Strong Interactions inside hollow core Fibers

Working on this experiment: Parvez Islam, Wei Li, Hu Di

Introduction

Typically, photons in vacuum do not interact with each other. However, if passing through an appropriate non-linear medium, photons can interact with each other. This opens up many exciting possibilities for creating strongly correlated non-classical states of light or using photonic systems as quantum simulators. In our approach, we use a cloud of laser-cooled atoms in a quasi one-dimensional geometry to create these photon-photon interactions.

For this, we have to make several experimental considerations:

  • We want a long interaction region where light field and atoms are perfectly overlapped. In our setup, we use a hollow core photonic crystal fiber to guide both light field and atoms. This allows for transporting atoms through the fiber and creates a tight quasi one-dimensional confinement for the atoms inside the fiber.
  • We want a strong nonlinearity to get strong effective photon-photon interactions. For this, we employ a coherent two-photon EIT process (see below).
  • Additionally, we want strong long-range atom-atom interactions for additional non-local effects. To do this, we excite our atoms to highly excited states, the so-called Rydberg states. They have an enormous polarizability so that dipole-dipole interactions between atoms play a role.

 

Long distance guiding in hollow core fibers

The working principle of an optical conveyor belt.
The working principle of an optical conveyor belt: One of the dipole trap beams is detuned with respect to the other which creates a moving optical lattice to transport the cold atoms inside the hollow core fiber.

With the advent of laser cooling techniques, atoms are readily cooled down to temperatures low enough to start showing their quantum behavior. This opens up a whole new area of physics based on quantum mechanics which offers tremendous increase in measurement accuracy, e.g. in gravitation sensing or in atomic clocks. However, these atoms are so sensitive to the environment that maneuvering is not an easy task. In this project, we establish techniques for guiding atoms for long distances maintaining their coherence properties. We transport our atom cloud with the help of an optical conveyor belt. It consists of two counter propagating laser beams which form a standing wave. The atoms are trapped in the dipolar field of this beam and are transported by changing the relative detuning between the two beams. The controlled transport and optimal loading of the atomic cloud into the fiber has been discussed in details in arXiv:1805.06333 (2018). The hollow core fiber carrying a Gaussian mode of light also keeps the atoms tightly confined in the center of the fiber to reduce any surface effects. The guiding distance is mainly limited by the fiber length and should scale favorably compared to free space implementations.  The Gaussian mode of the guided light has the advantage of good overlap with the atoms, thus enhancing the light-matter interaction.

Cold Rubidium atoms are transported inside the fiber (fiber tip visible on the right-hand side) using an optical conveyor belt.
Cold Rubidium atoms are transported inside the fiber (fiber tip visible on the right-hand side) using an optical conveyor belt.

 

 

Strongly interacting systems

Sketch of the two-photon EIT scheme, using Rydberg atoms. The dipolar potential shown is shifting the upper Rydberg level out of resonance. This is responsible for the so-called Rydberg blockade where within a certain radius only one atom can be excited into a Rydberg state.
Sketch of the two-photon EIT scheme, using Rydberg atoms. The dipolar potential shown is shifting the upper Rydberg level out of resonance. This is responsible for the so-called Rydberg blockade where within a certain radius only one atom can be excited into a Rydberg state.

As discussed above, interactions between photons are possible in the presence of certain non-linear media, which could be specially designed crystals, cold atoms, etc. In our experimental setup, photons and atoms can both stay confined in the fiber throughout its entire length and hence there is a strong enhancement in the interaction between them. Under certain conditions, this system can offer a strong non-linearity, which is highly controllable. One possibility is electro-magnetically induced transparency (EIT), a coherent two-photon process which renders an otherwise absorptive medium transparent in the presence of an additional control light beam.

In addition, long-range interactions between atoms can introduce correlations between non-local photons. This can help in realizing transfer of quantum information from one photon to the other one mediated via atoms. Because of their strong dipole moment, one of the most promising candidates for such a task would be Rydberg atoms (a short explanation for the interested layman is given in this video). Their dipole interaction is so strong that within a blockade radius, only one Rydberg atom can be excited (see picture on the right hand side). Relatively long lifetimes at higher quantum numbers makes Rydberg atoms even more suitable for studies of non-linear phenomena using ladder-type EIT. A typical two-photon EIT scheme is sketched on the right hand side.

Summing all up, EIT with tightly confined Rydberg atoms in a hollow-core fiber offers long-range interactions between photons. We have successfully demonstrated the feasibility of exciting cold Rydberg atoms inside a hollow-core kagome fiber and we studied the influence of the fiber on Rydberg electromagnetically induced transparency (EIT) signals (PRA 96, 041402(R) (2017)).

Also, photon crystallization and the generation of non-classical photons will be an immediate result of our approach.

