personality tests you take, the better you know qu-self). We could run the experiments multiple times with different frequencies. Aside from tuning the frequency, we could adjust other pulse parameters such as amplitude, duration, or phase for calibration. We could determine how the qubit reacts when those properties of the pulse are changed or when the qubit is recalibrated each time. Similar to the cardiac system, every time a heart beats, it creates pulses to keep the blood circulating. We might not need a resonant pulse to control how our bodies function, but we could manipulate the heart pulse’s amplitude, frequency, or other pulse parameters to obtain different heart rates.
The whole process of controlling qubits involves iterative calibration steps and constant evaluation of readouts from the experiments, then moves on to randomized benchmarking. If I were a qubit, although I would be susceptible to errors, noises, and uncertainties, we just need a routine to keep recalibrating, tuning life parameters, and setting personal benchmarks until we become a better qu.
