Quantum Computing

We will try to demystify quantum computing, a technology which will bring an exponential increase in computing power, in a few articles. We won’t talk about the technological part today but rather focus on the basic quantum Mechanics concepts needed to understand quantum computing.

Initial researches on Quantum Mechanics can be dated back to the 17th century but we had to wait until the late 19th century to see modern quantum mechanics take shape with Max Planck and its light quantum hypothesis. Shortly, quantum mechanics deals with the behaviour of matter and its interactions with energy on the scale of atoms and subatomic particles. There are three concepts that you must understand in order to begin your quantum computing journey: superposition, entanglement and decoherence.

Quantum superposition

This is a very counterintuitive phenomenon that refers to the ability of a quantum system to be simultaneously in multiple quantum states. For example, electrons possess a quantum feature called spin, a type of intrinsic angular momentum. In the presence of a magnetic field, the electron may exist in two possible spin states, usually referred to as spin up and spin down. Each electron, until it is measured, will have a finite chance of being in either state. Only when measured is it observed to be in a specific spin state. In common experience a coin facing up has a definite value: it is a head or a tail. Even if you don’t look at the coin you trust that it must be a head or tail. In quantum experience the situation is more unsettling: material properties of things do not exist until they are measured. It might be hard to get around this one so if you want a bit more information, you can check this video.

Entanglement

It refers to the idea that two or more particles are intrinsically parts of an inseparable quantum system. The individual constituents or particles cannot be fully described or considered without taking into account the entire entangled quantum system. Measurement of one part of the entangled quantum system will “collapse” the system. Namely, locally interaction on one constituent will affect the other constituents, even if the pieces of the entangled system are separated by large distances. Basically, if you have two entangled particles, measuring a property of one of them will determine the property of the other.

Decoherence

It describes the loss of quantum coherence. Quantum state are very easily disturbed, the slightest vibration or change in temperature can cause a particle to tumble out of superposition and “chose a state”. Maintaining quantum superposition if one of the biggest challenges faced by quantum computing.

What is a qubit?

Classical computers receive information in the form of binary numbers written with 0s and 1s. A qubits is a two-level quantum system where the two basic quantum states are 0 and 1. Therefore a qubit can be in state 0 or 1 like a classical bit, but also in a linear combination of both states. That’s the quantum superposition we talked about last week!

This is the one of the properties that theoretically will exponentially increase the computing power of quantum computers compared to normal computers. Rebecca Krauthamer, CEO of quantum computing consultancy Quantum Thought uses a good analogy to describe this difference. She says that “if you are trying to solve a maze, you’d come to your first gate, and you can go either right or left, but a quantum computer doesn’t have to choose one. It can go right and left at the same time”. In a sense, quantum computers can look at different options simultaneously and instantly find the most optimal path, which is very useful for optimization problems!

Furthermore, it is possible to generate pairs of qubits that are “entangled”, which means the two members of a pair exist in a single quantum state. Changing the state of one of the qubits will instantaneously change the state of the other one in a predictable way. It’s key to the power of quantum computers. In a conventional computer, doubling the number of bits doubles its processing power. But thanks to entanglement, adding extra qubits to a quantum machine produces an exponential increase in its number-crunching ability because measuring only one qubit gives you much more information than only measuring a bit.

Several different physical implementations of qubits are possible. A number of examples are the polarizations of a photon, two of the (multiple) discrete energy levels of an ion, a superconducting Transmon qubit, the nuclear spin states of an atom or the spin states of an electron. However, it is technically challenging to create superposition and entanglement and even more challenging to maintain it due to quantum decoherence.

Quantum computers relying on qubits will be able to run quantum algorithms (an algorithm that uses the quantum properties of qubits) and provide incredible results. However, it won’t be a complete revolution. Problems that are fundamentally unsolvable by classical algorithms (so called undecidable problems) cannot be solved by quantum algorithms either. The added value of quantum algorithms is that they can solve some problems significantly faster than classical algorithms. The best-known examples are Shor’s algorithm and Grover’s algorithm. Shor’s algorithm is a quantum algorithm for integer factorization. Simply put, when given an integer N, it will find its prime factors. It can solve this problem exponentially faster than the best-known classical algorithm can. Grover’s algorithm can search an unstructured database or unordered list quadratically faster than the best classical algorithm with this purpose.

