Quantum computers are amazing machines that can solve problems that are impossible for ordinary computers. They use the strange laws of quantum physics to manipulate bits of information called qubits, which can be in two states at once, or entangled with each other across distances.
But building a quantum computer that can do anything is very hard. Scientists have to overcome many challenges, such as keeping the qubits stable and errorfree, and finding ways to connect them and control them.
In this article, we will explore some of the latest advances and breakthroughs in quantum computing, and how they could lead to the ultimate goal of a universal quantum computer.
What is a universal quantum computer?
A universal quantum computer is a quantum computer that can perform any computation that is possible in principle, using a finite number of qubits and operations. It is like a Swiss army knife of computing, with a tool for every task. A universal quantum computer could solve problems that are too complex or timeconsuming for classical computers, such as factoring large numbers, simulating quantum systems, optimizing complex functions, or breaking encryption codes.
However, not all quantum computers are universal. Some are designed for specific purposes, such as quantum annealers, which use quantum fluctuations to find the lowest energy state of a system, or noisy intermediatescale quantum (NISQ) devices, which have a limited number of qubits and operations, and are prone to errors and noise. These quantum computers can still perform some useful tasks, such as sampling, machine learning, or chemistry simulations, but they cannot do everything that a universal quantum computer can.
How to build a universal quantum computer?
There are many ways to build a universal quantum computer, using different physical systems to encode and manipulate qubits. Some of the most popular ones are:

Superconducting qubits: These are tiny circuits made of
superconducting materials, which have zero electrical resistance at very low
temperatures. They can act as artificial atoms, with two energy levels that
correspond to the qubit states. Superconducting qubits can be coupled and
controlled with microwave or flux signals, which are electromagnetic pulses
that can change the energy levels of the qubits.
They are the basis of most of the current quantum computers, such as those from IBM, Google, and Rigetti. 
Trapped ions: These are atoms that have lost or gained one or more
electrons, and are trapped in a vacuum chamber by electric and magnetic
fields. They can store qubits in their electronic, vibrational, or hyperfine
states, which are the states of the nucleus and its interaction with the
electrons.
Trapped ions can be manipulated with lasers and detected with cameras. They are the basis of some of the most precise and stable quantum computers, such as those from IonQ and Honeywell. 
Photonic qubits: These are particles of light that can encode qubits
in their polarization, frequency, or timebin states. Photonic qubits can
travel long distances through optical fibers or free space, and interact
with each other through beam splitters and phase shifters.
Photonic qubits can be generated and measured with lasers and detectors. They are the basis of some of the most scalable and secure quantum computers, such as those from Xanadu and PsiQuantum.
There are also other ways to build a universal quantum computer, such as using nuclear magnetic resonance (NMR), diamond defects, silicon spin qubits, topological qubits, or neutral atoms. Each of these approaches has its own advantages and disadvantages, such as scalability, coherence time, error rate, connectivity, or complexity. Scientists are still exploring which one is the best for building a universal quantum computer.
What are the challenges and breakthroughs?
Building a universal quantum computer is not easy. It requires overcoming many technical and theoretical challenges, such as:

Qubit quality: Qubits are very fragile and sensitive to their
environment. They can lose their quantum state or get entangled with
unwanted particles due to noise or interference.
This causes errors and decoherence in the computation. To prevent this, scientists have to isolate the qubits from external sources of noise, such as heat or electromagnetic radiation, and use error correction techniques to detect and correct errors.  Qubit quantity: Qubits are also very scarce and expensive to produce. To perform complex computations, scientists need millions or billions of qubits, but so far they have only managed to create hundreds or thousands of them. To increase the number of qubits, scientists have to find ways to make them smaller, cheaper, and more reliable.

