Subtitles section Play video Print subtitles [MUSIC PLAYING] DANIEL SANK: Information is physical. Written letters are carbon grains on paper. Spoken words are vibrations of air molecules. Computer bits are electric charge. Each of these examples shares a common limitation-- they work under physics that was understood in the 1800s, known as classical physics. Science has progressed since then. We've discovered a new set of laws called quantum mechanics. Here's one of our chips designed to leverage the rules of quantum mechanics to process information in ways impossible on a computer based in classical physics. You may have heard that quantum mechanics only applies to microscopic objects, like atoms. So how does this chip bring out quantum behavior? I'm Daniel Sank, a research scientist working in the Google AI Quantum Computing Lab. In this video, we'll look at how our quantum bits are made physically. I want to explain the fundamental differences between classical and quantum information at the physical level so that you can understand why our quantum bits are made how they are. Physicists and computer scientists both think in terms of states. A physical state could be my position-- I can be on the left or on the right. And physical laws determine how nature goes from one state to another. Observe. If Sergio pushes me, my state changes. A computer's state is the value of its memory bits and computer programs determine how the computer goes from one state to the next. For example, when you hit the Play button, YouTube's program started manipulating your computer's memory to show this video. Where physics has physical states and natural laws, computer science has memory states and programs. Think of the state of computer memory as a string of bits. For end bits, there are two to the end possible strings. But because we're based in classical physics, the state of the computer is just one of these states at each point in time. On each step of a classical algorithm, we go from one state to the next. For example, the logic operation shown here takes the state 0-0-0 to 1-1-0. If we were to apply the same operation again, we'd go from 1-1-0 to 0-1-0. Compared to classical states, quantum states are more rich. They can have weight in all possible classical states, a situation physicists call superposition. Each step of a quantum algorithm mixes the states into complex superpositions. For example, starting in 0-0-0, we go to a superposition of 1-0-0, 1-0-1, and 1-1-1. Then each of those three parts of the super post state branches out to even more states. The extra complexity of quantum computers allows them to solve some problems faster than a classical computer ever could. We've discussed the computational difference between classical and quantum, but how do classical and quantum differ physically? How do we bring out quantum mechanics in our chip, which is so much bigger than the tiny atoms in which quantum mechanics was first discovered? Let's take a detailed look at classical bits at the physical level so that we can understand the physical difference between classical and quantum. Classical computer bits are stored in the presence or absence of charge on a capacitor in a circuit called dynamic RAM, or DRAM for short. If there's charge, it's a logical 1, and if there's no charge, it's a logical 0. But there's more going on here. Our logical 0 and 1 are actually made up of the presence or absence of 300,000 electrons. Why use so many? In principle, we could just use the presence or absence of one electron as our logical bid. Well, physical bits are noisy. Electrons are tiny and light, so they jiggle around and leak out of the DRAM. If we had only one electron and it were to leak out, our bit would change value, which is an error. By using lots of electrons, we're OK if a few leak out. DRAM circuits periodically check the logical level and replenish missing electrons. Encoding one logical bit in the state of so many physical bits gives classical information a level of reliability that we take for granted. We don't have to think about all those electrons bumping around when we write our programs. OK, so why can't we just put our DRAM into a quantum superposition of 0 and 1? Well, suppose we did have that superposition. It wouldn't last long. As soon as we do the first check to protect against a DRAM error, we've forced the bit into either 0 or 1, removing the quantum superposition state. In fact, that collapse happens even without us checking for errors. A single photon interacting with just one of our electrons can carry off information. When that happens, it's as if the photon observed the quantum state and the state collapses. You can think of this as nature observing and thus destroying our quantum states. Errors like this are unique to quantum information. In classical computing, you might be upset if somebody peeks at your bits, but that peek doesn't completely destroy them. Note that an error occurs whenever nature observes any one of our physical bits, so while we normally stack up more physical bits for redundancy, that approach actually makes quantum errors worse. That's the main difficulty in quantum computation-- the fundamental quantum constituents of matter are small and easily subjected to noise, but we can't brute force our way around that noise with redundancy because bigger systems are more subject to quantum errors. At Google, we use a technique that gets the best of both worlds. We use circuits with a huge number of electrons, but we prevent quantum errors with superconductivity. In regular metals, like with a conventional DRAM circuit, every individual electron does its own thing. As electrons move around, they can bounce off the positively charged ions of the metal, radiating vibrational waves that carry off quantum information about the electrons. This hectic, bustling cauldron of physical interactions generates a lot of quantum errors, and the information gets lost before we can use it. However, when certain metals are cooled down, their electrons join together in a single unit. The individual electrons no longer scatter and the rate of quantum errors drops to almost 0. Our quantum bits are, in fact, just electoral oscillators built from aluminum, which become superconducting when cooled to below 1 degree Kelvin. The oscillators store tiny amounts of electrical energy. When the oscillator is in the 0 state, it has 0 energy. When it's in the 1 state, it has a single quantum of energy. The two states of the oscillator with 0 or 1 quantum of energy are the logical states of our quantum bit, or qubit for short. Here's a picture of a superconducting qubit along with a circuit diagram. The crosses indicate Josephson tunnel junctions, which are nonlinear superconducting inductors. We pick the resonance frequency of our oscillators to be about 6 gigahertz, which sets the energy difference between the 0 and 1 states. That's a low enough frequency that we can build control electronics from readily available commercial parts, but also high enough that the ambient thermal energy doesn't scramble the oscillation and introduce errors. 6 gigahertz corresponds to 300 millikelvin. Fortunately, refrigerators that get to 15 millikelvin are relatively standard commercial products. For comparison, outer space is about 2.5 Kelvin. I think it's cool that the cryostats in our lab are colder than deep space. Now let's take a minute to make a few comments on how superconducting qubit architecture differs