Entangled photons represent a phenomenon in quantum mechanics where two or more particles become linked. This connection defies classical intuition, as the state of one entangled photon instantaneously influences the state of its partner, regardless of the distance separating them. This profound interconnectedness makes entangled photons a subject of intense scientific study and a potential resource for revolutionary technologies.
The Essence of Entanglement
Quantum entanglement describes a bond between particles where their individual quantum states are interdependent. If two photons are entangled, they exist in a shared, uncertain state until one of them is measured. For instance, if entangled photons are linked by their polarization—the orientation of their electromagnetic field oscillations—knowing the polarization of one immediately reveals the polarization of its partner, even if they are light-years apart. This instantaneous correlation, famously dubbed “spooky action at a distance” by Albert Einstein, highlights the non-local nature of quantum mechanics.
If one photon’s polarization is measured as vertical, its entangled counterpart will simultaneously be found to be horizontal, assuming they were prepared in a state where their polarizations are opposite. There is no classical information transfer between them; rather, their shared quantum state simply collapses for both particles at the moment of measurement.
The core of entanglement lies in the fact that the entangled particles do not possess definite individual properties before measurement. Instead, their properties are probabilistic and correlated. Only upon measurement does a specific outcome materialize for both, dictated by their entangled relationship. This non-separability is a defining characteristic, distinguishing entangled particles from merely correlated classical objects.
Creating and Observing Entangled Photons
Scientists primarily generate entangled photons through a process called spontaneous parametric down-conversion (SPDC). In this method, a high-energy pump photon, typically from a laser, passes through a specific type of nonlinear crystal, such as beta-barium borate (BBO). Within the crystal, the pump photon spontaneously splits into two lower-energy photons, known as the signal and idler photons. These newly created photons emerge from the crystal entangled, often in their polarization state.
The crystal is precisely cut and oriented to facilitate the down-conversion process, ensuring that the conservation laws of energy and momentum are upheld during the photon splitting. The signal and idler photons are then directed along separate paths, often using optical fibers or mirrors, to allow for independent manipulation and measurement. This separation is crucial for demonstrating their non-local correlation.
Verifying entanglement involves performing correlation measurements on the signal and idler photons. For polarization-entangled photons, scientists use polarizing beam splitters (PBSs) and detectors to measure the polarization of each photon. By comparing the measurement outcomes from both detectors, they can establish strong statistical correlations that exceed what is possible with classical physics. For example, if one photon is measured to be vertically polarized, its entangled partner will consistently be found to be horizontally polarized, confirming their shared quantum link.
Unlocking Quantum Technologies
Quantum Key Distribution (QKD)
Entangled photons are used in quantum communication systems, offering security. In quantum key distribution (QKD), entangled photon pairs generate encryption keys that are inherently secure. Any attempt by an eavesdropper to measure or intercept one photon in an entangled pair instantly disturbs its quantum state and consequently its entangled partner, revealing the intrusion and compromising the key.
Quantum Internet
Beyond QKD, entangled photons are foundational for a quantum internet. This future network aims to connect quantum processors and sensors globally, enabling distributed quantum computing and secure communication over vast distances. Entangled photons can act as quantum links, transmitting quantum information between distant nodes, facilitating tasks like quantum teleportation and distributed quantum sensing. Their unique properties allow for the transmission of quantum states rather than just classical bits.
Quantum Computing
Entangled photons serve as quantum bits or “qubits” in quantum computing. While other platforms like trapped ions or superconducting circuits are also explored, photon-based qubits offer advantages in robust coherence and ease of transmission over long distances. Entanglement between multiple photonic qubits is a prerequisite for building powerful quantum computers capable of solving complex problems intractable for classical machines. These systems leverage entanglement to perform parallel computations.
Quantum Sensing and Metrology
Entangled photons also enhance the precision of quantum sensing and metrology. By utilizing the correlated nature of entangled particles, scientists can achieve measurement sensitivities beyond what is possible with classical light sources. This includes applications in highly precise timing, improved imaging techniques, and more accurate measurements of physical phenomena. The enhanced sensitivity offered by entangled light allows for breakthroughs in fields ranging from medical diagnostics to fundamental physics research.
References
Pan, J., Chen, Z., Lu, C. et al. Multiphoton entanglement and its applications in quantum information processing. Rev. Mod. Phys. 84, 777–835 (2012).
Kwiat, P. G., Mattle, K., Schwindt, P. D., & Weinfurter, H. (1995). New High-Intensity Source of Polarization-Entangled Photon Pairs. Physical Review Letters, 75(24), 4337–4341.
Gisin, N., Ribordy, G., Tittel, W. et al. Quantum cryptography. Rev. Mod. Phys. 74, 885–905 (2002).