To understand quantum coherence, we need to first explore classical coherence. Coherence is a term that describes how predictably waves behave over time and space. If two light waves are coherent, they maintain a stable phase relationship. This means they rise and fall together in a synchronized manner.
In optics, coherence comes in two types:
- Temporal Coherence: How predictable the phase of a wave is over time. A laser, for instance, can maintain coherence over long periods, while sunlight loses it quickly.
- Spatial Coherence: How well the phase relationship is maintained across different points in space—like across the beam of light.
2. What Is Quantum Coherence?
Quantum coherence takes this idea and applies it to quantum states. In quantum mechanics, systems can exist in superpositions—being in multiple states at once. Quantum coherence refers to the ability of a quantum system to maintain these superpositions over time without being disrupted.
When a system is coherent:
- It exhibits clear quantum behavior.
- Interference patterns and correlations between parts of the system are visible.
- It can perform tasks like quantum computing or secure communication.
In quantum optics, coherence is the bridge between classical light and truly quantum states of light.
3. Light as a Quantum Object
Light behaves as both a wave and a particle. In quantum optics, we treat light as a stream of photons, and we can describe the state of these photons using quantum states.
These states can be:
- Coherent states (like laser light): The closest to classical behavior.
- Fock states (specific number of photons): Purely quantum, with no classical equivalent.
- Entangled states: Where photons are linked in such a way that one photon’s state depends on the other’s.
- Squeezed states: Where uncertainty is reduced in one variable at the expense of another.
Whether and how these photons interact, interfere, and evolve depends on the coherence of the system.
4. Coherence in Quantum Optics
In quantum optics, coherence refers to how pure and stable the quantum state of the light field is, especially in terms of superposition and phase relationships. This determines the degree to which the light shows quantum interference and behaves in non-classical ways.
A coherent quantum state:
- Can exhibit interference when overlapped with itself or other states.
- Maintains quantum correlations over time and space.
- Is necessary for quantum technologies like quantum key distribution or quantum computing.
5. Sources of Quantum Coherence in Optics
Quantum coherence in optics usually originates from specific sources:
- Lasers: Produce coherent light by stimulated emission. While mostly classical in nature, lasers are the starting point for creating more exotic quantum states.
- Spontaneous Parametric Down-Conversion (SPDC): A nonlinear optical process where one photon splits into two lower-energy entangled photons—often used to produce coherent, entangled photon pairs.
- Single-Photon Sources: Devices that emit one photon at a time with a well-defined phase and timing.
- Quantum Dots and Cavities: Can be engineered to emit highly coherent quantum light.
6. How Coherence Manifests in Experiments
In experiments, quantum coherence is seen through interference patterns and correlations that go beyond classical predictions.
Interference
When two coherent light waves overlap, they can interfere constructively or destructively. In quantum optics, this can even happen with single photons, producing effects like the Hong–Ou–Mandel dip, where two identical photons always leave a beam splitter together.
Entanglement Correlations
Quantum coherence allows the creation of entangled photon pairs, which show correlations that violate classical limits (Bell’s inequalities). These correlations are a direct result of quantum coherence.
Quantum Superposition
In a double-slit experiment with single photons, quantum coherence allows each photon to “interfere with itself,” creating a wave-like interference pattern even though photons arrive one at a time.
7. Quantum Decoherence: The Enemy of Coherence
Quantum coherence is fragile. When a quantum optical system interacts with its environment—like heat, vibrations, or electromagnetic noise—it can lose coherence. This process is called decoherence.
When coherence is lost:
- The quantum state becomes mixed or classical.
- Interference disappears.
- Quantum advantages (like speedups in computation or unbreakable encryption) are lost.
Preserving coherence is one of the biggest challenges in all quantum technologies.
8. Applications of Quantum Coherence in Optics
Quantum coherence isn’t just an interesting concept—it’s a fundamental resource for many modern technologies:
Quantum Computing
Quantum bits (qubits) rely on superpositions and entanglement. Coherence allows them to carry out complex calculations in parallel. Without coherence, a quantum computer becomes no better than a classical one.
Quantum Communication
Protocols like quantum key distribution (QKD) rely on coherent quantum states. If coherence is broken, the security of the communication is compromised.
Quantum Imaging and Metrology
Using coherent quantum states (like squeezed states), we can surpass classical limits of resolution and precision, improving telescopes, microscopes, and clocks.
Quantum Simulation
By maintaining coherence in a controlled system, researchers can simulate complex quantum systems like molecules or condensed matter systems.
9. Tools to Measure and Control Coherence
Controlling and measuring quantum coherence requires sophisticated techniques:
- Interferometers: Devices that split and recombine light to detect phase differences.
- Photon Correlation Measurements: Look at how photons are correlated over time—used in Hanbury Brown–Twiss setups.
- Quantum State Tomography: Reconstructs the full quantum state to analyze coherence.
- Cavity QED and Optomechanics: Help to isolate and manipulate photons with minimal loss of coherence.
10. The Future of Quantum Coherence in Optics
Quantum coherence in optics is a vibrant and rapidly advancing field. As scientists improve control over light and its interaction with matter, we move closer to real-world quantum technologies.
Future directions include:
- Integrated Quantum Circuits: On-chip systems that maintain coherence for scalable quantum computing.
- Quantum Internet: Using coherent photons to send quantum information over long distances.
- Ultra-Precise Sensors: Leveraging coherent quantum states for navigation, medicine, and fundamental science.