Quantum computers are often depicted as incredibly powerful machines, but they do not necessarily compute millions of times faster than traditional computers. Instead, they operate on fundamentally different principles and are only efficient for specific types of problems, meaning they will not replace conventional PCs.
The latest installment of "Quantum & After" delves into concepts like the coin in the air (superposition) and gloves in boxes (entanglement), clarifying that a quantum computer is not simply a "very fast processor."
In Brief
The core of quantum computing lies in its fundamentally different data processing approach, which significantly accelerates calculations. While a classical bit can hold a value of either 0 or 1, a qubit can exist in a superposition of these states. The complete description of a system with N qubits involves 2ⁿ complex amplitudes. The advantage comes not from simple "parallel processing," as often simplified, but rather from controlled interference of amplitudes that enhance the desired solutions.
This computational principle proves effective only for a certain class of problems, such as modeling quantum systems, solving specific optimization functions, and cryptanalysis. Consequently, these devices are highly specialized. Due to the probabilistic nature of measuring a quantum state, the same algorithm must be run multiple times, and programs designed for classical PCs are incompatible with quantum systems.
Why This Matters
A classical bit can be likened to a coin lying on a table showing heads (0) or tails (1). In contrast, a qubit resembles a spinning coin on its edge: while it spins, an observer cannot predict which side it will land on upon stopping. This superposition state is mathematically represented as |ψ⟩ = α|0⟩ + β|1⟩. Once measured, the superposition collapses (the wave function collapses), and the system adopts one of the two fundamental states. Typically, qubits are realized using atoms, ions, photons, or superconducting circuits.
Before measurement, a qubit exists in superposition, meaning its state is a combination of 0 and 1. This does not imply it is simultaneously both 0 and 1 in the traditional sense; rather, it lacks a definite classical value.
Another pivotal phenomenon is quantum entanglement. This is often illustrated using an analogy of gloves placed in two closed boxes: if one finds a left glove in the first box, they immediately know the second box contains the right glove. However, the connection between entangled qubits is far more complex: until measured, they possess no predetermined properties. Their state is described as a single quantum system, regardless of the distance separating them.
The third and frequently underestimated aspect is interference. The popular notion that a quantum computer "checks all possibilities in one step" is a significant oversimplification. Decoding all 2ⁿ states is impossible, as measurement will only yield a single outcome.
The essence of quantum computation is manipulating probability amplitudes: the algorithm is structured to cancel out incorrect answers through destructive interference while amplifying the correct one. This is why quantum speedup is not universal and depends on the specific structure of the problem at hand. For instance, Shor's algorithm provides exponential speedup in factoring and computing discrete logarithms, while Grover's algorithm only achieves quadratic speedup in unstructured search, and simulating molecules and catalysts is a foundational task for such architectures.
These limitations necessitate a cautious approach when discussing "quantum supremacy." Historically, this phenomenon has primarily been demonstrated on synthetic tasks (e.g., sampling random quantum circuits) that lack practical value. Often, following claims of achieving quantum supremacy, new classical algorithms emerge. At present, benchmarks confirm only the viability of physical concepts, not the development of commercially applicable products.
What This Means
When considering investments in quantum technology, it's crucial to focus not just on the number of physical qubits but also on operation accuracy, error rates, and the duration for which qubits can maintain their quantum state.
Currently, quantum computers cannot replace data centers. They serve as accelerators in hybrid configurations with classical supercomputers, tackling narrow tasks ranging from discovering new materials and molecules for pharmaceuticals to optimizing portfolio risks.
For the cryptocurrency market, this indicates that the threat to ECDSA does not stem from abstract computational power, but from the application of Shor's algorithm to compute discrete logarithms. Estimates suggest that breaking a 256-bit elliptic curve requires around 1200 logical qubits. Meanwhile, the requirements for breaking RSA-2048 have decreased over the last six years from 20 million physical qubits to less than 1 million. It is essential to monitor the dynamics of quantum advancements and the proportion of coins on addresses with exposed public keys, rather than being swayed by sensational headlines about new records.
Q-check ForkLog: The Purpose of Quantum Computers
Will they replace PCs?No, they are specialized devices.
What is the advantage?The ability to effectively solve specific problems that classical algorithms cannot address in a reasonable timeframe.
Where can we expect real effects?In quantum chemistry, materials science, catalyst development, optimization, and cryptanalysis.
Will speedup be universal?No. Shor's algorithm offers exponential speedup, Grover's algorithm provides quadratic speedup, and effective solutions for other problems are still lacking.
Is supremacy equal to usefulness?Not yet. Currently, it only demonstrates physical capabilities on artificial tests.
What’s Next?
Quantum computers face a fundamental challenge: qubits lose their quantum state in microseconds. The next issue will explore decoherence and error correction, explaining why hundreds of physical qubits are required to create a single logical qubit and how addressing this issue will determine the industry’s future.
For previous issues, read:
- Can one profit from quantum technologies?
- How blockchains are preparing for the "quantum" era.
- Can the quantum internet be hacked?
- Is the quantum computer a threat to Bitcoin?