Helium atoms in Rydberg states have been manipulated coherently with microwave radiation pulses near a gold surface and near a superconducting NbTiN surface at a temperature of
3K. The experiments were carried out with a skimmed supersonic beam of metastable
(1s)1(2s)11S0 helium atoms excited with laser radiation to
np Rydberg levels with principal quantum number
n between 30 and 40. The separation between the cold surface and the center of the collimated beam is adjustable down to $250\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Short-lived
np Rydberg states were coherently transferred to the long-lived
ns state to avoid radiative decay of the Rydberg atoms between the photoexcitation region and the region above the cold surfaces. Further coherent manipulation of the
ns Rydberg states with pulsed microwave radiation above the surfaces enabled measurements of stray electric fields and allowed us to study the decoherence of the atomic ensemble. Adsorption of residual gas onto the surfaces and the resulting slow buildup of stray fields was minimized by controlling the temperature of the surface and monitoring the partial pressures of
H2O,
N2,
O2, and
CO2 in the experimental chamber during the cool-down procedure. Compensation of the stray electric fields to levels below
100mV/cm was achieved over a region of
6mm along the beam-propagation direction which, for the
1770-m/s beam velocity, implies the possibility to preserve the coherence of the atomic sample for several microseconds above the cold surfaces.
Rydberg states of atomic helium with principal quantum numbers ranging from
n=20 to
n=100 have been prepared by non-resonance-enhanced single-color two-photon excitation from the metastable
23S1 state. Photoexcitation was carried out using linearly and circularly polarized pulsed laser radiation. In the case of excitation with circularly polarized radiation, Rydberg states with azimuthal quantum number $|{m}_{\ensuremath{\ell}}|=2$ were prepared in zero electric field and in homogeneous electric fields oriented parallel to the propagation axis of the laser radiation. In sufficiently strong electric fields, individual Rydberg--Stark states were resolved spectroscopically, highlighting the suitability of non-resonance-enhanced multiphoton excitation schemes for the preparation of long-lived high-$|{m}_{\ensuremath{\ell}}|$ hydrogenic Rydberg states for deceleration and trapping experiments. Applications of similar schemes for Doppler-free excitation of positronium atoms to Rydberg states are also discussed.
Circular Rydberg states with
n=70 have been prepared in helium using a modified version of the crossed-fields method. This approach to the preparation of high-
n circular Rydberg states overcomes limitations of the standard crossed-fields method which arise at this, and higher, values of
n. The experiments were performed with atoms traveling in pulsed supersonic beams that were initially laser photoexcited from the metastable
1s2s3S1 level to the
1s73s3S1 level by resonance-enhanced two-color two-photon excitation in a magnetic field of 16.154 G. These excited atoms were then polarized using a perpendicular electric field of 0.844 V/cm, and transferred by a pulse of microwave radiation to the state that, when adiabatically depolarized, evolves into the
n=70 circular state in zero electric field. The excited atoms were detected by state-selective electric field ionization. Each step of the circular state preparation process was validated by comparison with the calculated atomic energy-level structure in the perpendicular electric and magnetic fields used. Of the atoms initially excited to the
1s73s3S1 level, $\ensuremath{\sim}80%$ were transferred to the
n=70 circular state. At these high values of $n, \mathrm{\ensuremath{\Delta}}n=1$ circular-to-circular Rydberg state transitions occur at frequencies below 20 GHz. Consequently, atoms in these states, and the circular state preparation process presented here, are well suited to hybrid cavity QED experiments with Rydberg atoms and superconducting microwave circuits.
