What Is Quantum Supremacy? And Why Does It Matter?

The concept of quantum systems was first proposed by a Russian mathematician, Yuri Manin, in 1980.  However, it was Richard Feynman who conceived the possibility of quantum computers in the early 1980s.

Feynmann proposed that quantum computers would be effective in solving problems of chemistry and physics. Today’s computers use binary logic to perform tasks, but if we utilize the rules of quantum mechanics, many complex computational tasks will become feasible.

In 2012, an American theoretical physicist, John Preskill, coined the term “quantum supremacy” to describe a system far advanced than classical computers. It heralds the era of noisy intermediate-scale quantum technologies.

In this overview article, we have explained and what difference would “quantum supremacy” would make, what tech companies have achieved so far, why is it such a huge deal. Let’s start with the basics.

What Exactly Is Quantum Supremacy?

Quantum Supremacy is the goal of building a quantum computing system that can solve a problem that no classical computer can solve in a reasonable amount of time.

This involves the engineering task for developing a powerful quantum machine as well as the computational-complexity-theoretic task for classifying computational problems that can be solved by that quantum computer.

Quantum supremacy is an important step on the path to more powerful and useful computations. Several proposals have been made to demonstrate quantum supremacy. The most notable ones are:

  • Sampling the output of a random quantum circuits
  • Frustrated cluster loop problems designed by D-Wave
  • Boson sampling proposal introduced by Aaronson and Arkhipov

How will we tell for sure that quantum supremacy has been achieved? 

Verifying quantum supremacy is one of the trickiest tasks. It’s not like a nuclear explosion or a rocket launch, where you just watch and instantly know whether it succeeded.

You have to accurately demonstrate two things to verify quantum supremacy:

  1. The quantum device performs calculations fast.
  2. No classical computer could efficiently perform the same calculation.

The second part is quite complicated. It turns out that classical computers can perform specific types of problems very efficiently (better than scientists’ expectations). Until one has proved a classical computer cannot possibly perform a particular task effectively, there is always the chance that a more efficient, better classical algorithm exists. Proving that there is no such classical algorithm out there could be controversial, and it could take a lot of time.

Read: 12 Fastest Supercomputers In The World

The Battle Of Making A Quantum Computer

There have been working quantum devices for several years, but they outperform classical computers only under certain conditions. Most of the tasks performed by these quantum machines aren’t even useful in everyday lives.

In 2016, Google developed a fully scalable quantum simulation of a hydrogen molecule, using a 9-qubit quantum chip. In 2017, Intel manufactured a 17-qubit superconducting test chip for quantum computing, and IBM raised the bar with a 50-qubit chip that could preserve its quantum state for 90 microseconds.

17-qubit superconducting test chip developed by Intel

In 2018, Google revealed a 72-qubit processor named Bristlecone, and in 2019, IBM launched the world’s first commercial circuit-based quantum computer, IBM Q System One.

D-Wave Systems, a well-funded Canadian quantum computing company, remains an exception. In 2015, its 2X quantum computer with over 1000-qubits was installed at NASA’s quantum artificial intelligence lab. The company has subsequence shipped systems with 2048-qubits. Their devices rely on an alternative technique called quantum annealing to solve very specific problems.

Read: 5 Quantum Processors That Feature New Computing Paradigm

Google’s Big Announcement 

Out of the blue, by the end of 2019, Google researchers announced that they had achieved quantum supremacy. They developed a 54-qubit processor named Sycamore that performed the target computation (a random sampling calculation) in 200 seconds.

As per the research team, a classical supercomputer would take 10,000 years to perform the same calculations. This substantial increase in speed (compared to classical algorithms) is an experimental realization of quantum supremacy for this particular task.

What Did They Do? 

To demonstrate quantum supremacy, Google chose to solve a particular problem called “random circuit sampling.” A simple example of this problem is a program for simulating the roll of a fair die.

The program will run accurately if it appropriately samples from all possible outcomes. This means the program should generate each number on the die 1/6th of the time as it is executed repeatedly.

In a real scenario, instead of placing a die, a computer needs to properly sample from all possible outputs of a random quantum circuit. This sequence of actions is performed on a bunch of qubits. When qubits go through a circuit, its state becomes entangled (also known as a quantum superposition).

For example, when a circuit acts on 54 qubits, it causes 54 qubits to be a superposition of 254 possible states at the end of the circuit. This means the set of 254 possibilities collapses into one string of 54 bits. It’s like rolling a die, but instead of 6 possible outcomes, you are getting 254 outcomes, and not all are equally likely to occur.

