# The high stakes race to make quantum computers work

by **Lee Ho Sung**

양자 컴퓨팅의 원리와 현재 시도되고 있는 포획 이온 양자컴퓨팅, 초전도 회로 양자컴퓨팅에 대해 설명한다.

The success of quantum computing depends on whether we can control the strange physics of matter at microscopic scales. First, we have to control the environment precisely. The thick tabletop and legs guard against vibrations from footsteps, nearby elevators, and opening or closing doors. The cylinder is a vacuum chamber. Inside the vacuum chamber, is a smaller, extremely cold compartment, reachable by tiny laser beams. There are ultra-sensitive particles in the chamber that make up a quantum computer.

Is this effort worth it? Theoretically, quantum computers could overcome the limit of classical computer’s power. Traditional computers process data in the form of bits, which has two states labeled zero and one. On the other hand, a quantum computer uses something called a qubit, which can switch between zero, one, and what’s called a superposition. While the qubit is in its superposition, it has a log more information than zero or one.

You can think of these positions as poles on a sphere: the north represents one, and the south represents zero. A bit can only switch between these two poles, but when a qubit is in its superposition, it can be at any point on the sphere. The moment we measure it, the qubit resolves into a zero or a one by the characteristic of quantum. But even though we can’t observe the qubit in its superposition, we can manipulate it to perform particular operations while in this state.

As a problem grows more complicated, a classical computer needs more bits to solve it, while a quantum computer theoretically can handle it with few qubits. The unique properties of quantum computers result from the behavior of atomic and subatomic particles, which have quantum states corresponding to the state of the qubit.

Quantum states are incredibly fragile. That’s why quantum computers need such an elaborate setup, and the power of quantum computers remains mostly theoretical. For now, we can only control a few qubits in the same place at the same time.

There are two key components involved in controlling these fickle quantum states effectively: the types of particles a quantum computer uses, and how it manipulates those particles. For now, there are two leading approaches: trapped ions and superconducting qubits.

A trapped ion quantum computer uses ions as its particles and manipulates them with lasers. A trap made of electrical fields houses the ions. Inputs from the lasers tell the ions what problems to solve by causing the qubit state to rotate on the sphere. For a simplified example, the lasers could input the question: what are the prime factors of 15? In response, the ions may release photons - the state of the qubit determines whether the ion emits photons and how many photons it emits. An image detection system collects these photons and processes them to reveal the answer: 3 and 5.

Superconducting qubit quantum computers do the same thing differently: using electrical circuits instead of an ion trap. The states of each electrical circuits instead of an ion trap. The states of each electrical circuit transrate to the state of the qubit. Electrical inputs in the form of microwaves manipulate them.

Each approach had advantages and disadvantages. Ions in a trap can be manipulated very precisely, and they last a long time. But as more ions are added to a trap, it becomes increasingly difficult to control each precisely. We can’t currently contain enough ions in a trap to do advanced computations. But, connecting many smaller traps that communicate with each other via photons might be one possible solution.

Superconducting circuits, meanwhile, make operations much faster than trapped ions, and it’s easier to scale up the number of circuits in a computer. But the circuits are also more fragile and have a shorter overall life span.

To make an advance of quantum computing is the fight against preserving quantum states. But despite all these obstacles, we’ve already succeeded at making computations in a realm we can’t enter or even observe.

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