To understand squeezed states of light, we first need to change how we think about light. In classical physics, light is a wave that travels through space. In quantum mechanics, however, light is made up of photons—particles of light—but also behaves like a wave.
Now, in the quantum description, even the most “vacuum-like” state of light (completely empty space) isn’t truly empty. There are quantum fluctuations—random tiny variations—present all the time. These fluctuations represent uncertainty in certain properties of the light field, such as its electric and magnetic field components. These uncertainties arise from the Heisenberg Uncertainty Principle, which tells us that some quantities can’t both be known with perfect precision.
2. The Uncertainty Principle in Light
For light, two key properties are involved: the amplitude and phase of the electromagnetic field (or more precisely, two components called quadratures).
The Heisenberg Uncertainty Principle says you can’t simultaneously know both these components with perfect accuracy. In the most basic light state (called the vacuum state), the uncertainties in both components are equal and minimal—this is sometimes visualized as a symmetric circular “blob” of noise in a graph.
But what if you wanted to reduce the uncertainty in one property (say, the amplitude)? You’d then have to increase the uncertainty in the other (the phase). This trade-off is where squeezed states come in.
3. What Are Squeezed States of Light?
A squeezed state of light is a special quantum state where the uncertainty (or “noise”) in one property of the light (say, its electric field at a particular moment) is reduced below the level it would be in a normal vacuum or coherent state. However, this reduction comes at a cost—the uncertainty in the complementary property increases.
Imagine the uncertainty circle being “squeezed” into an ellipse. One direction gets tighter (less uncertainty), and the other expands (more uncertainty). This is why it’s called a squeezed state.
This squeezing doesn’t violate quantum laws. It simply redistributes the quantum noise, optimizing it for certain applications—especially where reduced noise in one direction is critical.
4. Why Squeezed Light Matters
Why go through the trouble of manipulating light in this way? Because squeezed light offers increased sensitivity.
- In quantum measurement, reducing noise in one aspect of the light field allows for more precise detection.
- In quantum communication, squeezed light can be used to enhance the security and fidelity of data transmission.
- In gravitational wave detection, like in the LIGO observatory, squeezed light helps detect extremely faint signals from space that would otherwise be lost in noise.
5. How Is Squeezed Light Produced?
Creating squeezed states of light isn’t easy. It requires specialized equipment and nonlinear optical processes. The most common method is using a nonlinear crystal, through a process called parametric down-conversion or four-wave mixing.
Here’s a simplified version of what happens:
- A strong laser (called a “pump”) is directed into the nonlinear crystal.
- Inside, some of the energy from the laser is converted into pairs of lower-energy photons in such a way that their combined energy and momentum are conserved.
- Because these photons are produced in a correlated way, and because of the special properties of the crystal, the output light emerges with reduced quantum fluctuations in one of its quadratures.
Careful tuning of the crystal and the laser parameters allows experimenters to control the degree and orientation of squeezing.
6. Types of Squeezing
There are different ways to define the squeezing, depending on what aspect of the light you want to reduce uncertainty in:
- Amplitude Squeezing: Reduces fluctuations in the amplitude (or intensity) of the light. Useful for precision intensity measurements.
- Phase Squeezing: Reduces fluctuations in the phase. Essential in applications like laser interferometry, where small phase shifts must be detected.
- Quadrature Squeezing: More general, applies to mathematical combinations of amplitude and phase.
Which type you use depends on the application and the measurement goals.
7. Visualizing Squeezed States
Although we’re avoiding formulas, it’s helpful to imagine the quantum state of light as a fuzzy blob in a two-dimensional plot:
- The horizontal axis represents one property (like amplitude).
- The vertical axis represents its conjugate (like phase).
- A coherent state (typical laser light) appears as a circular blob, equally uncertain in all directions.
- A squeezed state appears as an elliptical blob—compressed in one direction, stretched in the other.
This visualization helps understand what squeezing does: it reshapes the quantum noise.
8. Squeezed Light in Quantum Technologies
Squeezed light is at the heart of many quantum technologies. Some key areas include:
Quantum Metrology
- In ultra-sensitive measurements, such as those in gravitational wave detectors, squeezed light allows us to overcome the standard quantum limit and detect signals that would otherwise be buried under vacuum noise.
Quantum Communication
- Squeezed light is used in continuous-variable quantum cryptography, a form of quantum key distribution.
- Here, information is encoded in the quadratures of light, and squeezing enhances both the security and efficiency of communication.
Quantum Imaging
- Imaging systems using squeezed light can surpass classical limits of resolution and contrast, particularly in biological and low-light imaging scenarios.
Quantum Computation
- Squeezed states are also used in continuous-variable quantum computing, where information is stored and manipulated using quantum properties of light rather than discrete qubits.
9. Challenges with Squeezed Light
Despite its promise, working with squeezed light is technically demanding.
- It is highly sensitive to loss: any absorption or scattering tends to destroy the squeezing effect.
- Environmental noise and vibration can also degrade the quality of squeezed states.
- Generating high levels of squeezing requires precise control of optical components, often in very stable environments like cryogenic systems or vacuum chambers.
Researchers are actively working to develop more robust and scalable methods for producing and using squeezed light in practical devices.
10. Summary: Why Squeezed Light Is a Big Deal
Squeezed states of light are a perfect example of how quantum mechanics can do things that classical physics cannot. By redistributing quantum uncertainty, squeezed light enables higher precision, better security, and enhanced performance in a wide range of advanced technologies.
From the most delicate measurements of gravitational waves to the secure transmission of quantum keys, squeezed light demonstrates the practical power of quantum theory. It also highlights the depth and richness of the quantum world—where even “empty” space contains fluctuating fields, and those fluctuations can be controlled to push the boundaries of what we can measure and understand.