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Physicists here. In practice, a qubit is a two-level physical system. It can be spin state of an electron, polarization of a photon, lowest two energy states of an atom or an electron in quantum dot/well trap potential, the charge state (called charge qubit) -- whether you have 0 or 1 electrons in it, etc etc (if you have 3 levels, it's called qutrit, and for d levels qudit). This experiment uses charge qubits (a special variant which has some robustness against charge noise by design [by operating at a voltage level which is insensitive to 1st order fluctuations in the electric field, called "sweet spot"], called transmon). The main problem is achieving full control of these systems, which is extremely hard, because there are certain things (some random/stochastic) that you can't control at all and you have to fight+race against their influence: - qubits are tiny, and the energy splitting between these two states are typically minuscule: this means even a small vibration from a sneeze miles away can make the qubit flip. - qubits do not live in vacuum, they are typically hosted in solid-state systems and the qubits are coupled to their hosting environment, which have their own moving parts (two-level fluctuators which lead to charge noise, phonons which also couple to electrons typically via spin-orbit coupling, spinful defects in the material which have their own dynamics, etc etc) that you can't really control. it's extremely difficult to achieve full control of a qubit in the presence of things that you can't control and random in nature. - if the qubit is the lowest two-levels of a system with higher energy levels, one also needs to worry leakage errors to those higher states - there are ways of suppressing the influence of such unwanted interactions (dynamically corrected gates + quantum error correction codes) given that their strength is below certain thresholds. going below those thresholds is again an enormous engineering/material science problem (extremely low temperatures, isolation from vibrations, low/high-pass filters for the classical circuitry which is used to control/drive the qubits via electric/magnetic fields, design of the device itself which typically hosts two-level fluctuators, etc etc). this problem becomes harder in general as you increase the number of qubits though. - to do anything non-trivial, you need to have more than one-qubit and have controllable couplings between those (so you can't put them apart too far which makes it impossible to couple them). this doesn't work perfectly in practice, you can't completely control or turn off their couplings (a problem called cross-talk) which again leads to errors. so it doesn't work quite like modular classical circuit elements which you can "copy & paste" because the abstractions from the low-level, nitty-gritty physics of the underlying material fail for all these qubits. |