From: mk_thisisit
Very interesting experiments are being stimulated by attempts to build quantum computers [00:37:37]. While it’s sometimes said that we already have quantum computers, they are not yet available for general purchase and it will take some time for them to be [00:41:04]. The endeavor to build them is described as very ambitious [00:46:27].
Decoherence: The Main Obstacle
The primary engineering challenge and main problem for quantum computers is decoherence [02:49:09].
Understanding Decoherence
Decoherence is the process that “deprives” a quantum system of its coherence [00:04:53]. In quantum mechanics, an object can exist in multiple states simultaneously, known as a quantum superposition [00:03:07]. For example, a glass could be both “here and there” [00:00:00], or a coin could be both “obverse and reverse” [00:03:34]. These possibilities are considered coherent [00:04:43].
However, when information about a quantum state is measured or transferred to the environment, that quantum state ceases to exist [00:07:00]. This measurement doesn’t need to be by a person or device; any correlation that transfers information from an object to its environment (including observers like us) acts as a measurement [00:05:52].
For instance, the environment, such as photons and air particles, “knows” where a glass is, and we know that they know because we observe it [00:06:18]. This transfer of information causes the quantum system to cease to be in a superposition, leading to the disruption of coherence – this is decoherence [00:06:34]. Once information about where the glass is transferred, it prevents the glass from being in multiple places simultaneously; it must “decide” its location [00:06:42].
Impact on Quantum Computers
The challenge for quantum computers arises because if a quantum computer needs to have hundreds of “kits” (quantum bits or qubits), this scale makes it extremely difficult for the system to survive as a controlled quantum system, as the environment becomes very difficult to manage [00:24:55].
There is a fundamental contradiction that must be overcome experimentally in the design of quantum computers:
- Strong Internal Interaction: The different quantum systems (qubits) that perform calculations need to interact strongly with each other for the computer to advance quickly [00:25:39].
- Weak External Interaction: However, if they interact strongly with each other, they will also try to interact, even weakly, with the outside environment. Even very weak external interaction can and does derail quantum processes necessary for quantum computers [00:25:52].
Objects that are very small are easier to isolate, allowing them to maintain superposition for longer [00:19:45]. Larger objects, like those we perceive daily, cannot maintain superposition because their interaction with the environment is constant and unavoidable [00:19:53].
Alternative Approaches and Simulation
A slightly different and interesting approach to quantum computers involves setting different tasks. Instead of aiming for a quantum computer that functions like a quantized version of a classical computer, researchers can try to simulate or emulate various complex quantum systems [00:26:16]. This area shows more promising and interesting results, connecting quantum mechanics with solid-state physics [00:26:44]. For example, such quantum computers can simulate phase transitions, like those in liquid Helium [00:45:11].