Which new Measurements Provide a Glimpse of the Quantum Future

A multi-institutional team has created an efficient method for measuring high-dimensional quits encoded in quantum frequency combs, a kind of photon source, on a single optical chip using already available experimental and computational resources.   Despite the fact that the word “quit” may appear to be a typo, this less well-known relative of the quit, or quantum bit, has the ability to carry more data and is more noise-resistant, two crucial characteristics required to enhance the performance of quantum networks, quantum key distribution systems, and eventually the quantum internet. In contrast to traditional computer bits, which classify data as ones or zeros, quits can hold values of one, zero, or both. This is due to superposition, a phenomenon that enables several quantum states to exist simultaneously.                         Quit’s refers to the variety of levels or values that may be encoded on a photon.                      Traditional quits only have two levels, but by adding more levels, they become quiet We’ve always known that it’s possible to encode 10- or 20-level quits or even higher using the colors of photons, or optical frequencies, but the problem is that measuring these particles is very difficult, said Humanhood Lu, a postdoctoral research associate at ORAL. That’s the value of this paper — we found an efficient and novel technique that is relatively easy to do on the experimental side.                            Quits are even more difficult to measure when they are entangled, meaning they share correlations regardless of the physical distance between them. Despite these challenges, frequency-bin pairs — two quits in the form of photons that are entangled in their frequencies — are well suited to carrying quantum information because they can follow a prescribed path through optical fiber without being significantly modified by their environment.    We combined state-of-the-art frequency-bin production with state-of-the-art light sources, and then used our technique to characterize high-dimensional quit entanglement with a level of precision that hasn’t been shown before, said Joseph Likens, a Wigner Fellow and research scientist at ORAL.                                           The researchers began their experiments by shining a laser into a micro-ring resonator — a circular, on-chip device fabricated by EPFL and designed to generate       This powerful photon source takes up 1 square millimeter of space — comparable in size to the point of a sharpened pencil — and allowed the team to generate frequency-bin pairs in the form of quantum frequency combs.      Typically, quit experiments require researchers to construct a type of quantum circuit called a quantum gate. But in this case, the team used an electro-optic phase modulator to mix different frequencies of light and a pulse shaper to modify the phase of these frequencies. These techniques are studied extensively at the Ultrafast Optics and Optical Fiber Communications Laboratory led by Andrew Weiner at Purdue, where Lu studied before joining ORAL.                                                  These optical devices are commonplace in the telecommunications industry, and the researchers performed these operations at random to capture many different frequency correlations. According to Lu, this process is like rolling a pair of six-sided dice and recording how many times each combination of numbers appears — but now the dice are entangled with each other.