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Government of India | Department of Atomic Energy
Raja Ramanna Centre for Advanced Technology
Laser Physics Applications Division
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Atom Optics Laboratory (AOL)

Objective

The objective of the present activities of Atom Optics lab, LPAD, RRCAT is to develop quantum technologies using laser cooled atoms.

  • This includes development of cold atom based gravimeter for measurement of gravitational acceleration (g) of earth. This will have applications in mineral exploration, monitoring seismic activities, geodesy, etc.

  • The cold atom based UHV sensors are of importance to develop new quantum vacuum standards for pressure measurements in UHV and XHV regime. This is also an area in which the development in our lab at RRCAT is in progress.

  • The work is also being pursued for preparation of cold atom based qubits for development of quantum computing platforms.
Recent activities and achievements of the project : 

(i) Cold atom gravimeter development in RRCAT.

A cold atom gravimeter (CAG) for precision measurement of earth’s gravitational acceleration (g) has been developed which is presently working with precision of ~270 µGal (1 µGal = 10-8 m/s2).

The schematic and the photograph of the gravimeter setup are shown in Fig.1(I and II). The developed gravimeter is designed for atomic fountain geometry. The cold atoms were prepared at a temperature of ~ 25 μK in a magneto-optical trap (MOT) before launching in the atomic fountain. The apex height of the atomic fountain is ~ 25 cm. This allowed the maximum value of T ~ 25-30 ms in Raman pulses sequence π/2-T-π-T-π/2 for interferometry. The resolution of the chirping rate (α) of Raman beam frequency was ~6.9 Hz/sec. With this chirp rate resolution, the measurement resolution in the fringes becomes ~270 µG. The Raman pulse atom interferometry was performed with the sequence of pulses in π/2-T-π-T-π/2, where the π pulse duration is ~10 μs for our intensity. The interferometric fringes are shown in Fig. 1(III) with variation in chirp rate (α) in the frequency of one of the Raman beams. The central chirp-rate (αc) for the fringes (common maximum for fringes taken with different T values) gives the value of g (g = αc λ/2), where λ is wavelength. We estimated the local value of acceleration due to gravity ‘g’ in the lab is g = 9.786173 ± 0.000003 m/s2 with an accuracy of micro-Gal (~270 µG) level.

Fig.1: (I) Schematic and (II) photograph of the cold atom gravimeter setup developed at RRCAT. (III) The experimentally observed interferometric fringes. The fringes are measured for different values of T (plots (a) to (c)). The plot (c) shows only central peak in the fringes at higher resolution of chirp rate (α) for the Raman beam.
Fig.1: (I) Schematic and (II) photograph of the cold atom gravimeter setup developed at RRCAT. (III) The experimentally observed interferometric fringes. The fringes are measured for different values of T (plots (a) to (c)). The plot (c) shows only central peak in the fringes at higher resolution of chirp rate (α) for the Raman beam.


(ii) UHV pressure sensor development.

We have developed UHV pressure measurement system using the loading dynamics of magneto-optical trap (MOT). The loading time (τL ) and saturated number (Ns) for a MOT loaded from the background vapor depend on the pressure in the chamber. The partial pressure due to Rb-vapor can be varied by varying the dispenser source current. By measuring Ns and tL from MOT loading data at different values of dispenser current, the partial pressure due to Rb-vapor (PRb) and background pressure (P) due to non-Rb gases have been estimated.

The loading graph of magneto-optical trap is given in Fig. 2.

Fig. 2: Increase in MOT fluorescence signal (i.e. number of atoms in MOT cloud) with MOT loading time at different values of dispenser current (ref: V. Singh, V. B. Tiwari, S. R. Mishra, Laser Phys. Lett. 17 (2020) (035501)).
Fig. 2: Increase in MOT fluorescence signal (i.e. number of atoms in MOT cloud) with MOT loading time at different values of dispenser current (ref: V. Singh, V. B. Tiwari, S. R. Mishra, Laser Phys. Lett. 17 (2020) (035501)).


The loading graph of MOT follows the following equation,

N(t)=Ns[1-exp (-t⁄τ L)]

where, Ns=RτL=αPRbτL and τL=1/(βPRb +γb).

By measuring the variation in Ns with τL, the values of Rb pressure PRb and background pressure P (γb /P = 4.9x107 Torr-1 s-1) can be estimated.

Using this technique, the background pressure ranging from 1x10-8 to 1x10-10 Torr can be measured. For further lower pressure in XHV regime, magnetic traps are preferable, which will be the future programme in this direction.