Sketch of the experimental sequence. Cold atoms are collected in a MOT and are then guided inside the hollow core fiber with a dipole trap. Inside the fiber, they are excited into Rydberg states. The detection is typically done with a detection beam also coupled through the fiber.

 

Our Experiment

Vacuum chamber with fiber

In our experiment, we use a 2D – 3D MOT setup to trap and laser cool Rb-87 atoms. The atoms are first precooled in a 2D MOT and are then transported to the main experimental region, which is a custom-made vacuum chamber. There, they are trapped again in the 3D MOT and cooled down to temperatures of a few µK.

2D-3D MOT Setup.
2D-3D MOT Setup.

In the picture below, you can see the Rubidium 3D MOT inside the vacuum chamber, right in front of our hollow core fiber. The fiber itself is mounted horizontally on a special vacuum-compatible fiber mount. On the right hand side of the MOT, you can see one lens inside the vacuum chamber, which we use to couple light fields into the hollow core fiber. The broadband properties of our fiber makes it suitable to guide different wavelengths.

Rubidium 3D MOT (center) in front of the hollow core fiber (left) inside the vacuum chamber.
Rubidium 3D MOT (center) in front of the hollow core fiber (left) inside the vacuum chamber.

For example, we use a laser beam, which is red-detuned relative to the atomic transition to create a dipole trap for the atoms, which guides them into the fiber. We are also using a multi-color laser system to excite the atoms into the Rydberg states.

Typical laser system setup, consisting of optical elements and control electronics.
Typical laser system setup, consisting of optical elements and control electronics.

Previously, our experimental apparatus was located in Hamburg, where many experiments including the first demonstration of guiding cold Rubidium atoms through a 88 mm long hollow core photonic band gap fiber (S Vorrath et al 2010 New J. Phys. 12 123015) were performed in the group of Prof. Klaus Sengstock.

 

 

Open Positions - Bachelor / Master Theses

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We always have projects both for bachelor and master theses available. At the moment, projects are possible in the following areas:

-          Laser stabilization:

Stabilizing a multicolor laser system to a high finesse cavity; characterizing the laser linewidth; measuring the atomic (Rydberg) transitions after the laser has been stabilized

-          Imaging system:

Setting up a detection system for few to single photons including a fast data storage mechanism; measuring and characterizing photon-correlations of Rydberg polaritons

-          Phase locking of the dipole trap (DT) beams

Measuring the real-time phase noise of the beat-note of DT beams. Evaluating the influence of the phase noise on the transferring efficiency of the optical belt. Building a phase locking system to eliminate phase jetting between the DT beams caused by environmental disturbance.

-          Homodyne setup

A balanced homodyne detection scheme is necessary for a full state tomography of the photonic and in consequence the polaritonic states. For this, the laser beam is first divided into probe beam and local oscillator (LO). After the probe beam has interacted with the atoms inside the hollow core fiber, both beams are overlapped again on a beam splitter, whose outputs are then detected by balanced photodetectors.

 

If you are interested in joining our team as a bachelor or master student, please contact Prof. Patrick Windpassinger or either of the PhD students.

 

 

Past Theses

Optischer Transport von kalten Atomen in eine Hohlkernfaser, master thesis by Ronja Wirtz (2018)

Rydbergspektroskopie in Hohlkernfasern, master thesis by Chantal Voss (2017)

Programmierung und Implementierung einer flexiblen AOM-Ansteuerung, bachelor thesis by Florian Stuhlmann (2016)

Rubidium spectroscopy inside hollow core fibers, bachelor thesis by Maik Selch (2016)

Stabilitätsuntersuchungen eines Zweistrahl-Laserinterferometers, bachelor thesis by Christian Korn (2016)

EIT-Spektroskopie von Rydbergzuständen an Rubidium, bachelor thesis by Chantal Voss (2015)

 

 

Publications

Kalte Atome auf einem optischen Förderband
Maria Langbecker & Patrick Windpassinger
Physik in unserer Zeit 50, 10 (2019)

Micro lensing induced lineshapes in a single mode cold-atom hollow-core fiber interface
Mohammad Noaman, Maria Langbecker, Patrick Windpassinger
Opt. Lett. 43(16), 3925-3928 (2018), arXiv:1805.11391

Highly controlled optical transport of cold atoms into a hollow-core fiber
Maria Langbecker, Ronja Wirtz, Fabian Knoch, Mohammad Noaman, Thomas Speck, Patrick Windpassinger
New J. Phys. 20 083038 (2018)

Rydberg excitation of cold atoms inside a hollow-core fiber
Maria Langbecker, Mohammad Noaman, Niels Kjærgaard, Fetah Benabid, and Patrick Windpassinger
Physical Review A 96, 041402(R) (2017)

Efficient guiding of cold atoms through a photonic band gap fiber  
S. Vorrath, S. A. Möller, P. Windpassinger, K. Bongs, K. Sengstock
New J. Phys. 12 123015 (2010)

 

Funding

giryd    DFG

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