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.
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
The following are some highlights of our research accomplishments:
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).]
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
(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
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
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:
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.]
Near-field scanning optical microscope (NSOM). [Collaborating
Single-photon microscope with STM. [Collaborating with
We are developing advanced bioimaging techniques and apply them to
study various life science problems.
Super-resolution microscopy for bioimaging [Link to the
Imaging Center (SIC)].
Light-sheet microscopy for 4D living cell imaging.
High-resolution optical tweezer [Collaborating with
Toyotaka Ishibashi's group at Division of Life Science].
*** *** ***
Professor Du's wide research interests include:
Atomic, molecular and optical physics.
Light-matter quantum interaction
Quantum information and communication
Laser cooling and trapping
Bose-Einstein condensation (BEC)
Solid state lighting
Nano science and technology
Biophysics - optical microscopy