What is Quantum supremacy?

The term was defined by a professor at the California Institute of Technology, John Preskill to refer to the time when quantum computers will be able to perform a single calculation that no conventional computer, even the biggest supercomputer, can perform in a reasonable of time.

In October 2019, Google scientists made the news by announcing, through an article in Nature, that they had achieved quantum supremacy. According to the paper, Google 54 qubits (only 53 were working) quantum computer code-named “Sycamore” performed a task, that would allegedly take today’s fastest supercomputer, IBM’s Summit 10 000 years to perform, in 200 seconds. Its long-standing rival immediately refute Google’s claim, arguing that Summit could simulate the quantum system and perform the random quantum circuit sampling calculation (the task performed) in 2.5 days if correctly configured. To this day, not one side has really prevailed over the other, so let’s look at what has been achieved so far.

A very brief history of quantum computing

The first physical realization of a quantum computer was made by Yoshihisa Yamamoto and K.Igeta in 1988. Their approach uses atoms and photons and is the progenitor of modern quantum computing and networking protocols using photons to transmit qubits and atoms to perform two-qubit operations. You couldn’t do anything with it, but it lay down the bases for the future. We had to wait 10 years until 1998 to see the first quantum algorithm. Ran on a 2-qubit quantum computer, it was used to solve Deutsch’s problem. In the problem you have a black box quantum computer which implements a function taking n-digit binary values as input and produces either a 0 or a 1 as output and you must determine if the function is constant or balanced (half of the input are 0). The theory promised that the mentioned function is either constant or balanced. The problem is not really interesting in itself but it highlighted the usefulness of quantum computers since its very difficult to solve on classical computers but very easy on quantum ones.

We jump to 2007 when D-Wave and Google presented a 28-qubit computer running an image recognition algorithm. It was based on superconducting electronics and showed the usefulness of superconductors to build quantum computers as these structures naturally shield themselves from external noise. In 2014, D-Wave demonstrated a 512-qubit computer to perform the quantum annealing technique, which is a technique to find the global minimum of a complicated mathematical function. It might sound huge since we said above that Google might have achieved quantum supremacy with only 53 working qubits but the difference lies in the fact that D-Wave architecture differs from a “universal quantum computer”, it is only made to run the quantum annealing technique, it can’t run Shor’s algorithm (that could crack a lot of cryptosystem) for instance, and therefore it is much less complex to add qubits. In 2016, IBM made some of its gate-based quantum processors available on the Internet as a cloud service for anyone to use to experiment and in September 2019, it opened the IBM Quantum Computation Center in New York, the world’s largest fleet of quantum computing systems for commercial and research activity. In the meantime, they also introduced Quantum Volum, a metric to measure performance of a gate-based quantum computer.

In 2020, quantum computer became commercial “massively” as a cloud service, starting with IonQ 32 qubit trapped ion quantum computers and the launch of services such as Amazon Bracket or Azure Quantum that allow scientists, researchers, and software developers to experiment with computers from multiple quantum hardware providers.

We’ll wrap this up with the latest big news in the quantum computing world. In September, D-Wave launched its 5000 qubits computer that can natively solve problems in the 600–800 variable range, 5 times more than their 2000 qubits computer. And finally, in December, China announced that its 76-qubit photon-based quantum computer Jiuzhang achieved quantum supremacy by conducting a Gaussian Boson sampling task, which would take half a billion years for IBM’s Summit, in 200 seconds.

The most exciting stories about quantum computing have yet to come! With massive investments from all players, governments and companies, there will be plenty of practical implementations in the future, affecting all of our industries. And those who will come out on top in the Quantum Race will certainly have a major impact on the society. Will it be China, America, Europe, or another player that has yet to join the fight?