#10 Spooqy: The satellite that carried my signature

Literally and somewhat philosophically

Nov 14, 2021

A personal story

While leaving Ahmedabad for a postdoc position at the National University of Singapore, I did not have the slightest intention of working on a satellite, let alone sending my signature engraved on the sides of it! After my PhD defence, I started applying to different places for a post-doctoral fellowship. That is what everyone did in academics. The "normal" successful career path for an Indian academician is to have a foreign postdoc with a couple of papers in high impact journals and obtain a faculty position in any national institute. There is hardly any opportunity for private research in India. So, the mindset is always to get into an institution. Anyway, that is a digression from the topic.

I joined the group of Alexander Ling at the centre for quantum technology which is in the national university of Singapore. I was working on purely academic projects which involved designing experiments to test different theories. But the main project for the group was to build a satellite that contains a source of entangled photons! Once I finished my academic project, I moved to the satellite one.

To me, this was a different training. My PhD was more of a spontaneous one. I studied the subjects that were of my interest, working on projects that felt exciting. There was no proper plan to finish some experiment or project. If one experiment did not work, I used to jump to another one. Don’t get me wrong. This is an equally valid way to do your PhD. Such an approach will make you more independent and force you to come up with new ideas. However, one needs to be trained to do structured research as well. The work culture for the satellite project was a well-defined one that required disciplined, well-structured research. We know all the scientific background, we have made the experiment work in the lab. But making it work on a satellite is a whole new game.

Why do we do this

Sheldon: "Someone in Szechuan province, China is using his computer to turn our lights on and off.
Penny: "Huh, well that’s handy. Um, here’s a question, why?"
All together: "Because we can." - The Big Bang Theory

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Why do you need an entangled photon source on a satellite? The first intention is the scientific and technological curiosity itself. Entanglement is only possible when you ensure something called “coherence”. You can consider this as two instruments being in sync. If they are out of sync, we call it decoherence and we cannot observe the effects of entanglement. Building a system that ensures quantum coherence after surviving a harsh environment such as a rocket launch is a technological and scientific challenge. We need to have experimental designs that are less susceptible to vibration or thermal change. These endeavours push our scientific and technological limits. However, launching a satellite is a costly affair. Mere curiosity is not enough to justify spending millions. Fortunately, this has a not so far future use case: Quantum cryptography.

Some quantum ‘business’

I am saving the quantum cryptography part for another issue. But let me give a brief overview. The current cryptographic systems depend upon difficult mathematical problems that will take thousands of years to solve for any powerful supercomputer. However, a quantum computer, which has not been fully realised yet, will be able to crack this in minutes. The solution to this problem was already there in quantum mechanics. Use quantum states to send the secret key to encrypt your message.

The traditional classical communication encodes information as 0s or 1s known as bits. The MBPS and GBPS speed of a device referred to how many bits it can carry within a second. A movie of 1 Gb file size has 8 billion 0s or 1s. When you send information in quantum states, the states are called qubits, or quantum bits. It can take values that are superposition states of 0 and 1. Every state, depending upon how you measure, gives a result of 0 or 1 with a certain probability. More on the qubits later. Once measured, the state is lost. And you cannot keep a copy of the state, and send the original according to the ‘no-cloning theorem’. So any eavesdropper (one who hacks the system) will be visible if she tries to read the message and send it over to the receiver. Think of it as a super special seal you attach with a secret letter. Once opened, the receiver can detect it by carefully checking the seal.

Back in the 1990s, scientists have shown that photons, particles of lights, are the best candidate to send the qubits. Later many groups implemented this quantum communication through optical fibre cables between buildings and between adjacent cities. By the second decade of the 21st century, there were a handful of industries delivering Quantum key distribution systems (equipment to distribute the secret key using quantum states). By 2030 it is expected to be a billion-dollar industry with many giants developing new products and protocols. Most of these technologies use optical fibre cables to send and receive photons. However, the losses in fibre increase with the distance, and most of the systems cannot distribute these secret keys farther than 100 kilometres. So, how does a bank in India securely communicate to another bank in US or China? One answer is the use of satellites.

Sending a lab up in space

One of the main focuses of industrial research is to reduce the gap between the research and products in the market. The lab is a controlled environment. The temperature is maintained around 22-23 degrees, low humidity, and in most cases, the experiments sit on an optical bench which isolates the systems from any external vibrations. But in the outside world, we cannot ensure these conditions are maintained. So we need to make our experiments rugged enough to make them survive the harsh environments. Your laser pointer, fingerprint scanner or even the computer displays were once on a research lab mounted on a bulky optical table. You have to make some trade-offs on the performance and the ruggedness of the system.

FAQ: How Are Entangled Particles Created? [Video] - Spooky Action at a Distance — An initial design of entangled photons in lab. This is a setup from National University of Singapore. The red colour is the path of the entangled photons. It is not the photons itself. A laser is used for the visualisation.

Deploying an experiment on a satellite is more challenging. First, your instruments undergo intense vibrations due to the rocket launch. Second, it observes extreme temperature differences while orbiting the earth. On top of this, you need it to be running, collecting data without human intervention. Finally, we need radio communication to send or receive data.

