Nonlinear Optics for Quantum Information and Networking

Summary

Nonlinear optics offer ways to control and engineer the interconnects in a quantum network. In a hybrid quantum network where the quantum nodes operate at different wavelengths, interconnects such as quantum frequency converters or entangled photon pairs allow interfacing between the nodes. Both of these types of devices are based on nonlinear optical effects. We study applications of nonlinear optics to enable improvements to quantum networking, quantum-enhanced sensing and other aspects of quantum information.

Description

Quantum networking and distributed quantum sensing will require photonic links (so called flying qubits) to connect nodes and enable scaling to larger distances and network sizes. An interconnect may consist of a photon going from one node to another where both nodes operate at the same wavelength. However, in a heterogenous network where different nodes have different functions and different wavelengths, more complicated interconnects are needed.

To connect two nodes operating at different wavelengths, there are generally two strategies: (1) use quantum frequency conversion or quantum transduction to convert the wavelength of the photon emitted by one node to the wavelength of the second node, or (2) connect the two nodes using an entangled photon pair source whose two photons have wavelengths that match the two nodes. Both techniques involve nonlinear optics.

Quantum Frequency Conversion 

Figure 1. A 5-cm long PPLN waveguide used for QFC.

We study quantum frequency conversion (QFC) where photons are converted from one optical wavelength to a different optical wavelength. QFC is a type of quantum transduction, where the latter refers to the conversion of quantum information from one format to another (for instance, conversion from an optical photon to a microwave photon). Efficient QFC has been shown in second- and third-order nonlinear materials. Our group typically studies second-order materials for QFC such as periodically poled lithium niobate (PPLN), as shown in Fig. 1.

The strong, classical pump used in QFC can produce unwanted noise photons that are identical in wavelength to the desired transduced photons. We have studied the processes that produce these noise photons. For instance, we showed the noise generation process is temperature dependent. By reducing the temperature of the PPLN crystal, we can reduce rate of production of noise photons (Fig. 2).

Figure 2. Reducing the PPLN temperature reduces (a) the noise or dark count rate, and the (b) total photon counts. Changing the temperature also causes wavelength tuning. The filter in the experimental setup had slightly higher loss at the wavelengths for lower temperatures, leading to (c) decreased system detection efficiency at lower temperatures.

QFC also offers the opportunity to control and engineer the photons in a quantum network. For instance, we designed a PPLN waveguide that can convert input photons to photons at two possible output wavelengths (Fig. 3). Using this dual-channel device, we can route photons to different output paths, which is useful as a switch inside a quantum network.

Figure 3. Experiment setup for a two-channel quantum frequency converter. Signal photons can be routed to either channel 1 or 2 depending on the presence of pump 1 or 2 inside the wavelength. PC, polarization controller; VATT, variable attenuator; WDM, wavelength division multiplexer; EFDA, erbium-doped fiber amplifier; AL, alignment lens; VBG, volume Bragg grating; BPF, bandpass filter; Si APD, silicon avalanche photodetector

Entangled photon pair sources

Entangled photon pairs are produced by the process of spontaneous parametric downconversion (SPDC), where a higher-energy pump photon spontaneously splits into a pair of photons whose energies sum to the energy of the pump photon. The downconverted photon pair can have strong correlations in energy, arrival time, polarization and other degrees of freedom, which may be used for quantum entanglement. We study entangled photon pair sources for use in quantum networking, quantum communications and quantum sensing.

We demonstrated a polarization-entangled photon pair source based on a phase-modulated PPLN crystal. Pumped at 775 nm, the downconverted photons are generated at 1530 nm and 1569 nm. The phasematching conditions for [H1530nm, V1569nm] and [V1530nm, H1569nm] are different (where H and V refer to horizontal and vertical polarizations, respectively), which require two different grating periods. One simple way to generate polarization-entangled photon pairs is to place the two grating periods consecutively one after the other (Fig. 4a). After erasing “which path” information, there is ambiguity in whether the photon pair was produced in the first or second grating, which results in generation of the desired polarization-entangled state. In our work, we modify the poling pattern (using the phase-modulation technique) in the lithium niobate crystal to simultaneously phasematch both downconversion processes in the crystal (Fig. 4b). We observed high polarization-entanglement visibility using the phase-modulated SPDC crystal (Fig. 4c).

Figure 4. Normalized SPDC intensity of the horizontally polarized beam for (a) two consecutive PPLN gratings and (b) a phase-modulated grating. In the latter, both downconversion processes occur simultaneously in a distributed fashion throughout the crystal. (c) Fixing the 1569 nm photon polarization to horizontal (H), diagonal (D) or anti-diagonal (A), we observed the coincidence counts while rotating a polarizer placed before the detector for the 1530 nm photons. There is good polarization entanglement visibility in all three cases.