(iii) Cold atom based qubits:

The work is in progress to trap single 87Rb atom in an optical tweezer loaded from a magneto-optical trap. The single trapped atom will act as a qubit. Two atomic qubits in a close separation of few microns, it will be possible to induce Rydberg blockade between them. This will provide opportunity to prepare two qubits based C-NOT gate (Figure 3(a)). Presently trapping work is in progress to load low number of atoms in the optical tweezer as shown in absorption image of trapped atoms in Figure 3(b). The final goal is to reach single atom in the trap.

Fig. 3. (a) Schematic of the two qubits C-NOT gate preparation and (b) present status of atoms trapped in an optical tweezer operating at wavelength 811 nm.
Fig. 3. (a) Schematic of the two qubits C-NOT gate preparation and (b) present status of atoms trapped in an optical tweezer operating at wavelength 811 nm.


Earlier achievements of the activity

Bose-Einstein condensation of 87Rb atoms:

The ultracold atoms to produce Bose-Einstein condensate were prepared in a double magneto-optical trap (double-MOT) configuration. The double-MOT setup consists of a vapor chamber MOT (VC-MOT) at ~1-2 x 10-8 Torr pressure and an ultrahigh vacuum MOT (UHV-MOT) at ~3x10-11 pressure. This setup involves magnetic trap for cold atoms, RF evaporative cooling system, detection and characterization system, and a PC-based controller system to implement various cooling stages in a desired sequence. Here the UHV-MOT atoms are subjected to magnetic trapping and evaporative cooling to achieve BEC. Figure 4 shows an image and spatial profile of optical density of cold atom cloud after evaporative cooling performed using this setup in the lab. The sharp peak at the center in this profile shows the presence of Bose-condensate in the cloud.

Figure 4: (a) An observed image and (b) spatial profile of optical density of an ultracold atom-cloud with Bose-Einstein condensate at the center.
Figure 4: (a) An observed image and (b) spatial profile of optical density of an ultracold atom-cloud with Bose-Einstein condensate at the center.


Trapping atoms on an atom-chip

We have developed a setup for trapping atoms on atom-chip. The aim of this work is to develop expertise in trapping and manipulation of atoms on miniaturized scale so that more compact and sensitive atom-optic devices can be developed. Figure 5 (a) shows the photograph of chip. Atom chip was fabricated using Si substrate of size 25 mm x 25 mm (700 µm thick) after depositing the gold (Au) layer of 2.5 µm. The chip also serves as a mirror surface for mirror-MOT formation. On the chip, a magneto-optical trap called U-MOT is first prepared to trap ~ 5 x 107 87Rb atoms at temperature of ~300 µK.

Figure 5: (a) Photograph of in-house developed atom chip having single Z-shaped gold wires. (b) The CCD images of cold 87Rb atom cloud in U-MOT, G-MOT and micro-magnetic trap.
Figure 5: (a) Photograph of in-house developed atom chip having single Z-shaped gold wires. (b) The CCD images of cold 87Rb atom cloud in U-MOT, G-MOT and micro-magnetic trap.

Subsequently, this MOT cloud is brought closure to the chip surface to trap them in grey-MOT (G-MOT) before final trapping in Ioffe-Pritchard (IP) type magnetic trap formed by magnetic field due to a current carrying Z-shaped micro-wire (200 µm x 2.5 µm cross-section) on atom-chip. Figure 5 (b) shows the atom cloud in U-MOT, G-MOT and micro-magnetic trap.



Trapping atoms in optical dipole trap (ODT)

The cold 87Rb atoms from a magneto-optical trap have been trapped in a single beam optical dipole trap (ODT). The ODT was formed by focusing the output of a fiber laser system operating at 1064 nm wavelength.

Figure 6: The experimental optical density images of the trapped atom cloud obtained by absorption imaging.
Figure 6: The experimental optical density images of the trapped atom cloud obtained by absorption imaging.


Trapping cold 87Rb atoms in a RF-dressed magnetic trap

The RF-dressed potential are generated by applying a strong RF-field in presence of a static quad-rupole magnetic trap. By changing the RF field parameters, various atom trapping geometries can be demonstrated. We have demonstrated atom trapping in shell type geometry as shown in Figure 7.

Figure 7: Image of atom cloud in (a) quadrupole magnetic trap and (b) RF-dressed shell trap.
Figure 7: Image of atom cloud in (a) quadrupole magnetic trap and (b) RF-dressed shell trap.


