March 4, 2024

What is quantum advantage? A quantum computing scientist explains an approaching milestone that marks the arrival of extremely powerful computers

The quantum advantage is the milestone that the field of quantum computing is feverishly working towards, where a quantum computer can solve problems that are beyond the reach of the most powerful classical or non-quantum computers.

Quantum refers to the scale of atoms and molecules where the laws of physics as we experience them are broken down and a different, counterintuitive set of laws is applied. Quantum computers take advantage of these strange behaviors to solve problems.

There are some types of problems that are not practical for classical computers to solve, such as cracking state-of-the-art encryption algorithms. Research over the past few decades has shown that quantum computers have the potential to solve some of these problems. If a quantum computer can be built that actually solves one of these problems, it will have demonstrated a quantum advantage.

I am a physicist who studies quantum information processing and the control of quantum systems. I believe this frontier of scientific and technological innovation not only promises revolutionary advances in computing, but also represents a broader rise in quantum technology, including significant advances in quantum cryptography and sensing.

The source of quantum computing power

A central element of quantum computing is the quantum bit or qubit. Unlike classical bits, which can only be in states 0 or 1, a qubit can be in any state that is a combination of 0 and 1. This state of neither just 1 nor just 0 is known as quantum superposition. With each additional qubit, the number of states that can be represented by the qubits doubles.

This property is often confused with the source of quantum computing power. Rather, it all comes down to an intricate interplay of overlap, interference, and entanglement.

Interference involves manipulating qubits so that their states combine constructively during calculations to amplify correct solutions and destructively to suppress incorrect answers. Constructive interference is what happens when the peaks of two waves (such as sound waves or ocean waves) combine to create a higher peak. Destructive interference is what happens when a wave peak and wave trough combine and cancel each other out. Quantum algorithms, which are few and difficult to devise, establish a sequence of interference patterns that produce the correct answer to a problem.

Entanglement establishes a unique quantum correlation between qubits: the state of one cannot be described independently of the others, no matter how far apart the qubits are. This is what Albert Einstein dismissed as “spooky action at a distance.” The collective behavior of entanglement, orchestrated through a quantum computer, enables computational accelerations that are beyond the reach of classical computers.

The ones and zeros (and everything in between) of quantum computing.

Applications of quantum computing

Quantum computing has a variety of potential uses where it can outperform classical computers. In cryptography, quantum computers represent both an opportunity and a challenge. Most famously, they have the potential to crack current encryption algorithms, such as the widely used RSA scheme.

A consequence of this is that current encryption protocols must be redesigned to be resistant to future quantum attacks. This recognition has given rise to the burgeoning field of post-quantum cryptography. After a long process, the National Institute of Standards and Technology recently selected four quantum-resistant algorithms and began the process of preparing them so that organizations around the world can use them in their encryption technology.

Additionally, quantum computing can dramatically accelerate quantum simulation – the ability to predict the outcome of experiments operating in the quantum realm. The famous physicist Richard Feynman imagined this possibility more than 40 years ago. Quantum simulation offers potential for considerable advances in chemistry and materials science, assisting in areas such as complex modeling of molecular structures for drug discovery and enabling the discovery or creation of materials with novel properties.

Another use of quantum information technology is quantum sensing: detecting and measuring physical properties such as electromagnetic energy, gravity, pressure and temperature with greater sensitivity and precision than non-quantum instruments. Quantum sensing has countless applications in fields such as environmental monitoring, geological exploration, medical imaging, and surveillance.

Initiatives such as the development of a quantum Internet that interconnects quantum computers are crucial steps in bridging the worlds of classical and quantum computing. This network could be secured using quantum cryptographic protocols, such as quantum key distribution, which enables ultra-secure communication channels protected against computational attacks, including those using quantum computers.

Despite the growing set of applications for quantum computing, the development of new algorithms that take full advantage of the quantum advantage (particularly in machine learning) remains a critical area of ​​ongoing research.

A prototype quantum sensor developed by MIT researchers can detect any frequency of electromagnetic waves.
Guoqing Wang, CC BY-NC-ND

Maintain consistency and overcome errors

The field of quantum computing faces significant obstacles in hardware and software development. Quantum computers are very sensitive to any involuntary interaction with their environment. This leads to the phenomenon of decoherence, where qubits rapidly decay to the 0 or 1 states of classical bits.

Building large-scale quantum computing systems capable of delivering on the promise of quantum accelerations requires overcoming decoherence. The key is to develop effective methods to suppress and correct quantum errors, an area on which my own research focuses.

Addressing these challenges, numerous quantum hardware and software startups have emerged alongside well-established tech industry players like Google and IBM. This industry interest, combined with significant investments from governments around the world, underscores a collective recognition of the transformative potential of quantum technology. These initiatives foster a rich ecosystem where academia and industry collaborate, accelerating progress in the field.

The quantum advantage in sight

Quantum computing could one day be as disruptive as the arrival of generative AI. Currently, the development of quantum computing technology is at a crucial moment. On the one hand, this field has already shown early signs of having achieved a narrowly specialized quantum advantage. Researchers at Google and later a team of researchers in China demonstrated a quantum advantage for generating a list of random numbers with certain properties. My research team demonstrated a quantum speedup for a random number guessing game.

On the other hand, there is a tangible risk of entering a “quantum winter”, a period of reduced investment if practical results do not materialize in the short term.

While the technology industry is working to deliver a quantum advantage in products and services in the near term, academic research remains focused on investigating the fundamental principles behind this new science and technology. This ongoing basic research, fueled by enthusiastic cadres of bright new students of the kind I encounter almost every day, ensures that the field will continue to progress.

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