On the 23rd of October, Google came out with a bold claim: it had achieved quantum supremacy. For the first time in computing history, researchers had found an algorithm where a quantum computer would always outperform a classical computer and where no classical workaround could be found. More specifically, Google’s 53-bit Sycamore processor performed a computation in 200 seconds that would’ve taken the most powerful supercomputer 10,000 years.
IBM immediately refuted Google’s claim, saying that Google had mistakenly assumed the RAM storage requirements would be enormous for a classical supercomputer. IBM says its supercomputers have access to an enormous amount of RAM and hard disk space, to the point where the company claims it could reliably solve the same computation in 2.5 days (instead of 10,000 years). In such a scenario, it would be hard to maintain that quantum supremacy had been achieved.
But regardless of whether quantum supremacy has been achieved, there’s still an enormous difference between 200 seconds and 2.5 days. It’s another sign that the quantum era in computing is approaching fast. But let’s take a step back. What exactly is quantum computing and why is it such a big deal? For that, we need to understand the basic principles of quantum mechanics.
It’s a Wave… It’s a Particle… It’s Quantum Mechanics
Imagine you have a tennis ball machine that shoots tennis balls toward a panel with a single slit. Behind the panel is another panel that indicates where the tennis balls have hit. As you’d expect, all the tennis balls shot through the slit will predictably hit the back panel in a single band. If you swap the single-slit panel for a double-slit panel, tennis balls shot through both slits will predictably hit the back panel in two separate bands.
Now imagine you have a water wave generator that creates waves going through a single-slit panel. Behind the panel is another panel that measures where and how hard the waves hit. The water waves going through the slit will radiate outward from the slit. They will hit hardest directly behind the slit and less hard the further away from that center band. In that sense, it’s relatively similar to the single band of tennis balls.
The scenario is different when the waves go through a double-slit panel. When the waves radiate outward from their respective slits, the overlapping parts of the waves cancel out one another. So instead of seeing two bands where the waves hit hardest and gradually decline in intensity, you’ll see a pattern of stripes. Places on the panel where the waves hit and places where the waves don’t hit at all, because they canceled out one another. This is called an interference pattern.
So waves create an interference pattern while anything solid does not. Nothing strange so far. We’re still in classical physics territory. You would expect this behavior to hold in every experiment. But it doesn’t. Over the course of the twentieth century, scientists started to realize that very small particles follow an entirely different set of rules.
In 1927, Clinton Davisson and Lester Germer discovered that electrons, tiny bits of matter (like the tennis balls), create an interference pattern when they’re shot through a double-slit panel. Electrons aren’t waves, so this was a shocking finding. Even when the electrons were shot toward the double-slit panel one by one, an interference pattern appeared.
This meant that a single electron, somehow, turns into a wave and goes through both slits at the same time, after which it interferes with itself and creates an interference pattern. Extremely weird. But it’s even weirder mathematically: the single electron goes through both slits and it goes through neither. It also goes through just one slit and it goes through the other.
Physicists had discovered the mathematical idea of superposition. Something is in superposition when it’s in all its possible states at the same time. Until it’s observed, that is. When scientists try to pinpoint which slit the electron actually goes through, the electron acts as expected. It’s only when the electron was not directly observed that it acted as if it was in all possible states at once.
This strange behavior is called wave-particle duality. Scientists have since demonstrated this behavior in atoms and even molecules too. The larger the particle, however, the harder it becomes to demonstrate wave-particle duality. The rules of quantum mechanics seem to work only in the world of the very small.
There’s one last concept we need to cover before we return to quantum computing: entanglement. Quantum particles can become entangled with one another. This means that the state of one quantum particle immediately influences the state of the quantum particle(s) it’s entangled with. For example, if you have two entangled particles and you change the spin of one particle, the spin of the other particle will change instantly too.
This happens regardless of the distance between both particles. One particle can be in Switzerland and the other on Mars, yet they’ll interact instantaneously when entangled. This behavior seems to defy Einstein’s special theory of relativity that said nothing can go faster than the speed of light. How it happens, we don’t understand yet. But it’s a useful concept for quantum computing regardless.
Back to Quantum Computing
The smallest unit of data in a classical computer is a bit. The smallest unit of data in a quantum computer is a qubit (a quantum bit) – which mimics the behavior of a subatomic particle. A bit will always either be a zero or a one. A qubit can be a zero or a one, but, because of the principle of superposition, it can also be both a zero and a one.
Because of its bits, a regular computer can only ever be in one state at a time. It solves a problem by trying all possible combinations of bits one at a time (very rapidly, of course) until it arrives at the solution.
A quantum computer, however, can be in all its states at once. If you have n qubits, a quantum computer can be in 2n states at the same time. Exponentially more powerful than a classical computer. So Google’s 53-qubit quantum computer can be in 253 states at the same time (the equivalent of about 1016 states). Instead of trying out combinations one at a time, it can try out 253 combinations at the same time.
The difficulty with quantum computing, however, is that a quantum state is fragile. Whenever a qubit has even the slightest interaction with another particle that’s not a qubit, it turns into a regular bit. That’s why quantum computers are stored in room-sized refrigerators cooled to just above absolute zero temperatures. So you won’t have a quantum computer in your bedroom any time soon, I’m sorry.
Not that you’d want to have a quantum computer in your bedroom anyway. A quantum computer isn’t just a regular computer that’s more powerful. It’s an entirely different type of machine with an entirely different set of use cases. You wouldn’t use a quantum computer to play Call of Duty.
Quantum computers will be used to simulate the behavior of particles, atoms, and molecules in quantum physics and quantum chemistry (something that was hardly possible before). This, in turn, will help scientists discover better medicine, design better batteries, create better material, and so on…
Quantum computers will also have serious implications for cryptography. Modern cryptography relies on a classical computer’s inability to factor large numbers into primes. For example: 15 can be factored into 5 and 3, both of which are prime numbers (numbers that cannot be formed by multiplying two smaller numbers). That’s easy for a computer to solve. However, try the same for 2,301,592,102,21 and it becomes an impossible computational task.
Except for a quantum computer. It’s possible to create a quantum computer that could easily factor large numbers into primes. Given that nearly everything secured digitally relies on modern cryptography, we’ll need to find an encryption method that’s quantum-secure. Preferably sooner rather than later, too. You wouldn’t want a government or a company to be able to break the encryption method that all your online information is secured with.
Quantum computers consist of qubits, which mimic the behavior of a subatomic particle. Qubits can be in all possible states at once, making a quantum computer exponentially more powerful than the most powerful supercomputer. While the applications of quantum computers will be very different from those of a regular computer, they nonetheless unlock exciting opportunities for computers to solve what was previously considered unsolvable.