We have realized a single atom trap using a magneto-optical trap (MOT) with a high magnetic field gradient and a small optical dipole trap. Using this trap, we demonstrate the excitation to a highly excited Rydberg state (n=43) with a single Rubidium atom.
© 2009 Optical Society of America
The manipulation of single ions and atoms has benefitted greatly from the laser cooling and trapping tequniques[1, 2]. The internal degrees of freedom of single atoms can provide qubits for quantum information, and thus the control and manipulation of single atoms is now of great interest given the potential to create quantum registers, single photon sources and so on with single atom techniques[3, 4, 5, 6]. Indeed, the Meschede group have demonstrated a quantum register using a string of single atoms in dipole trap. Furthermore, the observation of entanglement between a single atom and a single photon provides the precondition for quantum communication and computation.
As we know, for a 2-qubit quantum gate, entanglement between two particles is needed. For ground state atoms, the interaction between atoms is generally very weak, and thus the distance between atoms must be less than 100 nm to realize quantum correlation between atoms. Using an optical lattice, where the atom can be trapped in a sub-micron potential, such quantum entanglement has been demonstrated. However, for Rydberg state atoms, Rydberg interactions are strong enough to realize the interference between two qubits. This weak interaction for ground state and strong interaction for Rydberg states can provide a controllable interaction between a few atoms thereby creating the right conditions to realize a 2-qubit gate.
Because atoms in highly excited Rydberg states have a strong dipole-dipole interaction, when several atoms are sufficiently close together the presence of a single excited atom can cause a shift in the energy of all the other atoms which is large enough to prevent resonant excitation of more than one atom in a sample. This so-called “Rydberg dipole blockade” could enable the realization of quantum information processing such as quantum gates and entanglement protocols[10, 11, 12, 13]. Recently, this Rydberg blockade has been demonstrated with two atoms separated by more than 1 micron. The Saffman group demonstrated that when atoms were excited from the Rb D2 line to the Rydberg n=79 state, with the distance between two atoms being around 10µm, because of the strong van der Waals interaction, excitation of one Rydberg atom blocked Rydberg excitation of the other one. Similarly, the Grangier group observed that for atoms excited from the Rb D1 line to the Rydberg n=43 state for a distance between atoms of around 4µm, the Rydberg dipole interactions caused a blockade. In effect, the above mentioned experiments made use of atoms trapped in two individual, narrow dipole traps. A standing wave[16, 17] or doughnut beam with a strong dipole trap can also provide narrow and strong confinement for two individual atoms.
To realize the Rydberg blockade effect, two or more atoms should be confined with a separation of less than 10 µm. The effect requires atom excitation to a Rydberg state and corresponding detection of atom loss within a very small distance. For very large atom number as more than 107, ion detection is very sensitive. For experiments such as ours with only a few atoms, ion detection is difficult. Instead, we detect the fluorescence signal. The basic necessity in order to realize the dipole blockade effect is the excitation of atoms from the ground state to a Rydberg state. Very recently, several groups have realized this Rabi oscillation[15, 19, 20]. In this paper, we will first detail the workings of our single atom trap and far-off-resonance optical dipole trap (FORT) for preparing a single atom in the dipole trap and then demonstrate its coherent excitation to a Rydberg state. Finally, we will detect the Rydberg excitation by observing the single atom Rabi oscillation. This is required step for future demonstrations of the dipole blockade, entanglement, 2-qubit quantum gates and so on.
