Entangled Optical Vortexes Survive "Trial by Fiber"

When it comes to computing, some see the future in quantum mechanics. Over the past two decades, single, short-lived bits of quantum information (called qubits) have grown up into full-blown quantum computers. Technology has matured to the point where it is possible to use quantum computers online via application programming interfaces.

The future is not here yet, though. Quantum computers are not large enough to outcompete their more traditional brethren. At present, most quantum computers are more like registers on which computations are performed rather than a traditional computer with memory and busses that transfer information. This is because transferring quantum information is tough: the states that encode the information are very easy to inadvertently destroy via environmental noise.

In the end, an interconnected set of quantum computers is going to require efficient and reliable quantum information transfer. A team led by Oxenløwe and Sciarrino, from Denmark and Italy, have demonstrated an important step here. They showed that orbital angular momentum (OAM) states of light retain their quantum properties after traveling through an optical fiber.

Why use optical fibers?

Each packet of quantum information must be encoded using just a single photon. To transport the photon, fiber optics are the only practical option. Yet, each photon must travel down an optical fiber without losing its quantum information.

For example, if a qubit is encoded in the polarization state of a photon, then the polarization describes the spatial orientation of the photon's electric field. Quantum information is lost if the fiber modifies the polarization state in an unknown way. This is exactly what ordinary optical fibers do—tiny temperature variations and stresses in the fiber change the polarization unpredictably.

What is orbital angular momentum, and why use it?

The fact that each packet of quantum information must be encoded in a single photon makes each photon very valuable. Using polarization, it is only possible to encode a single qubit, but it could be persuaded to hold much more information.

This is where the use of orbital angular momentum (OAM) comes in. Information must be encoded between two quantum states that are orthogonal to each other. For polarization, that limits us to two states. But, OAM is without limit. The photon can be encoded using polarization plus a number of OAM states called qudits and qutrits.

However, OAM is not preserved in an ordinary optical fiber, where even the slightest bend will destroy the OAM state. Instead, the researchers used an air-core fiber, designed such that, even when the fiber is stressed or bent, the photon cannot scatter into a different OAM mode. To show that quantum states were nicely preserved, the team measured the quality of entanglement between a pair of photons, one of which had travelled down the optical fiber.

Entanglement is a fundamental feature of quantum information. For instance, if two photons are produced by splitting a single photon, then the sum of polarizations of the two photons must add to that of the single photon. That means the polarization states of the two photons are correlated due to their common source. In fact, in terms of the polarization, we cannot consider there to be two independent photons. Instead there is one polarization object that is spread across two photons. This state is very delicate and is easily destroyed.

In the experiments performed by Oxenløwe and Sciarrino's team, one of the photons travels through the optical fiber. Remarkably, the entanglement was preserved with high fidelity. The researchers tested their entangled OAM states against the possibility that they were observing classical correlations and showed that this could not be the case. They had to be quantum.

The researchers also tested to see if they were obtaining a three-way entangled state using both polarization and OAM states, rather than three bilateral entangled states. This test also returned a positive result.

In the end, this is a remarkable achievement, considering that so much can go wrong in terms of preserving quantum states in optical fibers. And five meters, the length of fiber that the researchers used, is already very useful in terms of communication between a quantum memory and a quantum processor. Longer distances will still have to wait on better fibers, unfortunately.

Read the original research article in Advanced Photonics.