# Quantum Computer Technology: Architecture, Advantages and Disadvantages

Quantum computing emerging technologies leverage quantum mechanical principles to perform computation, communicate securely, or make precise measurements. Pertaining to computation, specifically, quantum algorithms promise to solve certain classes of very hard problems, including problems that take tremendous time and resources to solve. Some of these problems cannot be solved at all with today’s hardware and algorithms.

Though architectures of quantum computing systems vary wildly, most of them share some common, basic elements:

- Instead of the binary bits used by classical computers, which can only be 0 or 1 at any given time, the quantum bits (qubits) of quantum computers exist in a superposition of states, which encodes information into the probabilities of measurement outcomes
- Quantum computers perform computation by evolving the states of qubits through the application of microwave or laser pulses, referred to as “gates” when using digital quantum computing systems; sequences of gates are referred to as “quantum circuits”
- Because they are probabilistic, not deterministic like classical computers, quantum computers use constructive and destructive interference effects to increase the probability of obtaining correct solutions
- Extreme sensitivity to environmental noise, in all of its many forms, requires the application of quantum error correction (QEC) codes, which suppress, detect, correct, and mitigate errors, and will ensure the reliability of fault-tolerant quantum computation

**Advantages of Quantum Computer Technology**

- Although some quantum algorithms propose modest speed advantages over their classical counterparts, others propose highly-coveted exponential speed advantages, potentially enabling computation that is otherwise unfeasible or impossible
- The information that can be represented by entangled systems doubles with the addition of just one qubit, enabling a tremendous amount of compression that can not only contribute to computational advantages but also to advantages in the storage and transmission of data
- Beyond computation, quantum computers allow the simulation of quantum systems in ways that are otherwise impossible, particularly with the trapping and manipulation of individual or groups of atoms
- Although quantum computers gained notoriety for their potential to break the public cryptographic protocols that secure most communication, they can also generate truly random keys that help provide protection against various threats
- Even in scenarios where quantum algorithms might not outperform their classical equivalents, quantum computing technology consumes less energy than supercomputers, thus offering environmental and economic benefits
- Infrastructure does not necessarily have to be purchased; as noted in the article “Exploring the Advantages of Cloud-Based Quantum Computing,” quantum computers can be used for a fee, and in some cases for free, via the cloud from anywhere in the world with Internet access

An iCONEXT article titled “Quantum Computer Technology” notes experiments using Google’s “Sycamore” and China’s “Jiuzhang” that demonstrate that computational speedups are not just theoretical. Although the selected problems do not have real-world applications, the experiments nonetheless confirm that exponential advantages are at least possible.

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**Disadvantages of Quantum Computer Technology**

There are some disadvantages, as well, but it’s important to note that these can be classified as engineering challenges, as opposed to physics challenges, in that there are no known laws of physics that expressly prohibit these engineering challenges from being solved:

- Although some existing technologies are being applied to quantum computing, others are being newly developed, which introduces all of the various technical challenges associated with developing any new technology
- Quantum information is extremely sensitive to the environment, which not only incurs the aforementioned need for error correction codes, but also for shielding, cooling, and other preventative and protective measures
- Although quantum computers offer great promise for certain quantum computing technology applications, those applications are relatively few in number, especially compared to the broad applicability of classical computers
- Whereas classical laptop computers can be purchased for several hundreds of dollars or euros, the costs to build, operate, and maintain quantum computers is currently considerably higher than that
- Most mentions of scalability refer to the number of qubits available to solve large problems, but another issue is the amount of floor space that is currently required to house those qubits; most quantum computers need tables full of components or large enclosures

The points mentioned above are continually evolving. The pool of quantum computing emerging technologies is expanding. New advantages are being sought. And disadvantages are being mitigated.

