You Zhou, Giovanni Scuri, Dominik S Wild, Alexander A High, Alan Dibos, Luis A Jauregui, Chi Shu, Kristiaan De Greve, Kateryna Pistunova, Andrew Joe, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Mikhail D Lukin, and Hongkun Park. 6/26/2017. “Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons.” Nature Nanotechnology, 12, Pp. 856-860.Abstract

Transition metal dichalcogenide (TMD) monolayers are direct bandgap semiconductors that feature tightly bound excitons, strong spin-orbit coupling, and spin-valley degrees of freedom. Depending on the spin configuration of the electron-hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a new method for probing the optical properties of two-dimensional (2D) materials via near-field coupling to surface plasmon polaritons (SPPs), which selectively enhances optical transitions with dipole moments normal to the 2D plane. We utilize this method to directly detect dark excitons in monolayer TMDs. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly enhances experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials.

Lingbang Zhu, Xianxin Guo, Chi Shu, Heejeong Jeong, and Shengwang Du. 4/17/2017. “Bright narrowband biphoton generation from a hot rubidium atomic vapor cell.” Applied Physics Letters, 110, 16, Pp. 161101. Publisher's VersionAbstract

We demonstrate the generation of high-quality narrowband biphotons from a Doppler-broadened hot rubidium atomic vapor cell. Choosing a double-Λ atomic energy level scheme for optimizing both spontaneous four-wave mixing nonlinear parametric interaction and electromagnetically induced transparency (EIT), we achieve a biphoton spectral brightness as high as 14 000 s−1 MHz−1. Meanwhile, we apply a spatially tailored optical pumping beam for reduction of the Raman noise and obtain a violation of the Cauchy-Schwarz inequality by a factor of 1023.

Chi Shu, Peng Chen, Tsz Kiu Aaron Chow, Lingbang Zhu, Yanhong Xiao, MMT Loy, and Shengwang Du. 9/23/2016. “Subnatural-linewidth biphotons from a Doppler-broadened hot atomic vapor cell.” Nature Communications, 7, Pp. 12783. Publisher's VersionAbstract

Entangled photon pairs, termed as biphotons, have been the benchmark tool for experimental quantum optics. The quantum-network protocols based on photon–atom interfaces have stimulated a great demand for single photons with bandwidth comparable to or narrower than the atomic natural linewidth. In the past decade, laser-cooled atoms have often been used for producing such biphotons, but the apparatus is too large and complicated for engineering. Here we report the generation of subnatural-linewidth (<6MHz) biphotons from a Doppler-broadened (530MHz) hot atomic vapour cell. We use on-resonance spontaneous four-wave mixing in a hot paraffin-coated 87Rb vapour cell at 63°C to produce biphotons with controllable bandwidth (1.9–3.2MHz) and coherence time (47–94ns). Our backward phase-matching scheme with spatially separated optical pumping is the key to suppress uncorrelated photons from resonance fluorescence. The result may lead towards miniature narrowband biphoton sources.

Victor V Albert, Chi Shu, Stefan Krastanov, Chao Shen, Ren-Bao Liu, Zhen-Biao Yang, Robert J Schoelkopf, Mazyar Mirrahimi, Michel H Devoret, and Liang Jiang. 2016. “Holonomic Quantum Control with Continuous Variable Systems.” Physical Review Letters, 116, 14, Pp. 140502. Publisher's VersionAbstract

Universal computation of a quantum system consisting of superpositions of well-separated coherent states of multiple harmonic oscillators can be achieved by three families of adiabatic holonomic gates. The first gate consists of moving a coherent state around a closed path in phase space, resulting in a relative Berry phase between that state and the other states. The second gate consists of “colliding” two coherent states of the same oscillator, resulting in coherent population transfer between them. The third gate is an effective controlled-phase gate on coherent states of two different oscillators. Such gates should be realizable via reservoir engineering of systems that support tunable nonlinearities, such as trapped ions and circuit QED.

Xianxin Guo, Peng Chen, Chi Shu, MMT Loy, and Shengwang Du. 2015. “Frequency-bin entanglement with tunable phase.” Journal of Optics, 17, 10, Pp. 105201. Publisher's VersionAbstract

We describe a technique to produce narrow-band photon pairs with frequency-bin entanglement, whose relative phase can be tuned using linear polarization optics. We show that, making use of the polarization-frequency coupling effect, the phase of a complex polarizer can be transferred into the frequency entanglement.

Peng Chen, Xianxin Guo, Chi Shu, MMT Loy, and Shengwang Du. 2015. “Frequency-induced phase-tunable polarization-entangled narrowband biphotons.” Optica, 2, 5, Pp. 505–508. Publisher's VersionAbstract

Manipulating polarization entanglement of paired photons is always of great interest for understanding the quantum nature of photons and exploring their applications in quantum information processing and quantum communication. Narrowband biphotons with polarization entanglement are especially important for a quantum network based on efficient photon–atom interactions. In most demonstrated cases, the polarization-entangled states are manipulated through the birefringent effect. In this Letter, we produce narrowband polarization-entangled biphotons from spontaneous four-wave mixing in cold atoms and demonstrate a new method to tune the phase of the polarization entanglement by varying the frequency of one of the classical driving lasers. This is achieved through two-photon interference with a path-exchange symmetry. Our result represents a precision control of polarization entanglement from the frequency domain, and may have promising applications in quantum information and precision measurement.

Peng Chen, Chi Shu, Xianxin Guo, MMT Loy, and Shengwang Du. 2015. “Measuring the biphoton temporal wave function with polarization-dependent and time-resolved two-photon interference.” Physical review letters, 114, 1, Pp. 010401. Publisher's VersionAbstract

We propose and demonstrate an approach to measuring the biphoton temporal wave function with polarization-dependent and time-resolved two-photon interference. Through six sets of two-photon interference measurements projected onto different polarization subspaces, we can reconstruct the amplitude and phase functions of the biphoton temporal waveform. For the first time, we apply this technique to experimentally determine the temporal quantum states of the narrow-band biphotons generated from the spontaneous four-wave mixing in cold atoms.

Chi Shu, Xianxin Guo, Peng Chen, MMT Loy, and Shengwang Du. 2015. “Narrowband biphotons with polarization-frequency-coupled entanglement.” Physical Review A, 91, 4, Pp. 043820. Publisher's VersionAbstract

We demonstrate the generation of narrowband biphotons with polarization-frequency-coupled entanglement from spontaneous four-wave mixing in cold atoms. The coupling between polarization and frequency is realized through a frequency shifter and linear optics. When the polarization-frequency degrees of freedom are decoupled, it is robust to create polarization and frequency Bell states, confirmed by the polarization quantum-state tomography and the two-photon temporal quantum beating. Making use of the polarization-frequency coupling to transfer polarization phase retardation to the entangled frequency modes, we produce a frequency Bell state with tunable phase difference between its two bases.