Laser cooling and trapping of noble gas Krypton atoms

The experimental setup to cool and trap noble gas metastable Kr atoms along with the CCD images of the cold atom are shown in the Figures 8. The Krypton gas first flows into RF discharge glass tube through the gas inlet chamber, where metastable atoms of Kr are prepared. These metastable atoms, after slowing down in a Zeeman slower device, are transported to MOT chamber (~10-8 Torr). In this MOT chamber, the Kr atoms are cooled and trapped using cooling and repumping laser beams alongwith appropriate quadrupole magnetic field. The laser cooling of various bosonic (82Kr, 84Kr and 86Kr) as well as fermionic (83Kr) isotopes of Krypton has been successfully demonstrated. In addition, dual-isotope MOTs for cold bosonic-bosonic and bosonic-fermionic mixtures have been demonstrated. The dual-isotope MOTs are useful to study cold collisions among various species.

Figure 8: Photograph of the experimental setup for laser cooling of Kr atoms. (b) CCD fluores-cence images of separated and overlapped cold atom clouds of Kr* atoms in a dual-isotope MOT.
Figure 8: Photograph of the experimental setup for laser cooling of Kr atoms. (b) CCD fluores-cence images of separated and overlapped cold atom clouds of Kr* atoms in a dual-isotope MOT.


Publications

Some recent publications
  1. “Development of a pyramidal magneto-optical trap for pressure sensing application”,
    S. Supakar, V. Singh, Y. P. Kumar, S. K. Tiwari, C. Mukherjee, M. P. Kamath, V. B. Tiwari and S. R. Mishra
    Optical Engineering, 65, 054103, (2024).

  2. “Ultra-high vacuum pressure measurement using magneto-optical trap on atom chip”,
    S. Supakar, V. Singh, V. B. Tiwari, and S. R. Mishra
    J. Appl. Phys., 134, p. 024403, (2023).

  3. “Development and characterization of atom chip for magnetic trapping of atoms”,
    V. Singh, V. B. Tiwari, A. Chaudhary, R. Shukla, C. Mukharjee, S. R. Mishra.
    J. Appl. Phys., 133, 084402, (2023).

  4. "A method for loading magneto-optical trap in an ultrahigh vacuum environment",
    K. Bhardwaj, Sourabh Sarkar, S. P. Ram, V. B. Tiwari, and S. R. Mishra
    AIP Advances, 13, 015108, 2023.

  5. "Efficient quantum state preparation using Stern-Gerlach effect on cold atoms",
    V. Singh, V. B. Tiwari, and S. R. Mishra
    Measurement Science and Technology, 33, 095019, 2022.

  6. "Different atom trapping geometries with time averaged adiabatic potentials",
    Sarkar Sourabh, Ram S.P.*, V. B. Tiwari, and S. R. Mishra
    Eur. Phys. J. D, 75, 281, 2021.

  7. "A single laser operated magneto-optical trap for Rb atomic fountain",
    S. Singh, B. Jain, S. P. Ram, V. B. Tiwari, S. R. Mishra,
    PRAMANA, Journal of Physics, 95, 67, 2021.

  8. "Absorption imaging of trapped atoms in presence of AC-Stark shift",
    K. Bhardwaj, S. P. Ram, S. Singh, V. B. Tiwari, S. R. Mishra,
    Physica Scripta, 96, 015405, 2021.

  9. "Polarization enhanced tunable Doppler-free dichroic lock technique for laser frequency locking",
    V. Singh, V. B. Tiwari, S. R. Mishra,
    Journal of the Optical Society of America B, 38, 249-255, 2021.

  10. "On the continuous loading of a U-magneto optical trap on an atom-chip in an ultra high vacuum",
    V. Singh, V. B. Tiwari, S. R. Mishra,
    Laser Physics Letters,17, no. 3, p. 035501, Jan. 2020.

  11. "On electromagnetically induced transparency in N-systems in cold 87Rb atoms",
    Charu Mishra, A. Chakraborty, S. P. Ram, S. Singh, V. B. Tiwari, S. R. Mishra,
    Journal of Physics B: Atomic, Molecular and Optical Physics,53, no. 1, p. 015001, Dec. 2019.

  12. "Cooling of fermionic 83Kr and bosonic 84Kr isotopes in a magneto-optical trap",
    S. Singh, V. B. Tiwari, S. R. Mishra,
    PRAMANA, Journal of Physics,93:92, Dec. 2019.

