Hi, my name is Sanchar Sharma

Every experiment ends in a measurement. So, if we want to unlock quantum applications using magnets, we need to answer a fundamental question: how can we measure the quantum states of magnons?
The first step in measuring any system, whether it’s magnetic or not, is to connect it to something we already know how to measure. For magnets, I turned to optical photons. Why? Because we already have incredibly advanced tools to measure photons with quantum precision. Then, the idea is simple: if we can transfer information from magnons to photons, and then measure those photons, we can fully characterize the quantum state of the magnons. The process of fully characterizing a quantum state is called quantum tomography.
So, can we transfer information from magnons to photons? That’s where this process called Brillouin light scattering comes in. When a photon strikes a magnet, it can either absorb a magnon and gain energy or create a magnon and lose energy. Either way, the photon’s frequency changes as it scatters, although not every photon will scatter. What’s useful for us is that the number of scattered photons depends strongly on the phase of the photons (remember that photons are waves, just like everything else in this universe) and the quantum state of the magnons. By varying the phase of the photons and monitoring how many of them scatter, we can gather a lot of information about the magnons’ quantum state.
The actual task in this project was two-fold. First, to derive the precise relationship between the number of scattered photons, their phase, and the quantum state of the magnons. Secondly, to develop an algorithm to process this relationship to reconstruct the quantum state of the magnons that best matches the observed optical data.

When you turn on your phone, it it takes a moment to boot up, loading your home screen and all your apps. This kind of initialization isn’t just a quirk of your device, but a universal feature shared by all devices, big or small, classical or quantum. For magnons to be useful in quantum applications, we need a way to initialize them into specific quantum states, on demand. But how do you actually do that?
In this work, I developed an algorithm that provides a set of operations needed to generate any arbitrary quantum state of magnons. Let’s see what the algorithm spits out for an example. Suppose you want to use magnons to sense extremely tiny magnetic fields. To do this, it turns out that they should be prepared in a strange quantum state: a superposition of having no magnons and having six magnons at the same time (let’s call this state 0+6). Simply injecting magnons into the system will get you to six magnons, but how do you create that superposition?
The algorithm offers a neat solution. First, you inject magnons one by one until you have three magnons in the system. Then, you couple these magnons to another system that can exchange information with them. If you tune this coupling just right, there’s a 50-50 chance the system will either create a magnon or absorb one. This leaves you with a superposition of 2+4 magnons. Repeat this process two more times, and you end up with the 0+6 state you wanted.