Entanglement itself does not entangle light


Natalia Herrera-Valencia and her colleagues successfully entangled the entangled light after passing a 2-meter-long multimode optical fiber. The light returns to its original state. Under the leadership of Mayul Malik, the research team at Heriot-Watt University in Edinburgh used entanglement itself to solve this problem. Also involved in this study is a researcher from the University of Glasgow. A paper recently published in “Nature Physics” describes the research in detail.

Light passing through disordered (or “complex”) media (such as atmospheric fog or multimode optical fibers) scatters in known ways. The result is that although the information carried by light can be retained, it will be distorted. Therefore, additional steps are required to obtain this information. This becomes very tricky when entangled light is transmitted, because the medium disturbs the quantum correlation. The state is “scrambled”, and the original entangled state must be “descrambled” first .


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Entanglement Relief Entanglement

To understand complex media, physicists use a transmission matrix (ie, a two-dimensional array of complex numbers) to predict the result of any matter passing through the medium. Transmission matrix theory, coupled with some key developments in technology, has only recently allowed classical light to propagate through complex media. In this research, the Edinburgh research team extended the concept of transmission matrix to the field of quantum optics.

A property called “channel state duality” allows researchers to use only one quantum entangled state (a pair of photons with related properties) as a probe to extract the complete transmission matrix of the medium. This is different from the classic way of building a matrix. The classic method requires multiple optical probes to pass through the medium to obtain a complete matrix.

Once they know how the medium scrambles the information, Herrera-Valencia and her colleagues can use the same matrix to eliminate the influence of the medium. Here, entanglement once again used a clever trick: unlike descrambling the light passing through the optical fiber, researchers can scramble its “entangled twins” so that they can get the same result without passing through the medium. They use a device called a “spatial light modulator” (SLM) to scramble the light. The device can affect the light field distribution.

Deal with higher dimensions

Compared with two-dimensional qubits, high-dimensional entangled states have greater potential because they can carry more information and are more robust to noise. But these states are also more susceptible to environmental changes.

This research solved a major problem in quantum optics by describing the preservation of six-dimensional entangled states in space. “Qubit entanglement already has this technology, which can handle degrees of freedom that are not affected by the channel (such as polarization). However, when it comes to high-dimensional entangled states, there are many problems with spatial mode encoding,” Malik explained. Simple things like wavefront distortion can also disrupt information.

To create and measure high-dimensional entangled states, a concept often used by physicists is called spatial degrees of freedom. In this study, the research team is based on spatial “pixels.” They divide the continuous location space into discrete regions or pixels. In this way, if a photon is detected in the first pixel in one structure, the entangled twin of this photon should also be detected in the first pixel in another structure. The number of pixels determines the maximum entanglement dimension that may occur in the system. The pixel base point performs very well in terms of quality, speed and dimension. More importantly, because the spatial light modulator can achieve precise and lossless control.

Impact on quantum technology

In addition to increasing the dimensionality of entangled states and solving problems such as dispersion in long fibers, the research team is also exploring how to equate complex channels with quantum states to simplify the measurement of quantum states that carry large amounts of information.

The research team also mentioned in their paper that the technology can even be used to transmit high-dimensional entanglement in dynamic media such as biological tissues. Entangled light can also be sent through two independent channels. Controlling either channel can affect the entire state, and of course it will also affect the other channel. The researchers wrote: “This function may be useful in quantum network scenarios or non-invasive biological imaging. Because in these cases, reaching every part of a complex system may be impractical.”


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