2. Single atom trap
Our magneto-optical trap (MOT) is inside an ultra-high vacuum glass cell, (see Fig. 1). Prior to the experiment, a glass cell is filled with Rb atoms using a Rb dispenser for about 10 minutes, and then the dispenser current is switched off during the experiment. In order to control the Rb partial pressure in the glass cell, a light induced atom desorption (LIAD) technique is employed with a UV-LED light (400nm, 30 mW). UV LEDs are used to increase the Rb vapor pressure temporarily to increase the loading rate of the MOT. The glass cell is pumped by an ion pump (30 l/s) and a small non-evaporable getter. The background pressure in the cell is around 10-10 Torr. The MOT beams are introduced into the cell and are retroreflected. The beam size is reduced to about 2mm. The cooling laser detuned -6 MHz~-10 MHz drives the transition from |52 S 1/2,F=2〉 to |52 P 3/2,F=3〉 and the repump laser drives the transition from |52 S 1/2,F=1〉 to |52 P 3/2,F=2〉. The power of the cooling laser and repump laser is about 4 mW and 0.4 mW respectively. The MOT quadruple magnetic field is produced by water-cooled coils outside the cell. These coils can produce high magnetic field gradient of 260 G/cm with a current of 20 A. This high field gradient efficiently reduces the loading rate of the MOT to about 1 atom/s, and we can load single or very few atoms in the MOT. The fluorescence of the trapped atoms is collected by an AR coated aspherical lens (f=8mm, NA=0.5) inside the glass cell and detected by using a cooled CCD camera and an avalanche photodiode (APD). The magnification of the imaging system is about 9.4. The pixel size of the CCD is 7.4 micron ×7.4 micron. The effective spatial resolution of the imaging system is about 2 micron. For the APD, the total detection efficiency is 5-6%. A typical counting rate for a single atom is about 500-2500 counts/100ms.
Under the condition of low magnetic field (104 Gauss/cm (8 A)), we can catch less than 10 atoms in the trap and with high magnetic field (325 Gauss/cm (25 A)), we can obtain single atom, as shown in Fig. 2. The fluorescence distribution for a single atom is about 15 µm. The maximum life time of single atom can reach a few minutes.
Our far-off resonant trap (FORT) is formed by a Yb fiber laser (1080 nm), whose maximum power is 10W. The dipole laser goes through a 110 MHz switching AOM and is injected into the cell along the horizontal y direction (see Fig. 1). Transiting through a focus lens (focus=60 mm), its waist can be reduced to less than 2 µm. Normally we use a laser power of around 0.9W, which forms a trap depth of about 20 mK. The radial and axial confinement are ωx,z/2π=223 kHz and ωy/2π=27 kHz, respectively. With MOT and FORT, considering several energy levels close to the ground state 5S 1/2, the effective detuning between state F=2 and F’=3 is -63 MHz. By measuring the atom release retrap loss in the dipole trap, with appropriate strong confinement in the dipole trap, from Fig. 4, the temperature of atom in the dipole trap is found to be less than 4 mK. Correspondingly, the spatial distribution of the atoms is quasi one-dimensional with standard deviation of σx=σz=0.5µm and σy=4µm. Also, from this figure, we can get the appropriate dipole trap off time for the Rydberg excitation.
3. Rydberg excitation
The Rydberg excitation of trapped Rb atoms is realized by two-photon transitions with a coupling laser and a blue pump laser at 780 and 480 nm respectively as shown in Fig. 5(a). The 780 nm laser is frequency stabilized to the 5S 1/2(F=2)-5P 3/2(F′=3) transition using saturation spectroscopy. The 480 nm light is generated by a frequency doubled 960 nm diode laser. We obtain more than 100 mW of blue light at 480 nm using a periodically-poled potassium titanyl phosphate (PP-KTP) crystal and a power build-up cavity. The 480 nm laser is frequency stabilized to the excited state transition between the excited state 5P 3/2(F=3) and a highly excited Rydberg state nD 5/2(n=43-58) using electomagnetic induced transparancy (EIT). We obtain a frequency stability of a few hundred kHz. The linewidth of the 480 nm laser is narrower than 1 MHz, as verified by the Rydberg excitation spectrum measurement shown in the following section. Using acousto-optic modulators (AOM), both 780 nm and 480 nm excitation lasers are frequency shifted by 400 MHz from the intermediate 5P 3/2 state, after which they are made to intersect with the trapped atoms along the dipole trap beam axis(Fig. 5(b)).
In order to detect the Rydberg excitation of trapped atoms, we measure the atom loss in the ground state. A typical time sequence for the Rydberg excitation of trapped atoms is shown in Fig. 6. First we prepare a single or few atoms in the MOT and then we transfer the atoms into the dipole trap. After loading atoms to the FORT, we use an optical pumping beam and a bias field to excite the atoms to the mF=2 Zeeman state. Then, we excite the atoms using two photon excitation. During the Rydberg excitation, the dipole trap potential is switched off. After the excitation laser pulse, the dipole trap potential is switched on again, and we recapture the atoms in the dipole trap. When an atom is excited to the Rydberg state, it no longer feels the dipole trap potential, and additionally, these atoms can be photo ionized by the strong dipole trap laser field. The remaining atoms are transferred to the MOT to determine the number of atoms. Then we evaluate the probability of finding the atoms in the ground state.
