Wave function collapse in the context of Observation (physics)

Quantum entanglement is the phenomenon wherein the quantum state of each particle in a group cannot be described independently of the state of the others, even when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical physics and quantum physics: entanglement is a primary feature of quantum mechanics not present in classical mechanics.

Measurements of physical properties such as position, momentum, spin, and polarization performed on entangled particles can, in some cases, be found to be perfectly correlated. For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on a first axis, then the spin of the other particle, measured on the same axis, is found to be anticlockwise. This behavior gives rise to seemingly paradoxical effects: any measurement of a particle's properties results in an apparent and irreversible wave function collapse of that particle and changes the original quantum state. With entangled particles, such measurements affect the entangled system as a whole.

Quantum cryptography is the science of exploiting quantum mechanical properties such as quantum entanglement, measurement disturbance, no-cloning theorem, and the principle of superposition to perform various cryptographic tasks. Historically defined as the practice of encoding messages, a concept now referred to as encryption, quantum cryptography plays a crucial role in the secure processing, storage, and transmission of information across various domains.

One aspect of quantum cryptography is quantum key distribution (QKD), which offers an information-theoretically secure solution to the key exchange problem. The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic tasks that are proven or conjectured to be impossible using only classical (i.e. non-quantum) communication. Furthermore, quantum cryptography affords the authentication of messages, which allows the legitimates parties to prove that the messages were not wiretapped during transmission. For example, in a cryptographic set-up, it is impossible to copy with perfect fidelity, the data encoded in a quantum state. If one attempts to read the encoded data, the quantum state will be changed due to wave function collapse (no-cloning theorem). This could be used to detect eavesdropping in QKD schemes, or in quantum communication links and networks. These advantages have significantly influenced the evolution of quantum cryptography, making it practical in today's digital age, where devices are increasingly interconnected and cyberattacks have become more sophisticated. As such quantum cryptography is a critical component in the advancement of a quantum internet, as it establishes robust mechanisms to ensure the long-term privacy and integrity of digital communications and systems.

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum measurements are physically realized in different "worlds". The evolution of reality as a whole in MWI is rigidly deterministic and local. Many-worlds is also called the relative state formulation or the Everett interpretation, after physicist Hugh Everett, who first proposed it in 1957. Bryce DeWitt popularized the formulation and named it many-worlds in the 1970s.

In modern versions of many-worlds, the subjective appearance of wave function collapse is explained by the mechanism of quantum decoherence. Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s. MWI is considered a mainstream interpretation of quantum mechanics, along with the other decoherence interpretations, the Copenhagen interpretation, and hidden variable theories such as Bohmian mechanics.