Do science in an open and collaborative way.

Current projects

My research involves theoretical studies on the strong coupling between atoms and photons on an optical nanofiber (see figure below) and other nanophotonic platforms, especially to study the fundamental limits and possibilities of state preparation, control and readout (measurement) operations on an ensemble of atoms using waveguide interfaces as a data bus towards quantum computing, quantum communication and quantum sensing applications. This study was initially motivated as an upgrade to the free space atom-light quantum interface platforms implemented in cold atom labs towards a stronger coupling between atoms and light. My Research Statement for the purpose of some job applications can be found here.

Comparison of mode matching in free-space (a and b) and optical nanofiber (c) systems
Fig 1. Comparison of mode matching in free-space (a and b) and optical nanofiber (c) systems. Figure is adapted from X. Qi, B. Q. Baragiola, P. S. Jessen, I. H. Deutsch, Phys. Rev. A 93, 023817(2016).

As shown in the figure above, to enhance the coupling strength between an atomic ensemble and light propagated in free space as a Gaussian laser beam, one can either focus the laser beam very tightly so that the few atoms at the beam waist could couple to the optical field strongly (as shown in subfigure (a)), or by using a less focused laser beam so that more atoms can be coupled to the light (as shown in (b)). Obviously, the former approach doesn’t allow a large number of atoms couple to the light as strongly as the ones at the beam waist; and the latter approach makes the light-atom coupling weak per atom. Either way, the effective atom number that can be strongly coupled to the light is always far less than the total number of atoms trapped in the free-space atom-light interface, and the effective light signal collected in the quantum measurement process is far less than the input due to the poor mode matching nature of the free-space quantum interface caused by strong incoherent light scattering from atoms. In contrast, if the atoms are trapped around an optical nanofiber using the fiber’s evanescent field as shown in subfigure (c), not only all atoms could be all evenly and strongly coupled to the light, but also the forwarding light signal can be efficiently collected due to good mode matching. In the end, compared to the free-space case, far less number of atoms are needed using the nanofiber platform to generate a considerable amount of light-atomic ensemble coupling strength, which could open up possibilities, for example, to generate non-Gaussian collective spin states using quantum measurement.

We have outlined and proposed in our theory paper (Phys. Rev. A 93, 023817(2016)) that, in the dispersive regime, this nanofiber system can generate a strong coupling with the alkali metal atom ensembles, which can be used for precise quantum non-demolition (QND) measurement of atomic clocks as a Ramsey interferometer as well as for spin squeezing beyond the Gaussian state limit with fewer atoms than the free-space case.

In paper arXiv:1712.02916, we show that implementing strong atom-light coupling doesn’t necessarily mean the atoms have to sit in a strong optical field position by defining the cooperativity in the context of QND measurement induced spin squeezing on a nanophotonic waveguide interface. As one example, we show that the atom-light coupling in our Faraday interaction protocol becomes the strongest if the atoms are trapped at the weakest field position among all azimuthal angles outside of a nanofiber and a square waveguide geometry. The reason is that atom-light coupling includes two physical mechanisms, one is the “useful” interaction part (or the Faraday interaction in our case), and the other is the “damaging” part (or decoherence in our case). Despite the fact that the Faraday interaction can be nearly homogeneous in the azimuthal directions, the decoherence effect reaches the maximum value if the atoms are sitting in the strongest field position. We show that a square waveguide that has a darker local field compared to the cylindrical nanofiber geometry can help generate 13dB of squeezing with realistic experimental parameters, while the nanofiber can generate 7dB of squeezing. Our result might be counter-intuitive, but it could lead to new ideas of implementing quantum information processing protocols by avoiding some problems the community usually face at. In fact, a major barrier of manipulating atoms on a nanophotonic interface is the coupling field could be so strong that the incident photons kick atoms away from the trapping points before any meaningful quantum operations. Using our theoretical insight of cooperativity, one could potentially design a probing geometry or atomic state so that the photon scattering is weak enough that the probe field won’t disturb useful quantum operations on the atoms while still benefited from a strong atom-light coupling.

All of these findings imply a promising future for the nanofiber platform for atomic clocks, precise measurements, quantum data bus and other quantum information processing applications. During the study, our group has been closely working with our experimental collaborators, and the future of our project is tightly orientated by the new findings and inevitable challenges that our experimental collaborators have concluded and based on our theoretical insights which can make a fundamental difference for the frontier research of quantum information and quantum computing community. My current on-going theory research projects include the following and are open for new directions:

Spectroscopy on a nanofiber-trapped atom ensemble system
Fig 2. Polarization spectroscopy on an optical nanofiber coupled atomic ensemble system. Figure is adapted from X. Qi, B. Q. Baragiola, P. S. Jessen, I. H. Deutsch, Phys. Rev. A 93, 023817(2016).

In addition, through establishing the theoretical foundation of light-atom interaction on the nanofiber platform, I hope to reach the regime of nonlinear interaction between photons for quantum simulations/computations/communications, and to deeply understand the quantum behaviors of atoms and photons and the role of nanophotonic geometries for better integratable photonic systems for quantum information processing.
Stay tuned!

Note: If you think I might be able to contribute to your research as well as deepen my understanding on nature, we can talk.