Light can also be compressed Inventory of the application of compressed light in quantum technology

 

The "Nine Chapters" and "Nine Chapters II" optical quantum computing prototypes developed by the team of Pan Jianwei and Lu Chaoyang of the University of Science and Technology of China have achieved "quantum computing superiority" twice in a row. One of the key aspects is the preparation of compressed light.

 

We know that light does not have any volume or mass, can light be compressed? What does the compressed state of light mean in the quantum field, and what is its use? For these questions, today we are going to find out.

 

All this requires us to start with light itself. When we talk about light, we usually refer to visible light. Visible light is an electromagnetic wave of a specific frequency range that a person is physically limited to identify from the electromagnetic spectrum. Electromagnetic waves arise in the quantum realm. For example, electromagnetic waves are released from atoms when electrons leap to lower atomic energy levels. Photons are the fundamental particles of light, and the pattern of motion of photons follows the quantum mechanical model with wave-particle duality.

 

 

The above is only a brief explanation of what light is, we need to understand further, what is the compressed light?

 

01Background of the creation of compressed light

 

In the quantum realm, light that has been compressed and is in a compressed state is compressed light. But to further explain what compressed light is, we need to introduce some knowledge of quantum mechanics first.

 

The vacuum of the universe is not a void of nothing, even the vacuum is full of electromagnetic waves and radiant energy. Each vibration mode of each quantum field of the vacuum vibrates continuously, and this vibration is also called vacuum zero-point oscillation. And due to the interaction between different quantum fields, various virtual particles are created, disappear or transform, which is called quantum rise and fall.

 

If there is a single-mode light wave with definite frequency, definite polarization state and propagation direction, when the modulus and phase angle of its vibration are mutually incompatible operators, according to the uncertainty relation, the quantum coherent state in the fully coherent light condition no longer corresponds to a point in the amplitude plane, but a circular spot. The size of the circular spot is equal to the vacuum undulation rise and fall of the electric field.

 

 

Ordinary light waves go through a superposition of classical light waves and this vacuum upheaval, and this coherence creates a noise field. And scientists study compressed light in order to compress the effect of this quantum rise of vacuum on the measurement. When measuring light waves that have been compressed with noise, this accuracy may exceed the limits of Heisenberg's uncertainty principle. Compressing light, in short, is not about compressing the volume or mass of light, but about compressing noise and compressing uncertainty.

 

Back in 1981, Carlton Caves analyzed the limits of measurement accuracy due to the Heisenberg uncertainty principle. He proposed the idea of using compressed light to overcome the limit, allowing compressed light to be used to improve the sensitivity of laser interferometric gravitational wave detection.

 

02Principle of compressed light

 

We can use a simple mathematical formulation of the optical field as follows.

 

 

Because of the uncertainty principle, in the light field in phase space, the orthogonal amplitude component () and the orthogonal phase component () cannot be measured accurately at the same time, but if we can compress the component in one direction we can get a more accurate measurement in the other direction. It should be noted that because its does not happen in the common understanding of three-dimensional space, but in a special dimension in mathematics or physics. So for ordinary people to understand compression, we can only approximate it as a pat-flat compression of the previously mentioned circular spot, thus reducing the uncertainty.

 

 

In the above figure, when φ= 0, at this time, cos0 = 1,sin0 = 1,so =, the directional component disappears and is compressed; similarly, when φ= π/2, the  is compressed.

 

With the above foundation, we can refer to the two-mode compressed state (EPR entangled state) to further deepen our understanding.

 

 

The above is the theoretical principle of compressed light, so how is it prepared in reality? Scientists usually use devices involving laser-related processing, which look roughly like the figure.

 

 

03Applications of compressed light

 

Because the optical field of compressed light is a continuous variable entangled system with orthogonal amplitude components and orthogonal phase components as quantum variables, it is used in these related quantum fields because of its advantages such as deterministic generation, high efficiency quantum detection, and good compatibility with classical communication systems.

