The contents of this metal cylinder could either revolutionize technology or be completely useless— it all depends on whether we can harness the strange physics of matter at very, very small scales. To have even a chance of doing so, 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, devoid of all the gases in air. Inside the vacuum chamber is a smaller, extremely cold compartment,  reachable by tiny laser beams. Inside are ultra-sensitive particles that make up a quantum computer. So what makes these particles  worth the effort? In theory, quantum computers could  outstrip the computational limits of classical computers. Classical computers process  data in the form of bits. Each bit can switch between two states  labeled zero and one. 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 lot more information  than one or zero. You can think of these positions as  points on a sphere: the north and south poles of the sphere  represent one and 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. We can’t locate it exactly— the moment we read it, the qubit resolves  into a zero or a one. But even though we can’t observe the  qubit in its superposition, we can manipulate it to perform  particular operations while in this state. So as a problem grows more complicated, a classical computer needs correspondingly more bits to solve it, while a quantum computer will  theoretically be able to handle more and more complicated problems without requiring as many more qubits as a classical computer would need bits. The unique properties of quantum computers result from the behavior of atomic  and subatomic particles. These particles have quantum states, which correspond to the  state of the qubit. Quantum states are incredibly fragile, easily destroyed by temperature  and pressure fluctuations, stray electromagnetic fields, and collisions with nearby particles. That’s why quantum computers need  such an elaborate set up. It’s also why, for now, the power of quantum computers  remains largely theoretical. So far, we can only control a few qubits  in the same place at the same time. There are two key components involved in managing 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. The ions are housed in a trap made  of electrical fields. Inputs from the lasers tell the ions what  operation to make by causing the qubit state  to rotate on the sphere. To use 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 imaging system collects these photons and processes them to reveal the answer: 3 and 5. Superconducting qubit quantum computers do the same thing in a different way: using a chip with electrical circuits  instead of an ion trap. The states of each electrical circuit  translate to the state of the qubit. They can be manipulated with electrical inputs in the form of microwaves. So: the qubits come from either ions  or electrical circuits, acted on by either lasers or microwaves. Each approach has advantages  and disadvantages. Ions 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 with precision. We can’t currently contain enough ions  in a trap to make advanced computations, but one possible solution might be to  connect many smaller traps that communicate with each  other via photons rather than trying to create one big trap. Superconducting circuits, meanwhile, make operations much faster than trapped ions, and it’s easier to scale up the number  of circuits in a computer than the number of ions. But the circuits are also more fragile, and have a shorter overall lifespan. And as quantum computers advance, they will still be subject to the  environmental constraints needed to preserve quantum states. But in spite of all these obstacles, we’ve already succeeded at making  computations in a realm we can’t enter or even observe.