Schwettmann Group


Fig. 1

Fluorescence image of a cloud of ~1 billion atoms in our magneto-optical trap at OU.

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My main research interest is coherent quantum dynamics of ultracold atomic gases. Specifically, in my group we focus on ultracold spin-changing collisions in sodium spinor Bose-Einstein condensates (BEC). This research has applications in creating massive entanglement useful for quantum computing and in demonstrating new types of matter-wave quantum optics devices such as a phase-sensitive amplifier for atom number measurements. I am also interested in long-range interactions between ultracold sodium Rydberg atoms, with applications in precision measurements, quantum-assisted sensing, quantum computing and quantum simulation.

For a sodium atom in the ground state, the magnetic quantum number, m, related to the direction of the atomic spin, can be 0,+1 or -1. In a spin-exchange collision, two atoms with m=0 collide and transform into a pair with m=+1 and -1. In an ultracold gas, this causes the macroscopic spin populations to oscillate. This oscillation is a quantum implementation of the non-rigid pendulum. When prepared in a state of dynamical instability, the system is driven solely by small vacuum fluctuations. Over several tens of milliseconds, the collisions cause an amplification of initial vacuum fluctuations to the point were they become measurable by absorption imaging. The same dynamics also creates spin-squeezed quantum states, states with reduced uncertainty in one measurement variable which are useful for precision measurements.

Fig. 2

Sodium spinor Bose-Einstein condensate apparatus in our laboratory at OU.

Collisional dynamics in the antiferromagnetic spinor BEC are quite rich. Apart from the quantum pendulum analogy, they can also be understood in terms of the Josephson effect known from superconducting devices, as well as in terms of optical four-wave mixing processes known from nonlinear optics. While there are intriguing similarities and analogies, there are also important differences between the atomic, the solid state and the photonic systems that can be exploited in novel ways.

Recently, we learned that we can exert precise control over the collisional dynamics in a spinor BEC [1-5]. Using microwaves, RF fields, and applied magnetic fields, we can tune the system to behave, for example, like an interferometer in spin space with reduced noise, or like a phase-sensitive amplifier for sensitive atom number measurements, and explore the field of spin-based matter-wave quantum optics with applications in quantum sensing.

At OU, we have designed and built a custom state-of-the art spinor BEC experiment, and observed OU's first BEC in 2018. Since then, we have calibrated the apparatus, automated the process, and are taking competitive data on matter-wave quantum optics and atom interferometry.

My research group at OU consists of four graduate students working on their Ph.D. degrees: Qimin Zhang, Shan Zhong, Hio Giap Ooi, and Isaiah Morgenstern. One graduate student, Anita Bhagat, graduated with a successful thesis-based M.S. degree from my group. Her thesis title was "An apparatus to study matter-wave quantum optics in sodium spinor Bose-Einstein condensates". Many undergraduate students are involved every year in the form of successful senior research projects (CAPSTONE, 7 students) and NSF summer research projects (NSF-REU, 4 students).

In addition to my current research at OU and former research at NIST involving spinor BECs and exotic spin states, I investigated highly excited states of cesium and rubidium atoms in Jim Shaffer's research group as part of my dissertation research. These so-called Rydberg atoms are exotic because the electron is extremely far away from the nucleus. This leads to many exaggerated properties. For example, using laser-cooling and trapping and photo-association, we created cold ultralong-range molecules of weakly-bound pairs of cesium Rydberg atoms with internuclear distances of several micrometers, and we showed that these exotic molecules are exquisitely sensitive to external electric fields [6,7].

Fig. 3a

Research group.

Fig. 3b

Photos of our spinor Bose-Einstein condensate laboratory at OU. The top two images show the lab when I arrived at OU in August 2014 and after renovations when the optical tables arrived in January 2015. Our state-of-the art spinor BEC apparatus is custom-designed and built by our research group with help from the OU machine shop.

Later, we performed quantum optics experiments using Rydberg atom electromagnetically induced transparency (EIT). In EIT, a medium that is normally opaque becomes transparent when illuminated with a strong pump laser beam. The effect is due to a quantum interference between two different excitation pathways that the atoms can undergo in presence of the applied light fields. Specifically, we employed a 4-level ladder-type Rydberg EIT scheme with rubidium atoms, where two different Rydberg states are resonantly coupled by a microwave field, and one of the Rydberg states is also coupled to the atomic ground state through the first excited state by two light fields. With this setup, we were able to combine the high sensitivity of Rydberg atoms to external electric fields with the quantum interference effect of EIT in a quantum-assisted sensing approach to create a microwave sensor with high sensitivity [8,9].

We are also always highly interested in the application of modern technologies such as field-programmable gate array (FPGA) technologies to atomic physics experiments. For example, a while ago, I designed and programmed a low-cost FPGA-based laser locking circuit using off-the-shelf parts and a Nexys-2 prototyping board [10]. The VHDL source code is here. We recently designed a custom FPGA-based microwave source that we use in my research group at OU [2].

[1] J. Jie, Q. Guan, S. Zhong, A. Schwettmann, and D. Blume, "Mean-field spin-oscillation dynamics beyond the single-mode approximation for a harmonically trapped spin-1 Bose-Einstein condensate," Phys. Rev. A 102, 023324 (2020).

[2] I. Morgenstern, S. Zhong, Q. Zhang, L. Baker, J. Norris, B. Tran, and A. Schwettmann, "A Versatile Microwave Source for Cold Atom Experiments Controlled by a Field Programmable Gate Array," Rev. Sci. Instrum. 91, 023202 (2020).

[3] Q. Zhang and A. Schwettmann, "Quantum interferometry with microwave-dressed F=1 spinor Bose-Einstein condensates: Role of initial states and long-time evolution," Phys. Rev. A 100, 063637 (2019).

[4] J. P. Wrubel, A. Schwettmann, D. P. Fahey, Z. Glassman, H. K. Pechkis, P. F. Griffin, R. Barnett, E. Tiesinga, and P. D. Lett, "Spinor Bose-Einstein-condensate phase-sensitive amplifier for SU(1,1) interferometry," Phys. Rev. A 98, 023620 (2018).

[5] H. K. Pechkis, J. P. Wrubel, A. Schwettmann, P. F. Griffin, R. Barnett, E. Tiesinga, and P. D. Lett, "Spinor dynamics in an antiferromagnetic spin-1 thermal Bose gas," Phys. Rev. Lett. 111, 025301 (2013).

[6] A. Schwettmann, J. Crawford, K. R. Overstreet, and J. P. Shaffer, "Cold Cs Rydberg-gas interactions," Phys. Rev. A 74, 020701(R) (2006).

[7] K. R. Overstreet, A. Schwettmann, J. Tallant, D. Booth and J. P. Shaffer, "Observation of electric-field-induced Cs Rydberg atom macrodimers," Nature Physics 5, 581 - 585 (2009).

[8] J. A. Sedlacek, A. Schwettmann, H. Kübler, R. Löw, T. Pfau, and J. P. Shaffer, "Microwave electrometry with Rydberg atoms in a vapour cell using bright atomic resonances," Nature Physics 8, 819-824 (2012).

[9] J. Sedlacek, A. Schwettmann, H. Kübler, and J. P. Shaffer, "Atom based vector microwave electrometry using rubidium Rydberg atoms in a vapor cell," Phys. Rev. Lett. 111, 063001 (2013).

[10] A. Schwettmann, J. Sedlacek, and J. P. Shaffer, "Field-programmable gate array based locking circuit for external cavity diode laser frequency stabilization," Rev. Sci. Instrum. 82, 103103 (2011).