A surface-electrode decelerator consisting of 44 electrodes has been used to decelerate supersonic beams of helium Rydberg atoms moving with an initial velocity of 1200 m/s. Prior to the deceleration, the helium atoms were excited from the
1s2s1S0 metastable state to selected Rydberg-Stark states with a narrow-band tunable UV laser. Complete deceleration could be achieved over a distance of 36 mm and in 60 $\ensuremath{\mu}$s, corresponding to an acceleration of $\ensuremath{-}2.0\ifmmode\times\else\texttimes\fi{}{10}^{7}$ m/s
2. After deceleration, the atoms were held in stationary electric traps above the chip surface, reaccelerated off the chip, and detected by pulsed electric-field ionization. The decelerator was also used to generate helium Rydberg-atom beams with a final velocity tunable between 0 and 1200 m/s. Transitions between low- and high-field-seeking Rydberg-Stark states were observed during trap loading and are attributed to adiabatic Landau-Zener dynamics at electric fields exceeding the Inglis-Teller field. By comparing the experimental results with the results of particle-trajectory simulations, the velocity distribution of the decelerated atoms was found to be characterized by temperatures ranging between 50 and 200 mK, depending on the magnitude of the electric dipole moment of the Rydberg-Stark states selected prior to the deceleration.
Driving atoms from an initial to a final state of the same parity via an intermediate state of opposite parity is most efficiently done using STIRAP, because it does not populate the intermediate state. For optical transitions this requires appropriate pulses of light in the counter-intuitive order - first coupling the intermediate and final states. We populate Rydberg states of helium (n = 12 ~ 30) in a beam of average velocity 1070 m/s by having the atoms cross two laser beams in a tunable dc electric field. The &ldquo red &rdquo light near &lambda = 790 ~ 830 nm connects the 33P states to the Rydberg states and the &ldquo blue &rdquo beam of &lambda = 389 nm connects the metastable 23S state atoms emitted by our source to the 33P states. By varying the relative position of these beams we can vary both the order and the overlap encountered by the atoms. We vary either the dc electric field and fix the &ldquo red &rdquo laser frequency or vary the &ldquo red &rdquo laser frequency and fix the dc electric field to sweep across Stark states of the Rydberg manifolds. Several mm downstream of the interaction region we apply the very strong bichromatic force on the 23S &rarr 23P transition at &lambda = 1083 nm. It deflects the remaining 23S atoms out of the beam and the ratio of this signal measured with STIRAP beam on and off provides an absolute measure of the fraction of the atoms remaining in the 23S state. Simple three-level models of STIRAP all predict 100% excitation probability, but our raw measurements are typically around half of this, and vary with both n and l of the Rydberg states selected for excitation by the laser frequency and electric field tuning on our Stark maps. For states with high enough Rabi frequency, after correction for the decay back to the metastable state before the deflection, the highest efficiencies are around 70%. An ion detector readily detects the presence of Rydberg atoms. We believe that the observed signals are produced by black-body ionization at a very low rate, but sufficient to ionize about 0.5 ~ 1.0 % of the atoms in a region viewed by our detector. Many measurements provide support for this hypothesis.
Helium atoms have been prepared in the circular $|n=55,\ensuremath{\ell}=54,{m}_{\ensuremath{\ell}}=+54\ensuremath{\rangle}$ Rydberg state using the crossed electric and magnetic fields method. The atoms, initially traveling in pulsed supersonic beams, were photoexcited from the metastable
1s2s3S1 level to the outermost, ${m}_{\ensuremath{\ell}}=0$ Rydberg-Stark state with
n=55 in the presence of a strong electric field and weak perpendicular magnetic field. Following excitation, the electric field was adiabatically switched off causing the atoms to evolve into the circular state with ${m}_{\ensuremath{\ell}}=+54$ defined with respect to the magnetic-field quantization axis. The circular states were detected by ramped electric-field ionization along the magnetic-field axis. The dependence of the circular state production efficiency on the strength of the excitation electric field, and the electric-field switch-off time was studied, and microwave spectroscopy of the circular-to-circular $|55,54,+54\ensuremath{\rangle}\ensuremath{\rightarrow}|56,55,+55\ensuremath{\rangle}$ transition at $\ensuremath{\sim}38.5$ GHz was performed.