Spectrum, Spectra, Spectroscopy
In optics, a spectrum is a range of colors formed when a beam of white light is dispersed into a resultant rainbow. One of the common spectrums, the electromagnetic spectrum, shows different regions of light radiate different energies. It ranges from low-energy radio waves (long wavelength, low frequency) microwave, infrared, ultraviolet to high-energy X-rays and gamma rays (short wavelength, high frequency). In everyday life, we employ electromagnetic spectrum in a wide range of technologies, for example, microwave ovens, Wi-Fi, GPS, cellular communication, medical imaging, etc.
In a lab where scientists work on qubit control, they use electromagnetic spectrum, particularly microwaves, to manipulate qubits. The microwaves with resonant frequencies match the energy difference between the qubit’s ground and excited states. If we view a qubit as a spinning magnet, the resonant microwave pulse “pushes” the qubit and flips the qubit state back and forth between 0 and 1. During the resonance, the duration and amplitude of the microwave pulse help control the qubit probability in either the 0 (ground) or 1 (excited) state.
Researchers often run qubit spectroscopy which entails spectroscopic techniques to study the energy spectrum of a qubit system. Qubit spectroscopy sweeps the frequency of a microwave signal applied to the qubit. The “sweep” means gradually changing the frequency of the microwave signal to measure the resonant frequency. Since microwaves are not visible to human eyes, we utilize instruments such as network analyzers to obtain an estimated qubit frequency range. It can be a continuous linear sweep, a logarithmic sweep, or a sweep with certain frequency steps.
This is a plot of the spectrum of a qubit showing the spectral response of the qubit to a swept-frequency microwave pulse, and the peak is the resonant frequency of the qubit. Analyzing spectra (plural form of the spectrum) is a crucial step to understanding and controlling qubits systems. By studying the spectral responses, they could identify specific frequencies that trigger different energy levels of transitions. They use that spectral information to fine-tune the frequencies for higher accuracy.
It is spectra-cular to see how the analysis of the energy spectrum of individual qubits intertwines with pulse-level qubit control. I’m wrapping up this post with a photo taken a while ago.
