Hi! My name is Sophie Hermans and I am a master student in the group of Ronald Hanson. I have started my MSc project about five months ago in the ‘cavity team’. In this blog post I will take you along and show you what I do on a regular day at QuTech.
08:30 Arriving at the Applied Physics building
My day starts at half past eight. I hang my coat, turn on my computer and say good morning to the others in the student room, well… the ones who are already in. In the student room there are eight students, both master and bachelor students from different groups within QuTech. The second thing I do is get a cup of coffee. That is one of the advantages of doing your project at QuTech; coffee made from freshly ground beans. Yeah, this coffee is much better than what comes out of the machines near the lecture halls.
Now we are all set to go, I will give a short introduction on what my master’s project entails. In the Hanson Lab we are looking at nitrogen-vacancy (NV) centres in diamond. The spin states of the NV centre can be used as a qubit, that can be entangled over a large distance. To do this the NV centres need to emit resonant photons. However, the probability that the NV centre emits such a photon is very small, only ~3%. This makes the entanglement rate low: only one per hour over a distance of 1.3 km. In the cavity team we are trying to increase this fraction of resonant photons by placing the diamond membrane with the NV centre in a ….. cavity! Our optical cavity consists of two tiny mirrors spaced only a few micrometers apart. A human hair would not fit in the cavity.
09:00 Getting a helium vessel
At the moment I am working on finding an appropriate sample that can be used in the cavity set-up. To be used in the cavity, we have to make a very thin diamond membrane. We do this in the cleanroom, where we use an etching method to make a thin membrane out of a bigger piece. I don’t do this myself. This work is done by two of the PhD students in our group, Max and Stefan. Apart from the thickness we have another requirement, related to the characteristics of the NV centres. The wavelength of the resonant photons should not fluctuate too much, less than 1 gigahertz. This feature only shows up at low temperature. In order to measure these fluctuations we must cool down the set-up to 8 K (or -265 oC).
The cooling down is done with liquid helium. QuTech has a facility, which can make liquid Helium out of helium gas. The only thing we have to do is to bring a vessel down to our lab in the basement, connect it to our set-up and wait. The liquid Helium flows in our cryostat and as it evaporates it cools down the diamond. The residual Helium gas is transported back to the liquefier and can be recycled. After approximately 4 hours the set-up is at the desired temperature.
While the set-up is cooling down, I have some time to read scientific papers. It is needless to say that I need to read literature as part of my thesis. But besides this, in the group we read a paper once a week collectively. The subject of the papers ranges from results of other groups who work on similar topics to something that is not directly related to our work but is still interesting. In the time of one hour, we discuss the paper and see whether we can learn something from it for our experiments.
12:00 Lunch time!
And of course, lunch! Every day at 12 o’clock Team Diamond gathers next to the coffee machine for lunch. During lunch we discuss our work, but also (most of the time even) weekend plans, news facts and the terrible food of Sodexo.
ONE DAY I WORK ON THE THEORY AND THE NEXT DAY I AM TROUBLESHOOTING IN THE LAB!
12:30 Back to the lab to do measurements
To find the NV centres in the diamond, we make confocal microscopy images of the membrane. In this way we can identify the NV centres; they light up as bright spots in these images. This sounds easy, but this is only true when all the optical elements in the set-up are well-aligned. This alignment may vary slightly day by day. A set-up can consist of more than 50 elements. Fortunately, I do not have to realign all these elements every day. But before doing a measurement I should check the alignment of one or two elements and maybe adjust them a bit.
After cooling down and aligning it is now finally time to do the actual measurements! First I use the green laser to find the NVs and focus the laser on one of them. Next, I have to check that I am looking at an NV centre, and not just at a spot of dirt. Afterwards, I measure whether this sample meets our requirement to be used in the cavity. The light emitted by the NV centre may not fluctuate more than 1 gigahertz in frequency. We can do this by using a spectrometer, an apparatus which measures the frequency of all the light that is coming in. Unfortunately, we have not yet found a sample and we continue searching.
When I told my parents about my project and showed them around in the lab, my mother asked worriedly: ‘Do you sit here in the dark all day?!’. Fortunately I can also control the lab PC from my computer upstairs. But sometimes we even stay in the basement a little longer by choice and end the week by drinking a beer at the faculty pub, which is also located in the basement. Physicists have a strange preference for basements apparently.
My MSc project is a lot of experimental work but also requires detailed understanding of what you are measuring so the theory is also an important part. All in all, the day of a master student is pretty versatile!
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