In order to evaluate the atomic population probability in the ground state from the fluorescence signal, we use an Agilent Technologies DSO 6052A oscilloscope (250 ps resolution) to count the signal (500000 points) from the APD before and after the injection of the coupling laser. For the two measurements, we take the rising edge of coupling as the trigger and count the signal during 500 ms before and after this trigger, respectively. The FORT is turned off for several microseconds during the measurement which is long enough to perform the Rydberg excitation and short enough to not lose the atoms. According to the atom temperature obtained in the previous section, we chose 6µs as the dipole trap off-time.
4. Single atom excitation
Using this Rydberg excitation system, we first measure the spectrum of the two photon transition with a single atom. As mentioned in the previous section, by detecting the ground state probability before and after the atom is excited to the Rydberg state for different frequencies of the coupling laser and calculating the ratio between the two measurements of the atomic ground state population (including background subtraction), we get the spectrum of the single atom resonance between ground and Rydberg state. As shown in Fig. 7, the linewidth is around 0.5 MHz. Considering the Zeeman shift of the ground state and the excited state, excitation lasers injected into the MOT should have a compensation detuning of around -2 MHz compared with the EIT locked spectroscopy signal. From this spectrum, we can also confirm that our laser linewidth is less than 1 MHz. Next we measure the Rabi oscillation of the Rydberg excitation by changing coupling excitation pulse width.
By measuring the ground state probability for different excitation times, we can observe Rabi oscllation of atoms between the ground and Rydberg states, (see Fig. 8). We measure a two-photon Rabi oscillation frequency ΩR≈2π×0.5 MHz, which is close to the theoretical Rabi frequency defined by ΩR=Ω780Ω480/2Δ≈2π×0.56 MHz. The maximum excitation efficiency for atoms from the ground state to the Rydberg state is about 60%.
5. Discussion and summary
In this paper we observed single atom Rydberg excitation in a dipole trap. This is the first important step to study Rydberg interactions between two atoms. Compared with other similar experiments, there are some different advantages with our scheme. With our special configuration for the laser setup and locking system, the laser linewidth is narrow enough and laser frequency is stable enough to observe single atom Rabi oscillation clearly. Controlling the magnetic field gradient to change the loading rate of the MOT, we can reliably control the number of atoms in the dipole trap. With a unique lens to generate the dipole trap, it is easy to form a standing wave potential. By this potential, we can confine several atoms in a very small region and control the distance between atoms of less than 1 µm. So our scheme also provides a good way to study dipole blockade.
We have also observed the Rabi oscillation with more than one atom and found the slightly different signal. However so far we can not confirm the differences are due to Rydberg interactions or some experimental parameters that need to be improved. The signal is influenced by many parameters, such as variation of atom spatial distribution, inhomogeneous dephasing caused by fluctuation of the laser intensity and the trapped atom loss due to background collisions.
Next, we plan to fix the distance between atoms at around µm order by using a standing wave potential trap or doughnut beam trap or two close, independent dipole traps. In order to realize the blockade effect with strong Rydberg interactions, we will change our blue laser frequency to be resonant with a higher Rydberg state, such as n=79 or change the energy level configuration. Additionally, since the damping is mainly influenced by the spatial motion and the high temperature of the atoms in the dipole trap, we will reduce the temperature of the atoms to extend the decay time of the Rabi oscillations. Also, we need to improve our experimental stability, by stabilizing the laser intensity and frequency to reduce spontaneous decay and noise. In the future, we plan to demonstrate Rydberg blockade caused by strong atomic dipole-dipole interactions and ultimately to construct a 2-qubit gate and generate entanglement between atoms. Additionally, since the blockade effect allows only one atom to be excited, we believe that another interesting experiment would be to generate single photons by four-wave mixing using a few atoms in a collective Rydberg excitation. Such a scheme would be useful for fast quantum-state detection or transmission.
We would like to thank Philippe Grangier, Antoine Browaeys, Matthias Weidemüller, Mark Sadgrove for their helpful discussions. This work was supported by Grants-in-Aid for Science Research (Grants No. 21244063) from the Ministry of Education, Science, Sports, and Culture, and the 21st Century COE program on coherent Optical Science.
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