Quantum Computing with Superconducting circuits
Overview: A short lab tour aimed at exposing the general public to quantum computing with superconducting circuits. I have tried a few different ways, but the following sequence has been my most successful way to keep the audience engaged and stay within 10-15 minutes. This is, of course, most entertaining when you have access to the physical lab, but pictures/slides can work well too.
Tour Script & Flow:
- Classical Computing & its limits
In this current era, every one of us strongly relies on computers. We rely on them to tell us whether it’s going to rain tomorrow, to send money, stay connected, and so on. How do computers actually work? Inside them, billions of microscopic silicon transistors act like tiny switches. Just as a physical switch has two distinct states, ON and OFF, a transistor has 0 and 1. Computers chain these switches together to store information and process it. They are incredibly fast, but they hit a wall with specific problems. For example, your phone can instantly multiply two large numbers. But if you ask it break a big number into two prime numbers, it can take a very long time.
- The Quantum Advantage
That’s where quantum computing steps in. A quantum computer processes information very differently. Instead of storing information strictly as a 0 or a 1, a quantum computer can use a ‘superposition state’ that is part 0 and part 1. Think of it like this: Picture a globe, where the North Pole is 0 and the South Pole is 1. While a standard transistor is stuck pointing strictly at the North or South Pole, a quantum version can point anywhere on the surface of the sphere.
Instead of trying every key on a massive keyring one by one, superposition lets us test every key in the lock at the exact same moment. That’s how a quantum computer can break down a prime factorization problem dramatically faster.
- Building Artificial Atoms
Now, how do we actually build one physically? We need something that obeys quantum mechanics: Such as atoms! The very same things that make up everything, including ourselves. But trapping single atoms is notoriously difficult. So, we engineer artificial atoms on a silicon chip using microscopic circuits, made with superconductors and a special element called Josephson junction. [Action: Pass around a patterned chip] It’s hard to see the patterns on this chip. It’s because they are a fraction of the width of a human hair. To see them, we need special microscopes that shoot electrons instead of light. [Action: Show SEM images on a screen or printout]
- The Dilution Refrigerator
For this microscopic circuit to work like an atom, we need to cool it down to a very low temperature. We do so with a dilution refrigerator [Action: Point to the dilution refrigerator or show images] What temperature do you think this refrigerator reaches? It gets colder than outer space! About 7 millikelvin, or -273 degrees Celsius (near absolute zero). Just like your kitchen fridge, the dilution refrigerator starts by compressing and expanding gas to pump heat out. But instead of standard refrigerants, it uses helium, which is the only element that doesn’t freeze at the very cold temperatures we want to reach. Then, to get down to those ultra-cold levels, it takes advantage of the quantum thermodynamics of mixing two different isotopes of helium.
- Computing with artificial atoms
So, when the circuit is finally cold and behaves like an artificial atom, how do we actually use it to compute? Normally, we think of energy on a smooth, sliding scale. But for an atom, energy is quantized—meaning it works like steps on a ladder. At the lowest step, it’s in state 0. If we shine a microwave pulse to give energy to the atom, it jumps up to the next step, state 1. Depending on exactly how long you shine this pulse, you can leave the atom in a mixture of 0 and 1. That is how we create the superposition state I mentioned earlier. Quantum computers aren’t meant to replace your phone. They are designed for specific, massive challenges—like prime factorization and other important things like modeling complex molecular bonds for new life-saving drugs.