What is a quantum computer? Explained with a simple example

You may have heard that quantum computers have the potential to revolutionize computing as we know it. Tech giants like Google, IBM, and Microsoft are investing billions into developing quantum computing technology. But what exactly is a quantum computer, and how does it differ from the computers we use today? In this article, I‘ll provide an intuitive explanation of quantum computing using a simple example.

How do regular computers work?

First, let‘s review how traditional, non-quantum computers work. A regular computer stores and processes information in the form of bits. A bit is like a tiny switch that can either be in the "on" position (represented by a 1) or in the "off" position (represented by a 0). Any piece of data, whether it‘s numbers, text, images, or video, is ultimately stored as a sequence of many bits, each set to either 0 or 1.

To perform a computation, a regular computer takes in some data (a sequence of 0s and 1s), applies logical operations to transform that data step-by-step, and produces new data as the output. While modern computers can perform these logical operations extremely quickly, they can still only do one operation on one piece of data at a time.

Bits vs Qubits

Quantum computers, on the other hand, use quantum bits or qubits to store information. While a regular bit can only be set to either 0 or 1, a qubit can be in a quantum superposition of the 0 and 1 states. In other words, a qubit can represent a 0, a 1, or both 0 and 1 at the same time!

Think of a coin. A regular bit is like a coin that can only be heads (0) or tails (1). But a qubit is like a spinning coin that is simultaneously in a mix of heads and tails states, with a certain probability of landing on heads or tails when you stop it and look at it. The spinning coin represents a superposition of heads and tails.

This mind-bending property comes from the principles of quantum mechanics. In the quantum world, a particle like an electron or photon can be in multiple states at once, like different positions, momentums, or spins, until it is measured.

Exponential speedup

Why are qubits useful for computing? Since a qubit can be in a superposition of multiple states, a group of N qubits can represent a superposition of 2^N different states simultaneously! With each additional qubit, the number of states represented doubles.

For example:

• 1 qubit = superposition of 2 states
• 2 qubits = superposition of 4 states
• 3 qubits = superposition of 8 states
• 4 qubits = superposition of 16 states
• 20 qubits = superposition of 1,048,576 states
• 300 qubits = superposition of 2^300 states (more than the number of atoms in the observable universe!)

So with just a few hundred qubits, a quantum computer could perform more calculations simultaneously than there are atoms in the universe. This allows quantum computers to solve certain problems much faster than even the most powerful supercomputers today.

Wedding seating example

Let‘s illustrate this with a simple example. Suppose you are planning the seating arrangements for your wedding. You have a list of N guests and their relationships to each other (e.g. who knows who, which people want to sit together, which people don‘t get along). The challenge is to find an optimal seating arrangement that maximizes the total happiness of all guests.

To solve this on a regular computer, you would need to generate all possible combinations of seating charts and evaluate each one to find the optimal arrangement. If you have N guests and each one can be seated in one of two seats, then there are 2^N possible seating combinations to check:

• With 5 guests, there are 2^5 = 32 combinations
• With 10 guests, there are 2^10 = 1024 combinations
• With 20 guests, there are 2^20 = 1,048,576 combinations
• With 300 guests, there are 2^300 = 2×10^90 combinations (more than atoms in the universe)

As you can see, the number of possibilities grows exponentially with the number of guests. For a regular-sized wedding with 100-300 guests, finding the optimal seating chart would take trillions of years, even with the fastest supercomputers.

But a quantum computer with just 300 qubits could represent all 2^300 seating combinations simultaneously in a superposition! By applying a series of quantum logical gates to manipulate this huge collection of states, a quantum algorithm could find the optimal seating arrangement in seconds. This is an exponential speedup over classical methods.

Real-world applications

The wedding example illustrates the main advantage of quantum computers: the ability to perform many calculations in parallel by leveraging superpositions of qubits. But what complex problems could we actually solve with this power?

Some promising real-world applications of quantum computing include:

• Modeling complex molecules for drug discovery: Classical computers have trouble simulating large molecules because the interactions between atoms grow exponentially with the molecule size. Quantum computers could handle this exponential complexity to help design new drugs and materials.

• Solving optimization problems: Many real-world situations involve finding the best solution among an enormous number of possibilities, like optimizing supply chains, financial portfolios, flight schedules, and delivery routes. Quantum computers could search these huge solution spaces much faster.

• Boosting machine learning: Quantum algorithms could speed up the training of machine learning models on large datasets. Quantum computers could also generate more robust and complex models in areas like speech recognition, natural language processing, and recommendation systems.

• Cracking cryptography: Some public-key cryptography methods used today rely on the difficulty of factoring large numbers. Quantum computers could break these encryption schemes by efficiently factoring numbers, which would disrupt cybersecurity. Post-quantum cryptography aims to develop new encryption methods resistant to quantum attacks.

Challenges of building qubits

While quantum computing is very exciting, there are still many challenges in building practical quantum computers. The biggest challenge is that qubits are extremely delicate and easily disrupted by external noise in the environment, causing errors in calculations. Even small vibrations, temperature changes, or stray electromagnetic fields can cause qubits to lose their quantum properties in a process called decoherence.

To maintain superpositions and entanglement, qubits need to be isolated from the environment as much as possible. This requires advanced hardware like superconducting circuits cooled to near absolute zero, vacuum chambers, and precise lasers or microwave generators. Even with this protection, current qubits still have significant error rates that limit the complexity of achievable computations. More robust quantum error correction techniques are an active area of research.

Additionally, to be useful for real-world problems, quantum computers will need to scale up to thousands or millions of qubits. Currently, the most advanced quantum computers have only a few dozen to a few hundred qubits. Wiring up and coordinating that many qubits is a formidable engineering challenge.

State of the industry

Despite the challenges, there has been substantial progress in quantum computing over the past few years. In 2019, Google announced that its 53-qubit quantum processor had achieved "quantum supremacy" by performing a specific calculation faster than the best classical supercomputers (although this claim is disputed by IBM). China recently announced a 66-qubit processor and plans to build a 1-million-qubit system by 2030.

Many tech companies including IBM, Microsoft, Intel, Honeywell, IonQ and startups like Rigetti are developing quantum computers based on different technologies like superconducting circuits, trapped ions, photonics, and topological qubits. Some of these systems are available to researchers and developers through cloud platforms. Open-source quantum software frameworks like Qiskit, Cirq, Q#, and OpenQASM make it easier to write quantum algorithms.

Misconceptions about quantum computing

There are some common misconceptions about the capabilities of quantum computers. Quantum computers are not simply faster versions of today‘s computers. They won‘t speed up all computations, but rather are suited for specific problems in areas like optimization, simulation, and machine learning that have an exponentially large number of solutions.

Quantum computers also don‘t break all encryption schemes, only those that rely on integer factorization or discrete logarithms. Symmetric-key algorithms like AES (used for secure communication) and hash functions (used for digital signatures) are still considered quantum-resistant with large enough key sizes.

Finally, quantum computers are not a replacement for classical computers. Instead, they will likely work in tandem, with classical computers handling most computations and quantum processors called in to accelerate specific complex workloads.

Conclusion

I hope this article gave you a sense of the amazing potential of quantum computers, which leverage the strange properties of quantum mechanics to perform certain computations much faster than regular computers. While there are still many technical challenges to overcome, quantum computing is an exciting field that could transform medicine, finance, logistics, and artificial intelligence in the coming decades. We are still in the early days of this quantum revolution, but the future looks bright!