• Nandan V

Google claims Quantum Supremacy, IBM disputes it

Updated: Oct 24, 2019

Google has claimed that its quantum computer, dubbed Sycamore can complete a task in 200 seconds which would take conventional supercomputers millennia. However, IBM has disputed the claim stating that it would only take them 2.5 days. Let's dig deeper.

Sycamore, Google's quantum computer.

Before we talk about supremacy, we need to understand what a quantum computer is and how it differs from what we usually associate the word computer with. Fundamentally, the phone you use, your laptop, that PlayStation are all computers meant for certain purposes. They all have a processor, RAM and some storage. All the operations you do on them, watching a meme, playing a game do the exact same thing as far as the processor is concerned, flip some bits or don't (completely inaccurate but sufficient for this context). A bit or a binary digit can have any of the two values at a particular instant, a one or a zero.

Quantum computers are fundamentally different from these as they use qubits or quantum bits for carrying out various operations. A qubit at an instant can be both zero and one due to quantum superposition. Sounds weird? Well, put together two and they can have 4 possible states that you can have in superposition. The number of states a quantum computer has is two to the power of the number of qubits it has.

The quantum computer engineered by the physicists at google had 54 qubits, which equals a lot of states (knowing the number won't do you any good).

"It is the enormous amount of parallel processing that gives it the power" - John Martinis | Chief Scientist

A quantum computer uses interference patterns to detect the state of qubits. A qubit, unlike a bit, does not work alone, they are linked together (quantum entanglement) using superconducting loops. The sycamore used 53 of its 54 qubits as one of them got damaged. Now that I have told what a quantum computer is, let's dive into the matter at hand.

The task that Sycamore achieved is really easy to say in a single sentence. It predicted the output of a quantum random-number generator. It did this in 3 minutes and 20 seconds. A random number generator is literally a random number generator but doesn't stack up to that claim. Quantum random number generators are truly random as it utilizes the inherent uncertainty of the quantum universe. However, the setup here is just a random arrangement of quantum gates which are likely to produce certain bit strings as output than others.

Here is how it predicted it. The feat can be thought of as analogous to rolling a loaded die. Not all outputs have equal probability and with enough throws, you could figure out which are most likely and which are not. Sycamore took a mere 200 seconds to demystify the quantum random number generator.

As of the claim that it would take 10,000 years for a supercomputer, it is true the way Google approximated it.

Sycamore vs a supercomputer

A quantum random number generator uses a quantum circuit which in turn is consists of quantum gates. A quantum gate is to a quantum circuit what logic gate is to a digital circuit. Unlike logic gates, quantum gates have to be reversible. This means if an input x gives an output y, an input y must give an output x.

The physicists at Google cannot just let a supercomputer run and wait for it to crack the quantum random number generator. To approximate the results, they calculated the time needed by the supercomputer (which I forgot to mention is IBM's) to crack a fraction of the quantum circuit and then extrapolated the results to the scale of the complete quantum circuit.

IBM's claim of 2.5 days has not been validated and even if it is true, this is definitely a massive breakthrough but would be a PR stunt to attract more engineers into the exciting new world of quantum computing.

The task handed to Sycamore might feel devoid of any practical applications. However, random number generators are key to security and privacy. Cloudflare uses lava lamps to generate random numbers because it is programmatically impossible to generate true random numbers using conventional computing algorithms, the numbers generated this way are termed "pseudo" random numbers.

What possibilities lie ahead? Creating a useable quantum computer is paramount and we are a lot closer to it than we earlier thought.

"...figure out something cool to do with it. Mankind is pretty good at that" - Marissa Giustina | Senior Research Scientist


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