The series of samples from this random circuit (following the proper distribution), can be efficiently generated on quantum computers. However, there isn’t any classical algorithm for producing these samples on start-of-the-art supercomputers. Therefore, as the number of samples increases, digital supercomputers rapidly get overwhelmed by the calculations.

In this experiment, Google researchers ran random simplified circuits from 12 to 53 qubits, keeping the number of gate cycles (quantum logic gates) constant. They then used classical simulations to check the performance of the quantum computer and compared it with a theoretical model.

Once they confirmed that the system is operating correctly, they ran random hard circuit with 53 qubits and increased gate cycles, until they reached a point where classical simulation became unworkable.

Process for demonstrating quantum supremacy | Credit: Google 

The experiment was performed on a completely programmable 54-qubit chip, Sycamore. It contains a 2D grid where each qubit attached to 4 other qubits, enabling enough connectivity for qubit states (so they instantly interact throughout the processor) and making it unfeasible to perform the same calculations on classical computers.

To achieve this level of performance, they utilized a new kind of control knob that could switch off interactions between nearby qubits, significantly reducing errors in the multi-connected qubit system. They also developed new control calibrations to avoid qubit defects and optimized the chip design to lower crosstalk, which further improved the performance of the quantum chip.

Did Google Really Achieve Quantum Supremacy?

Google’s Sycamore chip is kept cool inside the quantum cryostat. Image Credit: Eric Lucero/Google 

Although Google claimed that it had achieved quantum supremacy and a classical supercomputer would take about 10,000 years to perform the equivalent task, IBM disputed this claim, saying that an ideal simulation of the same task can be performed on a classical computer in 2.5 days with far greater fidelity.

Google’s experiment shouldn’t be viewed as proof that quantum devices are ‘supreme’ over classical computers. However, it perfectly demonstrates the progress in superconducting-based quantum computing, unveiling state-of-the-art gate fidelities on a 53-qubit system.

Headlines that contain some variation of ‘quantum supremacy achieved’ are eye-catching and interesting to read, but they completely mislead the general public.

As per the definition of the quantum supremacy, the goal hasn’t been met. And even if someone demonstrates it in the near future, quantum computers will never reign ‘supreme’ over classical computers. Instead, quantum systems will work alongside classical supercomputers, since each has its unique strengths and benefits.

Naming Controversy

Some scientists do not agree with the term ‘quantum supremacy.’ As per their perspective, the word ‘supremacy’ has overtones of violence, neocolonialism, and racism through its association with ‘white supremacy.’ They have suggested that the alternate phrase’ quantum advantage’ should be used instead.

However, John Preskill, who came up with this phrase, clarified that he wanted to emphasize that this is a privileged time in history when information technologies based on quantum laws are ascendant. He also explained that “quantum supremacy” best captured the point he wanted to convey. Other words, such as ‘advantage,’ lack the punch of ‘supremacy.’

Applications and Future

Recent advancements in quantum computing have inspired a whole new generation of computer scientists and physicists to fundamentally change the aspect of information technology.

Currently, scientists are working on fault-tolerant quantum machines that would be able to correct computation errors in real-time, enabling error-free quantum calculations. Considering the current state of the art in quantum computing, this goal is several years away from realization.

Tech companies are investing hundreds of millions of dollars to develop fault-tolerant quantum devices as quickly as possible. However, the big question is whether quantum machines will need to be fault-tolerant before they can perform a useful task.

Such machines promise a variety of valuable applications. For example, quantum computing could improve weather forecasting, strengthen cybersecurity, and help design new material for airplanes and lightweight batteries of vehicles. It could precisely map individual molecules, which in turn, could potentially open opportunities for pharmaceutical research.

It could also have a strong impact on the banking sector. Quantum computing may handle financial issues related to investment strategy optimization, which involves analyzing a vast number of portfolio combinations to figure out the best-fit criteria or to recognize fraudulent transactions.

Read: 22 Most Interesting Facts About Quantum Computers

At present, it is tough to predict which industry quantum computing will impact the most, because it has been tested on a very limited set of tasks. We will need to be patient for some years (or even decades) before we can appreciate the full splendor of the quantum era.

Written by
Varun Kumar

Varun Kumar is a professional science and technology journalist and a big fan of AI, machines, and space exploration. He received a Master's degree in computer science from Indraprastha University. To find out about his latest projects, feel free to directly email him at [email protected] 

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