A biphoton refers to a pair of photons that are quantum mechanically linked. Their properties are correlated in a way that goes beyond classical physics.
Biphotons are defined by quantum entanglement. This phenomenon means the quantum state of each photon in the pair cannot be described independently of the other, regardless of the distance between them. For instance, if one entangled photon’s polarization is measured as horizontal, the other photon’s polarization will instantaneously be known, often as vertical, even if they are far apart.
This correlation extends to various properties, including momentum, energy, and time of arrival. The measurement of one photon’s property seems to instantaneously influence the other, a concept Albert Einstein called “spooky action at a distance.”
Creating Biphotons
The primary method for generating biphotons is Spontaneous Parametric Down-Conversion (SPDC). This technique involves a high-energy “pump” photon interacting with a nonlinear crystal. As the pump photon passes through the crystal, it can spontaneously split into two lower-energy photons, forming the biphoton pair.
In this process, the original pump photon’s energy and momentum are conserved and distributed between the two newly created “daughter” photons. While SPDC is the most common method, other techniques exist, such as using quantum dots or excited atoms to produce entangled photon pairs.
Applications of Biphotons
Biphotons, with their unique quantum correlations, are powerful tools across various fields of quantum technology. Their applications leverage entanglement to achieve capabilities not possible with classical light sources.
Quantum Computing
In quantum computing, entangled photons can serve as qubits, the fundamental units of quantum information. Unlike classical bits that are either 0 or 1, qubits can exist in a superposition of both states simultaneously, and entanglement allows for complex relationships between multiple qubits. This property is harnessed in photonic quantum computers, where the entangled state of biphotons can encode and process information.
Building on the concept of qubits, entangled photons facilitate quantum gates, which are the building blocks of quantum circuits. These gates manipulate the quantum states of qubits to perform computations. By exploiting the non-classical correlations of biphotons, researchers can design and implement sophisticated algorithms that could solve problems intractable for even the most powerful supercomputers, including tasks like factoring large numbers and simulating complex molecular structures.
Quantum Communication and Cryptography
Biphotons are important for secure quantum communication, particularly in Quantum Key Distribution (QKD). QKD protocols utilize the entanglement of biphotons to create cryptographic keys that are inherently secure. Any attempt by an eavesdropper to intercept the key would disturb the entangled state, immediately alerting legitimate users to the intrusion. This “non-clonability” of entangled states ensures communication remains private and tamper-proof.
The secure transmission of information relies on the integrity of the quantum link. Biphotons enable the distribution of these secure keys over significant distances, with ongoing research focused on extending these ranges. Fiber optic cables are commonly used for terrestrial QKD, while satellite-based systems are being explored to overcome distance limitations and enable global quantum networks. The robust nature of biphoton entanglement is what underpins this high level of security.
Quantum Metrology
Quantum metrology leverages biphotons to achieve enhanced precision in measurements, surpassing the limitations of classical physics. One notable application is super-resolution imaging, where entangled photons can resolve details beyond the diffraction limit that typically restricts conventional microscopes. This capability allows for imaging with greater clarity and finer detail, beneficial in fields like biology and materials science.
Beyond imaging, biphotons are used in enhanced sensing applications. Their quantum correlations can improve the signal-to-noise ratio in various measurement setups, leading to more sensitive detection of subtle changes in physical parameters. This includes measurements of magnetic fields, temperature, or even gravitational waves, where minute disturbances need to be accurately detected. The unique quantum properties of biphotons offer a pathway to develop highly sensitive sensors.
Quantum Imaging
Quantum imaging utilizes biphotons to create images in challenging environments or with very low light levels. Ghost imaging, for example, is a technique where an image is formed by correlating two light paths: one that illuminates the object and another that never interacts with it. The entangled nature of the biphoton pair allows for the reconstruction of an image even if the light interacting with the object is never directly detected by the imaging sensor.
This approach is particularly useful in situations where the object is fragile or sensitive to light, or when imaging through scattering media like biological tissue. By separating the illumination and detection paths, quantum imaging with biphotons can minimize the light exposure on the object while still acquiring high-quality images. The ability of biphotons to maintain their correlations over distance and through different environments makes them uniquely suited for these specialized imaging tasks.