Highly adaptable entanglement: The new technique makes it possible to alter both the flux and the polarization state of the photon pairs simply by changing the orientation of the molecules in the liquid crystal. This can be done either by engineering the sample geometry or applying an electric field.
Researchers in Germany and Slovenia have found a new, more adaptable way of generating entangled photons for quantum physics applications. The technique, which relies on liquid crystals rather than solid ones, is much more tunable and reconfigurable than today’s methods, and could prove useful in applications such as quantum sensing. The usual way of generating entangled photon pairs is in a crystal such as lithium niobate that exhibits a nonlinear polarization response to an applied electric field. When a laser beam enters such a crystal, most of the photons pass straight through. A small fraction, however, are converted into pairs of entangled photons via a process known as spontaneous parametric down-conversion (SPDC). Because energy is conserved, the combined energy and momenta of the entangled photons must equal those of the original photons.
This method is both cumbersome and inflexible, explains team leader Maria Chekhova. “First they grow a crystal, then they cut it in a certain way, and after it’s cut it can only be used in one way,” says Chekhova, an optical physicist at the Friedrich-Alexander Universität Erlangen-Nürnberg and the Max-Planck Institute for the Science of Light, both in Germany. “You cannot generate pairs at one wavelength with one sort of entanglement and then use it in a different way to generate pairs at a different wavelength with a different polarization entanglement. It’s just one rigid source.” In the new work, Chekhova, Matjaž Humar of the Jožef Stefan Institute in Slovenia and colleagues developed an SPDC technique that instead uses liquid crystals. These self-assembling, elongated molecules are easy to reconfigure with electric fields (as evidenced by their widespread use in optical displays) and some types exhibit highly nonlinear optical effects. For this reason, Noel Clark of the University of Colorado at Boulder, US, observes that “liquid crystals have been in the nonlinear optics business for quite a long time, mostly doing things like second harmonic generation and four-wave mixing”.
Generating and modifying entanglement Nobody, however, had used them to generate entanglement before. For this, Chekhova, Humar and colleagues turned to the recently developed ferroelectric nematic type of liquid crystals. After preparing multiple 7-8 μm-thick layers of these crystals, they placed them between two electrodes with a predefined twist of either zero, 90° or 180° between the molecules at either end. When they irradiated these layers with laser light at 685 nm, the photons underwent SPDC with an efficiency almost as high as that of the most commonly used solid crystals of the same thickness. What is more, although individual photons in a pair are always entangled in the time/frequency domain – meaning that their frequencies must be anti-correlated to ensure conservation of energy – the technique produces photons with a broad range of frequencies overall. The team believes this widens its applications: “There are ways to concentrate the emission around a narrow bandwidth,” Chekhova says. “It’s more difficult to create a broadband source.” The researchers also demonstrated that they could modify the nature of the entanglement between the photons. Although the photons’ polarizations are not normally entangled, applying a voltage across the liquid crystal is enough to make them so. By varying the voltage on the electrodes and the twist on the molecules’ orientations, the researchers could even control the extent of this entanglement — something they confirmed by measuring the degree of entanglement at one voltage and twist setting and noting that it was in line with theoretical predictions. Potential extensions The researchers are now exploring several extensions to the work. According to their calculations, it should be possible to use liquid crystals to produce non-classical “squeezed” states of light, in which the uncertainty in one variable drops below the standard quantum limit at the expense of the other. “We just need higher efficiency,” Chekhova says.
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