Qubit connectivity: Qubits also need to communicate with each other
to perform operations and share information. This requires creating
connections or links between them, either physically or virtually. However,
connecting qubits is not trivial, as it can introduce noise or errors, or
limit the scalability or flexibility of the system.
To improve the connectivity of qubits, scientists have to find ways to create more and better links, such as using quantum buses, networks, or teleportation.
Despite these challenges, scientists have made remarkable progress and breakthroughs in quantum computing in recent years, such as:

Quantum supremacy: In 2019, Google claimed to have achieved quantum
supremacy, which means that their quantum computer performed a task that
would be impossible for a classical computer in a reasonable time. They used
a 53qubit superconducting device called
Sycamore
to sample from a random quantum circuit in 200 seconds, which they estimated
would take 10,000 years for a stateoftheart supercomputer.
However, this claim was disputed by IBM and others, who argued that the task was not useful or realistic, and that the classical computation could be done faster with better algorithms or hardware. 
Quantum advantage: In 2020, Chinese researchers claimed to have
achieved quantum advantage, which means that their quantum computer
performed a task that is much faster or cheaper than a classical computer.
They used a 76photon device called
Jiuzhang
to sample from a boson sampling problem in 200 seconds, which they estimated
would take 2.5 billion years for the world’s fastest supercomputer.
However, this claim was also challenged by some experts, who questioned the validity or relevance of the problem, and the accuracy or scalability of the device. 
Quantum error correction: In 2021, IBM researchers demonstrated for
the first time a quantum error correction protocol on a 7qubit
superconducting device called IBM Quantum Eagle, which is part of the IBM
Quantum fleet of processors. They used a technique called surface code to
encode one logical qubit into four physical qubits, and correct any errors
that occurred during the computation.
They achieved a logical qubit fidelity of 85%, which is higher than the physical qubit fidelity of 79%. This means that the error correction improved the quality of the qubit. However, this protocol is still far from practical, as it requires thousands of physical qubits to encode one logical qubit.
What is the future of quantum computing?
Quantum computing is still in its infancy, but it has enormous potential and promise for the future. Scientists are working hard to overcome the challenges and achieve the breakthroughs that will enable them to build a universal quantum computer that can do anything. Some of the possible applications and impacts of quantum computing are:

Cryptography: Quantum computers could break some of the most widely
used encryption schemes, such as RSA or ECC, which rely on the hardness of
factoring large numbers or finding discrete logarithms. This could pose a
threat to the security and privacy of online communication and transactions.
However, quantum computers could also enable new forms of encryption, such as quantum key distribution (QKD), which uses quantum entanglement to generate and share secret keys that are provably secure against eavesdropping. 
Simulation: Quantum computers could simulate complex quantum systems,
such as molecules, materials, or particles, with high accuracy and
efficiency. This could lead to new discoveries and innovations in fields
such as chemistry, physics, biology, or medicine.
For example, quantum computers could help design new drugs, materials, or catalysts; understand photosynthesis, superconductivity, or dark matter; or test quantum theories or hypotheses. 
Optimization: Quantum computers could optimize complex functions or
problems, such as traveling salesman, knapsack, or scheduling problems,
which involve finding the best solution among many possible ones.
This could improve the performance and efficiency of various systems and processes in fields such as logistics, manufacturing, engineering, or finance. For example, quantum computers could help optimize routes, schedules, portfolios, or resources; minimize costs, risks, or emissions; or maximize profits, outputs, or satisfaction. 
Machine learning: Quantum computers could enhance machine learning
algorithms and models, such as neural networks, support vector machines
(SVMs), or kmeans clustering. This could improve the speed and accuracy of
data analysis and processing in fields such as computer vision, natural
language processing (NLP), or recommender systems.
For example, quantum computers could help classify images, translate languages, or generate recommendations.
Conclusion
Quantum computing is an exciting and fascinating field that is changing the world of computing and beyond. It is also a challenging and complex field that requires a lot of research and development. By understanding how to make a quantum computer that can do anything, we can unlock new possibilities and opportunities for science, technology, and society.
These are some of the most significant breakthroughs in quantum computing that have happened in recent years, but there are still many more to come. Quantum computing is a rapidly evolving field that promises to transform many aspects of science, technology, and society.
What do you think of these achievements and challenges? Do you have any questions or opinions about quantum computing? Share your thoughts in the comments below.