# Zhe-Yu Jeff Ou, Ph.D.

Professor, Department of Physics## Education

- 1990 Ph.D. Physics, University of Rochester.
- 1986 M.A. Physics, University of Rochester.
- 1984 B.S. Physics, Beijing University, Beijing, China.

## Research

My research mainly concerns the quantum behavior of light with emphasis on multi-photon interference effects and quantum entanglement, quantum noise, precision measurement of phases, and interaction of photons with an collective ensemble of atoms for quantum state transfer. My recent emphasis is on quantum state transfer in frequency conversion via Raman process and parametric down-conversion process.

Multi-photon interference effects are direct manifestation of quantum entanglement and are often used to confirm the establishment of quantum entanglement in a quantum state. The interference arises from multiple indistinguishable paths for all the photons involved as a whole. In this case, the photons as one entity interfere with themselves, thus extending Dirac's famous single-photon interference statement. Indistinguishability among all the photons involved is critical for the multi-photon interference. It turns out that indistinguishability for multi-photon case is somewhat similar to the concept of coherence in traditional optical interference. It can be quantified via multi-photon interference experiment.

Fluctuations in electromagnetic fields exist even in vacuum due to the Heisenberg uncertainty principle. This noise is responsible for a number of quantum phenomena of light including spontaneous emission. However, quantum theory does not prevent us from suppressing this noise. Thus, there exists fundamental quantum noise in basically every measurement process. I am particularly interested in the quantum noise in phase measurement. It turns out that quantum noise leads to a fundamental limit, i.e., the Heisenberg limit, in the precision of phase measurement. The Heisenberg limit is the ultimate limit and is pretty hard to achieve. By far, only a few known quantum states can achieve this limit. There is another limit associated with phase measurement using classical sources. This is the so-called standard quantum limit, which can be surpassed with quantum sources such as squeezed state and number entangled state (NOON state).

A parametric down-conversion process is a nonlinear optical process, in which a higher energy photon is split via nonlinear interaction with media into two lower energy photons. This process is responsible for a number of novel quantum states, such as the two-photon state, the squeezed state, and EPR entangled state, etc. Raman process in a collective atomic medium is another interesting process that can lead to novel quantum systems. It entangles photons with atoms. Four-wave mixing in optical fiber is another process that is very similar to a parametric down-conversion process and may be used to produce quantum states in telecom wavelength. All the above mentions processes are used in our experimental study of quantum optics.

## Publications & Professional Activities

C. K. Hong, Z. Y. Ou, and L. Mandel; "Measurement of Subpicosecond Time

Intervals between Two Photons by Interference", Phys. Rev. Lett. 59, 2044

(1987).

Z. Y. Ou, S. F. Pereira, H. J. Kimble, and K. C. Peng; "Realization of the

Einstein-Podolsky-Rosen Paradox for Continuous Variables", Phys. Rev. Lett. 68,

3663 (1992).

Z. Y. Ou; "Complementarity and Fundamental Limit in Precision Phase

Measurement", Phys. Rev. Lett. 77, 2352 (1996).

Z. Y. Ou, "Temporal Distinguishability of an N-Photon State and Its

Characterization by Quantum Interference", Phys. Rev. A 74, 063808 (2006).

G. Y. Xiang, Y. F. Huang, F. W. Sun, P. Zhang, Z. Y. Ou, and G. C. Guo,

"Demonstration of Temporal Distinguishability in a Four-Photon State and a

Six-Photon State", Phys. Rev. Lett. 97, 023604 (2006).

F. W. Sun, B. H. Liu, Y. X. Gong, Y. F. Huang, Z. Y. Ou, and G. C. Guo,

"Stimulated Emission as a result of Multi-Photon Interference", Phys. Rev.

Lett. 99, 043601 (2007).

Z. Y. Ou, "Efficient conversion between photons and between photon and atom by

stimulated emission", Phys. Rev. A 78, 023819 (2008).

Xiaoying Li, Lei Yang, Xiaoxin Ma, Liang Cui, Zhe Yu Ou, and Daoyin Yu, "An

all fiber source of frequency entangled photon pairs", Phys. Rev. A 79, 033817

(2009).

Yu Ding and Z. Y. Ou, "Frequency down-conversion for a quantum network",

Opt. Lett. 35, 2591 (2010).

L. Q. Chen, Guo-Wan Zhang, Cheng-ling Bian, Chun-Hua Yuan, Z. Y. Ou, and

Weiping Zhang, "Observation of the Rabi oscillation of light driven by an

atomic spin wave", Phys. Rev. Lett. 105, 133603 (2010).

## Honors, Awards and Grants

- 2004 Research Award of the School of Science, IUPUI.
- 1995 Young Investigator Award of the Office of Naval Research.
- 1990 Minoru and Ethel Tsutsui distinguished graduate research award of New York Academy of Sciences.