 

1) Quantum precision measurement

 

As we know from the previous article, compressed light was first invented to reduce photon counting noise in optical high-precision measurements, so it is widely used for precision measurements. For example, laser interferometric gravitational wave detectors, which can detect ripples in the bending of space-time caused by the merging of distant black holes, neutron stars, etc. In 2017, American scientists Rainer Weiss, Barry C. Barish and Kip S. Thorne were awarded the Nobel Prize in Physics for their decisive contributions to the LIGO detector (Laser Interferometric Gravitational Wave Observatory) and gravitational wave detection. Nobel Prize in Physics.

 

Since April 2019, all gravitational wave observatories around the world use lasers that produce compressed light as an additional light source. The compressed light overlaps spatially with the traditionally stronger beams in the interferometer arms, producing compressed photon statistics at the photodiode detectors and thus reducing noise.LIGO and Virgo record more than one gravitational wave event per week on average when collecting data, and quantum noise compression improves the signal-to-noise ratio of these events. It also increased the average detection rate of binary neutron star mergers by 50% for LIGO and 20% for Virgo. Discoveries from compressed light also include black hole mergers and binary star systems composed of black holes and neutron stars.

 

Image source: wiki

 

In addition, compressed light has good applications in biological microscopic imaging. 2021, Professor W. Bowen's group at the University of Queensland, Australia, experimentally demonstrated that the signal-to-noise ratio of excited Raman scattering could be improved by 1 dB using the compressed-state light field of the orthogonal amplitude component of picosecond pulses, and quantum-enhanced microscopic imaging of yeast cells was performed.

 

Image source: Quantum-enhanced nonlinear microscopy by Catxere A. Casacio

 

2) Quantum communication

 

I believe that we have seen a lot of news about quantum communication in the past few years. Simply put, EPR entangled light can be generated by the compressed state of light, which can be used for quantum key distribution, invisible transmission, quantum coding and other fields.

 

Image source: nature

 

3) Quantum computing

 

The quantum computing industry is also very hot at present. Quantum computers, due to the special nature of their quantum bits, can quickly solve some big problems of classical computers in some specific mathematical problems, such as large number decomposition, discrete logarithm calculation, etc.

 

For example, the compressed state light field is applied to solve the classical computational puzzle of Gaussian bosonic sampling. Compared with the single photon state, the diversity of the photon number distribution of the compressed state light field itself further enhances the quantum computing advantage of Gaussian bosonic sampling.

 

The following figure shows the principle of the optical path system of the "Nine Chapters" quantum computing prototype: the laser system on the top left generates high peak power femtosecond pulses; the 25 light sources on the left generate 50 single-mode compressed states through the parametric down-conversion process to the 100-mode optical quantum interference network on the right; finally, 100 high-efficiency superconducting nanowire single-photon detectors are used to interfere with the output optical quanta of the interferometer. Finally, 100 high-efficiency superconducting nanowire single-photon detectors are used to detect the output optical quantum states of the interferometer. The "Nine Chapters" successfully detected 76 photons and processed the Gaussian bosonic sampling one trillion times faster than the fastest supercomputers of the time.

 

 

In 2021, the team carried out a series of conceptual and technological innovations based on the "Nine Chapters". The researchers designed and implemented an excited dual-mode quantum compression light source, which significantly improved the yield, quality and collection efficiency of the quantum light source. As a result, the number of photons detected by Juzhang II has increased to 113. Further, by dynamically adjusting the phase of the compressed light, a reconfiguration of the Gaussian bosonic sampling matrix has been achieved, demonstrating the ability to program the Ninth Chapter II to solve mathematical problems with different parameters. The processing speed of "Chapter 9-2" in Gaussian Bose-sampling problem was billion billion billion times faster than the fastest supercomputer at that time.

 

4）Photodetector

 

A photodetector is an optical device that converts light energy into an electrical signal to measure the power of a light beam. In the case of perfect quantum efficiency (100%), this detector should convert every photon of energy of the incident light into a photoelectron.

 

Conventional detectors, however, need to know how many photons hit the surface of the photodetector, i.e. they need to be calibrated for radiance. Compressed light takes advantage of the decoherence sensitivity of the compressed state of light, reducing detection losses and improving the efficiency of the photodetector.