## The Foundations of Quantum Computing

Quantum computers are not the only inventions based on human knowledge of quantum mechanics, but they stand out for requiring, until higher-level user interfaces are developed, a certain amount of knowledge about quantum mechanics just to operate. A researcher cannot discover a quantum algorithm without a basic understanding of superposition and entanglement, for example. An Ericsson blog article titled “An introduction to quantum computer technology” provides the following high-level overview:

- Quantization: The states that are referred to as 0 and 1 are energy levels of quantum systems; these energy levels have discrete values, meaning there are no in-between values
- Wave-particle duality: Objects behave as waves while in superposition and as particles when they are measured
- Heisenberg principle: measurement precision is limited due to the inverse relationship between what can be known about a particle’s position and its momentum at the same time
- Superposition: A particle can be in multiple states, but it is not definitely in any of them; the particle has some probability of being in each state
- Entanglement: A quantum system can consist of multiple particles, none of which can be mathematically described independently, but only as the whole system
- No-Cloning: Quantum information can not be precisely copied, but it can be moved by destroying, via measurement, the previous copy of the information
- Measurement: a superposition of states probabilistically collapses into a single state, extracting useful information in binary strings of zeroes and ones
- Quantum State: All possible measurement outcomes have complex probability amplitudes; the probability of measuring any one outcome is the square of its amplitude
- Parallelism: The nature of superposition allows solutions to be computed in parallel, potentially offering speed advantages over classical computation
- Interference: The probability amplitudes of wave functions add constructively and destructively to increase the probability of measuring a correct solution
- Decoherence: qubits are extremely sensitive to environmental noise, therefore reliable computation requires the use of error correction codes

Quantum algorithms must exploit these properties in order to promise speed advantages over classical computation.

## Quantum Algorithms: Shaping the Future of Problem Solving

Quantum algorithms have the potential to disrupt and reshape entire industries by solving problems that are challenging, or even impossible, for classical computers. The use cases under investigation fall into just a few broad categories:

- Cryptography: Quantum computers are best known for their potential to break cryptosystems, but they can also generate truly random keys to help safeguard communication
- Search: Grover’s algorithm offers a quadratic speedup for a broad range of problems, including constraint satisfaction problems
- Simulation: The original inspiration for quantum computers to be built is the simulation of quantum systems themselves; this has applicability in drug design and material science
- Machine Learning: Large state spaces may be able to work with complex data patterns effectively and efficiently, including sampling and the generation of probability distributions
- Optimization: Many real-world problems can be formulated as optimization problems; neutral atoms, in particular, are a natural fit for solving certain classes of optimization problems

It is important to note that many classes of problems are not listed above. There are many problems for which quantum computing will not be advantageous, as well as many for which quantum computing is inefficient or even impractical. But, when quantum algorithms are advantageous, some might be total game-changers.

## Advancements in Qubit Stability and Error Correction

In order for quantum computers to have practical uses, many more qubits are required. But, error-prone qubits will not be enough. Qubits will have to become far more stable, and error correction codes will need to mature.

- Modalities: Quite a few qubit technologies are in development and being researched, each with advantages and disadvantages regarding scalability and coherence
- Error Correction: Research is ongoing in surface codes for the encoding of logical qubits, as well as in concatenated codes, which implement multiple levels of error correction codes
- Error Mitigation: Software tools are already available to analyze errors coming from real hardware and then mitigate those errors algorithmically
- Cryogenics: Most qubits are cooled to near absolute zero, and the technology continues to improve to get signals into and out of these environments
- Metrics: Quantum Volume (QV) is one popular metric that can help indicate qubit stability and, thus, the overall performance of a quantum computer
- More Qubits: Large numbers of physical qubits will be needed in order to create the numbers of logical qubits needed to enable fault-tolerant quantum computation

Advances in these areas will bring about the stability, reliability, and scalability needed to fulfill the great promise of quantum computing.

## Quantum Computing Technology Challenges on the Horizon

Quantum computing faces several significant challenges before its full potential may be realized:

- As previously noted, reducing error rates is going to be critical, and research into how to do this is a field of study in its own right
- Scalability is not just a matter of fabricating a chip with more qubits; these additional qubits must be fully controllable
- Individual chips and vacuum chambers may have scalability limits; interconnections will be required to continue scaling any single device
- Similar to LANs, MANs, WANs, and the Internet, quantum processors will need to be able to reliably interact over long distances
- Quantum algorithms will need to demonstrate computational advantages that can stay achieved in the face of continually improving quantum-inspired algorithms
- Though not all modalities are expected to survive into the future, standardization and interoperability will be required among those that do
- Due to potential threats, even with emerging precautions in cryptography and key distribution, ethical and security concerns need to be addressed as hardware matures

These challenges are complex and interrelated. Addressing them requires collaboration among researchers, engineers, and policymakers.