Research

 

Experimental Atom and Quantum Optics Lab

We are exploring fundamentals in the field of atomic, molecular, and optical (AMO) physics. Current research activities focus on experimental and theoretical study of quantum (linear & nonlinear) interaction between single photons and laser-cooled atomic ensembles, as well as developing optical spectroscopy for nano material and structures.

  • Quantum Optics with Cold Atoms

The two most recently developed advanced technologies make the near- and on-resonance nonlinear optical processes possible. The first is laser cooling and trapping of neutral atoms. Because cold atoms with a temperature of about 100 uK have negligible Doppler broadening and very long coherence time, their atomic hyperfine structures can be resolved without need of a Doppler-free setup. The second is the electromagnetically induced transparency (EIT) which not only can eliminate absorption on resonance but also enhance nonlinear interactions at low light level. We make use of these two technologies to generate nonclassical light sources such as narrow-band (~MHz) biphotons from cold atomic ensemble with high optical depth (OD~60-100). These narrow-band biphotons offer a benchmark tool for the fundamental research and potential applications in quantum optics, for example, single photon interferometry, single photon waveform engineering, quantum storage and teleportation, quantum communication that require long coherence time and length. Narrow-bandwidth biphotons with subnatural linewidth are also ideal for interacting with and entangling atomic ensembles.

The following are some highlights of our research accomplishments:

  1. Optical coherent transients.
    (i) Optical precursor. For the first time, we successfully produced freely-standing optical precursors and thus fully verified the theoretical prediction by Sommerfeld and Brillouin in 1914. The observation of optical precursors also verified that the information velocity of a light pulse does not follow the group velocity description and the Einstein causality is not violated in a superluminal medium. Before our study, there is a debate on the existence of optical precursors. Our research does not only make a closure of this debate, but also opens new research opportunities. [Phys. Rev. Lett. 103, 093602 (2009) (highlighted as Editors' Suggestion); Phys. Rev. Lett. 104, 223602 (2010).]
    (ii) Two-photon free-induction decay (FID). We demonstrated, for the first time, the direct observation of two-photon FID without having to use the traditional heterodyne means. We obtain FID signals with a temporal length more than four times longer than the atomic lifetime of the excited state. [Opt. Lett. 35, 1923 (2010).]
    (iii) Theory: from FID to optical precursors. We show, in both theory and experiment, the connection between FID and optical precursors which has been previously considered as two completely unrelated transient phenomena for many decades. [Phys. Rev. A 81, 033844 (2010).]

  2. Narrow-band entangled paired photons with controllable quantum states. Generating single photons with controllable quantum states is of particular interest to quantum communication, quantum information processing and quantum computation. Traditionally, biphotons generated from spontaneous parametric down conversion (SPDC) in nonlinear crystals have very wide bandwidth (> THz) and ultra short coherence time (<ps). Using spontaneous four-wave mixing (SFWM) in cold atoms, we produce narrow-band (~ MHz) biphotons with a long coherence time (0.1-1.0 us). Such a long coherence time allows us access and manipulate the biphoton quantum waveform in time domain directly. Meanwhile, we have also developed a technique for engineering biphoton entanglements in time-energy, polarization, and position-momentum spaces. Our major research outcome in this direction includes:
    (i) Biphoton temporal waveform generator. We have proposed and also experimentally demonstrated a technique for producing biphotons with arbitrary quantum waveform shapes with modulated classical fields. In other words, we have developed a biphoton waveform function generator. [Phys. Rev. A 79, 043811 (2009); Phys. Rev. Lett. 104, 183604 (2010).]
    (ii) Narrow-band hyperentangled paired photon generation. [Phys. Rev. Lett. 106, 033601 (2011).]
    (iii) Nonlinear optical frequency conversion with entangled photon pairs. [Phys. Rev. A 83, 033807 (2011).]                           

  3. Optical Precursor of a Single Photon: a single photon obeys the speed limit. Most recently, we have made a breakthrough in measuring and determining propagation of a single-photon waveform. While classical light propagation has been intensively studied in the past century, the motion of a single photon remains still unclear to the physics community due to the difficulty in understanding correctly its particle-wave duality with limited experimental evidences. Using the heralded single photons with well controlled waveform produced from one cold atomic ensemble, we studied its propagation through a second cold atom cloud whose optical properties can be varied in a wide range. We obtained the first observation of optical precursor of a single photon. Our experimental results indicate that the optical precursor traveling at c is always the fastest part of the single-photon wave packet in any medium. Even in a superluminal medium, there is no probability for a single photon moving faster than c. It thus brought a closure to the long-standing debate on the true speed of information carried by a single photon. Our work was published in Physical Review Letters, selected as Editor’s Suggestion and highlighted as Physics Synopsis. This work was also reported by HKUST press release and many international media. [Phys. Rev. Lett. 106, 243602 (2011)(highlighted as Editors' Suggestion and Physcis Synopsis)]
     

  • Optical Spectroscopy for Nano Material and Structures

Taking the research strength and sources of the department in nano science and technology, my group’s another ongoing research direction is to develop new optical spectroscopy tools for probing nano materials and structures, and study their bulk or local electro-optical properties for possible applications. Currently, we have the following two on-going projects:

  1. Hyperfine spectral study of iodine molecules trapped in nano-size channels of zeolite crystals. [Appl. Phys. Lett. 98, 043105 (2011); Collaborating with Zikang Tang's group.]

  2. Near-field scanning optical microscope (NSOM). [Collaborating with Ning Wang's group.]

  3. Single-photon microscope with STM. [Collaborating with Nian Lin's group]
     

  • Biophysics

We are developing advanced bioimaging techniques and apply them to study various life science problems.

  1. Super-resolution microscope for bioimaging [Link to the Super-resolution Imaging Center (SIC)]

  2. High-resolution optical tweezer [Collaborating with Toyotaka Ishibashi's group at Division of Life Science].

 

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Professor Du's wide research interests include:

  • Atomic, molecular and optical physics.

  • Light-matter quantum interaction

  • Quantum optics

  • Quantum information and communication

  • Quantum imaging

  • Nonlinear optics

  • Atom optics

  • Laser cooling and trapping

  • Bose-Einstein condensation (BEC)

  • Atom chip

  • Superconducting electronics

  • Solid state lighting

  • Nano science and technology

  • Biophysics - optical microscopy

 

 
   

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