Entangled light is unscrambled using entanglement itself

Light speckle
Chaotic speckle: a pattern resulting from light being scrambled by a complex medium such as a multimode optical fibre. (Courtesy: M Malik and S Goel)

Natalia Herrera-Valencia and colleagues have successfully unscrambled entangled light after it has passed through a  2 m long multimode fibre. Led by Mehul Malik, the team at the Heriot-Watt University in Edinburgh tackled the challenge using entanglement itself. The research was done in collaboration with a colleague at the University of Glasgow and is described in a recent paper in Nature Physics.

Light passing through a disordered (or “complex”) medium like atmospheric fog or a multimode fibre gets scattered, albeit in a known manner. As a result, the information carried by the light gets distorted but is preserved, and extra steps are needed to access it. This gets especially tricky for the transport of entangled states of light because the medium muddles up the quantum correlations. The states get “scrambled” and “unscrambling” becomes necessary to retrieve the original entangled states.

Entanglement rescues entanglement

To understand a complex medium, physicists use a transmission matrix, which is a 2D array of complex numbers that predicts the fate of any input going through the medium. The transmission matrix theory, along with some key developments in technology, has only recently enabled propagation of classical light through complex media. In this work, the Edinburgh team has extended the idea of the transmission matrix to quantum photonics.

A property called the “channel-state duality” allows the researchers to use just a single quantum entangled state – a pair of photons that are correlated in their properties – as a probe to extract the entire transmission matrix of the medium. This is different from the classical way of constructing the matrix, where multiple light probes must be sent in through the medium to get the full matrix.

Once they know how the medium scrambles information, Herrera-Valencia and colleagues could undo its effects using the same matrix. Here again, entanglement offers a neat trick: instead of unscrambling light going through the fibre, the researchers can instead scramble its “entangled twin”, that does not go through the medium, to get the exact same results. They scramble light using a device called the spatial light modulator (SLM) which shapes the light field profile.

Dealing with higher dimensions

Compared to 2D qubits, higher dimensional entangled states have great potential because they can carry more information and are more robust to noise. But such states are also much more susceptible to changes by the environment.

Reporting the preservation of six-dimensional entanglement in space, the research tackles a significant challenge in quantum photonics. “Qubit entanglement already has the technology and deals with degrees of freedom [like polarization] that are not affected by the channel. When it comes to high-dimensional states, there are many issues with spatial mode encoding”, Malik explains. Something as simple as wavefront distortion could scramble the information.

To create and measure high-dimensional entangled states, physicists often use the spatial degree of freedom. In this work, the group uses a spatial “pixel” basis. They divide the continuous position space into discrete regions, or pixels, so if a photon is detected at the first pixel for one arm, its entangled twin will be detected at the same pixel in the other arm. The number of pixels determines the maximum dimension of entanglement that is possible in the system. The pixel basis works great in terms of quality, speed and dimensionality, more so because the SLM enables a precise and lossless control.

Implications for quantum technologies

In addition to increasing dimensionality of states and addressing issues like dispersion in longer fibres, the team is exploring how the idea that a complex channel is equivalent to a quantum state can simplify measurement of quantum states carrying a lot of information.

In their paper, the team also mentions that the technique can be used to transport high-dimensional entanglement even through dynamic media like biological tissues. Entangled light can also be sent through two independent channels, where manipulating any channel would affect the whole state, hence the other channel as well. They write, “Such an ability could be useful in quantum network scenarios or for non-invasive biological imaging, where access to all parts of the complex system may be limited”.

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