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Daily Quantum Learning #5 - Physical Basis of Quantum Gates

Qubit Representation: A qubit can be physically understood using different systems like:

Superconducting Circuits - Qubits are made from superconducting materials which correspond to different energy levels.

Trapped Ions - Ions are isolated and trapped and then manipulated with lasers. The states of each ion then act as a qubit.

Photons - The polarization of a photon can serve as a qubit.
Spin of Elections or Nuclei - The spin states of electrons and nuclei if measured can represent qubits.

Each of the listed systems has distinct quantum states that can represent |0⟩ and |1⟩, or any superposition of these.

Manipulating Quantum States:

Quantum Gates as Operations - Quantum logic gates change the state of a qubit or multiple qubits through a series of operations. This can be achieve as follows:

Electromagnetic Fields - Superconducting qubits have an electric wave pulse used which changes the energy level of the qubit to perform the operation.

Laser Pulses - For trapped ions or atoms a laser can be tuned to interact at a certain energy level which will perform these operations.

Magnetic Fields - For spin based systems a magnetic field can be applied to influence the spin which serves as a quantum gate operation.

Optical Devices - For photonic qubits a beam splitter or phase shifter can implement these quantum gate operations by manipulating the polarization or the path of photons.

Challenges:

Quantum Coherence and Control - One of the major difficulties is maintaining quantum coherence long enough to perform the operations. This involves using error correction techniques and isolating the qubits from as much environmental noise as possible.

Measurement - After gate operations qubits are measured after collapsing from their superposition state to a definite state. This involves high precision techniques such as charge sensing or fluorescence detection.

Imperfections - Issues like gate errors or cross-talk between qubits can influence a quantum system in the real-world in a manner that is difficult to predict mathematically. This puts further emphasis on the importance of error correction and fault tolerant protocols.

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