This is the second part of quantum computing. Will be explained the quantum effects which difficult the construction of a quantum computer.

To see the first part, click in the button below.

**Uncertainty Principle**

The Uncertainty Principle says that it is not possible to measure the speed and the position of a particle at the same time. Can define position or speed. To measure a particle’s properties, must send an electromagnetic wave to interact with the particle. A long wave length (λ) allow to know the particle’s speed, but not the position. While a short wave length allow to know the position, but not the speed.

It happens because in quantum mechanics, particles have dual behavior, sometimes act like wave, sometimes like particle. Below we have the uncertainty equation. h is the Plank constant (6,626 \cdot 10^{-34}J \cdot s), \Delta x_i is the variation of position and \Delta p_i is the variation of linear momentum. The linear momentum pi is the speed multiplied by particle’s mass.

\Delta x_i\Delta p_i\geq \frac{h}{4\pi}

Although can’t measure the position and speed of particles, it is possible to to define the energy of orbitals, which are areas with a probability to find the particle. This figure shows orbitals of an electron from a hydrogen atom, lighter areas show greater probability to find the electron. The d3 orbital has more energy and the s1 has less energy.

It is also possible to measure the spin of particles, therefore, the information must be stored in energy or spin.

**Superposition and decoherence**

Qubits have the superposition property which allows to assume 0 and 1 values. However, the superposition only works if the qubit is completely isolated from external environment. The lowest interference from external environment end the superposition and the qubit have to assume 0 or 1. The act of measure itself end the superposition.

The loss of superposition is called decoherence. The quantum computers need error corrector methods to compensate the decoherence and spread the information to many spins. The qubits need structures and topologies more resistant to noise.