Elephant in the room: Chinese Micius satellite

When you google satellite and quantum, the first few results will be about the Chinese satellite called Micius or Mozi. The satellite was named after the ancient Chinese philosopher Mozi. What the Chinese did was remarkable. They have sent a whole lab up there. The total satellite weighed roughly 600 Kgs, and the budget was about $100 million. The satellite was launched in 2016 to a 500km orbit. They had an entangled photon source, a weak coherent laser source (this is another way you can do quantum cryptography) and two telescopes to send the quantum light to the earth. Many telescopes were set up in China and Austria to receive the signal and perform the measurement. They have demonstrated entanglement distribution, teleportation and quantum key distribution. Don't be alarmed; these are not as difficult concepts as you think. I will touch up some of them in later posts.

photograph of the quantum link between satellite Micius and a ground station in china — The green line is the satellite track. The light is from its beacon Image credit: The Guardian

Now they have demonstrated that it is possible to do quantum communication from satellite to earth. Now, why do we worry about building another satellite? First of all, it is not only about science. Most of the technology they have developed is not accessible, and other parts of the world need access to the quantum-satellite world. But there was another striking pursuit. It was to use a nano-satellite to carry the quantum experiment.

Cube sates are made up of 10cm cubes and this technology-enabled many universities to be part of space programs. The cost and the mission time required for such a satellite are much less compared to the big satellites.

A comparison of the Chinese satellite and the satellite developed at NUS, Singapore

Spooqy -1 our nano-satellite

No, it was not a Halloween launch; if that you were suspecting after hearing the name. Albert Einstein called entanglement '“spooky action at a distance”. Hence the name spooqy (with a q for quantum !). Spooqy -1 contained an entangled photon source and a measurement set up to verify the entanglement. It did not have any light transmitting to earth for quantum communication. The aim was to test whether we can get the instrument working in harsh conditions with strict size, weight and power (SWaP) limitations.

My colleagues Aitor and Zonkan with the main payload of Spooqy-1 Pic https://www.quantumlah.org/

There are a lot of optimisations and research that goes into making the instrument work, but let me spare those details. When I joined CQT, most of the designs of Spooqy-1 were completed. However, I had the chance to be involved in some final calibration and later run the experiment after launching into the space.

Spooqy was launched to the international space station using its supply rocket on 17th April 2019. Then the satellite was deployed from ISS after two months on 17th June 2019. You can watch the deployment video here.

Spooqy deployed from ISS. The big solar panels are ISS solar panels.

Sleepless nights of satellite tracking

Now the satellite is in space. But how do we run experiments on it? We talk to the satellite through radio communication. We had radio antennas at NUS and in Switzerland to communicate to Spooqy. However, we could talk to spooky only if it is flying over you (it's called a pass). You can predict when the satellite appears on your horizon beforehand. Once it is on the horizon, we have less than 10 minutes to talk to it until it goes out of your reach. It comes back after 90 mins.

We made contact with Spooqy, the very next day. We did not want to waste any good passes to know about the status and run some simple measurements. Some time the passes were in the middle of the night or early morning.

Some self-promotion. The top right screen shows the track of the satellite and the bottom one is the terminal to talk to spooqy. If you are interested in satellite tracking look up https://celestrak.com/

Fingers crossed

In the initial days of spooqy, we did not run the main experiment. To switch the laser on, the payload needs to be above 14 degrees. So we monitored the daily temperature changes, power usage and detector noise. Every day, the whole team will meet in a conference hall (remember, it was pre covid time) and discuss yesterdays result and the next steps. We decided to use a heater and run the experiment the next day. In the lab, we have operated the machine for different temperatures. During the experiment, the temperature remains almost the same. This is not the case in orbit where temperature can change rapidly. So, we were not expecting any great result

I was in charge of making the flight planner, which decides the experiment run on the satellite. Alex, another postdoc, who made a code to analyze the result, and Aitor, PhD student who built the payload, were doing the analysis and summary. Once I ran the experiment, we had to wait for a good pass to download the data. Though we did not expect it to work on the first go, the stakes were high.

Data arrived. I and Aitor were eagerly looking at the screen while Alex compiled the results. We could not believe what we saw. The characteristic Bell curves show the photons are entangled. At first, we did not believe it and checked if we had run some old experimental data. But it was indeed the experiment ran in the space. Since then, we had run hundreds of experiments on spooqy which showed a violation of Bell’s inequality, which tells you the photons are entangled. You can access the main publication here. After two years of its service, Spooqy became space dust and fell back to the ocean on 16th October 2021.

A figure from the paper. The top is the curve before the launch. The middle one is the experiment run in space. And the bottom one is the S parameter, which is an indication of whether the photons are entangled or not, for different temperatures in space

Hold your horses! There is a signature

I used the signature line just for a nice heading. I don’t consider it to be a bigger deal than working on the satellite and being part of the mission. However, we decided to engrave our signatures and a quote from the centre of art (that is a story for another time) on the side of the satellite. So I had my signature in space for the last two years, until it got buried underwater.

What’s next?

Spooqy has demonstrated that we can put a delicate instrument in space on a small satellite. Currently, many missions are planned to establish quantum communication from space to earth. You can go through the links if you are interested. The ultimate aim is to connect the whole world with quantum signals. We can expect constellations of satellites to give this global quantum coverage. And there is a vision of quantum internet where multiple quantum computers are connected via satellite links.

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