  13. "Coupling field dependent quantum interference effects in a Λ-system of 87Rb atom",
    Charu Mishra, A. Chakraborty, V. Singh, S. P. Ram, V. B. Tiwari, S. R. Mishra,
    Physics Letters A,382, no. 45, p. 3269-3273, Nov. 2018.

  14. "On loading of a magneto-optical trap on an atom-chip with U-wire quadrupole field",
    V. Singh, V. B. Tiwari, K. A. Singh, S. R. Mishra,
    Journal of Modern Optics,65, no. 21, p. 2332-2338, Aug. 2018.

  15. "Electromagnetically induced transparency in Λ-systems of 87Rb atom in magnetic field",
    Charu Mishra, A. Chakraborty, A. Srivastava, S. K. Tiwari, S. P. Ram, V. B. Tiwari, S. R. Mishra,
    Journal of Modern Optics,65, no. 20, p. 2269-2277, Jul. 2018.

  16. "Effect of Zeeman Slower Beam on Loading of a Krypton Magneto-Optical Trap",
    S. Singh, V. B. Tiwari, S. R. Mishra, H. S. Rawat,
    Journal of Experimental and Theoretical Physics,126, no. 4, p. 441–445, Apr. 2018.

  17. "Resonance enhancement of two photon absorption by magnetically trapped atoms in strong rf-fields",
    A. Chakraborty, S. R. Mishra,
    Physics Letters A,382, no. 4, p. 157-161, Jan. 2018.

  18. "A Floquet formalism for the interaction of magnetically trapped atoms with rf fields",
    A. Chakraborty, S. R. Mishra,
    Journal of Physics B: Atomic, Molecular and Optical Physics,51, no. 2, p. 025002, Dec. 2017 .

  19. " Dependence of in-situ Bose condensate size on final frequency of RF-field in evaporative cooling",
    S. R. Mishra, S. P. Ram, S. K. Tiwari, H. S. Rawat,
    Pramana – J. Phys.,88:59, (2017).

  20. "A tunable Doppler-free dichroic lock for laser frequency stabilization",
    V. Singh, V. B. Tiwari, S. R. Mishra, and H. S. Rawat,
    Appl. Phys. B:Lasers and Optics, 122, 225, (2016).

  21. "Electromagnetically induced absorption and transparency in degenerate two level systems of metastable Kr atoms and measurement of Lande g-factor", 
    Y. B. Kale, V. B. Tiwari, S. R. Mishra, S. Singh, and H. S. Rawat,
    Opt. Commun. 380, 297 (2016).

  22. "A toroidal trap for the cold 87Rb atoms using a rf-dressed quadrupole trap",
    A. Chakraborty, S. R. Mishra, S. P. Ram, S. K. Tiwari, H. S. Rawat,
    J. Phys. B: At. Mol. Opt. Phys. 49, 075304, (2016).

  23. "Investigation of cold collision in a two-isotope Krypton magneto-optical trap",
    S. Singh, V. B. Tiwari, Y. B. Kale, S. R. Mishra and H. S. Rawat,
    J. Phys. B: At. Mol. Opt. Phys., 48, 175302, (2015).

  24. "Resolution of hyperfine transitions in metastable 83Kr using electromagnetically induced transparency",
    Y. B. Kale, S. R. Mishra, V. B. Tiwari, S. Singh and H. S. Rawat,
    Phys. Rev. A, 91, 053852, (2015).

  25. "Generation and focusing of a collimated hollow beam",
    S. K. Tiwari, S. P. Ram, K.H. Rao, S.R. Mishra, H. S. Rawat,
    Opt. Eng., 54, 115111, (2015).

  26. "Velocity selective bi-polarization spectroscopy for laser cooling of metastable Krypton atoms",
    Y. B. Kale, V. B. Tiwari, S. Singh, S. R. Mishra, H. S. Rawat,
    J. Opt. Soc. Am. B, 31, 2531, (2014).

  27. "Loading of a Krypton magneto-optical trap with two hollow laser beams in Zeeman slower",
    S. Singh, V. B. Tiwari, S. R. Mishra, H. S. Rawat,
    J. Exp. Theo. Phys., 146, 464, (2014).

  28. "The effect of laser beam size in a zig-zag collimator on transverse cooling of a krypton atomic beam ",
    V. Singh, V. B. Tiwari, S. Singh, S. R. Mishra, H. S. Rawat,
    Pramana-J. Phys., 83, 131, (2014).

  29. "An atomic beam fluorescence locked magneto optical trap for Kr atoms",
    S. Singh, V. B. Tiwari, S. R. Mishra, H. S. Rawat,
    Laser Phys., 24, 025501, (2014).
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