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Controlled Single-Photon Emission from a Single Trapped Two-Level Atom

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Science  15 Jul 2005:
Vol. 309, Issue 5733, pp. 454-456
DOI: 10.1126/science.1113394

Abstract

By illuminating an individual rubidium atom stored in a tight optical tweezer with short resonant light pulses, we created an efficient triggered source of single photons with a well-defined polarization. The measured intensity correlation of the emitted light pulses exhibits almost perfect antibunching. Such a source of high-rate, fully controlled single-photon pulses has many potential applications for quantum information processing.

Implementing a deterministic or conditional two-qubit quantum gate is a key step toward quantum computation. Deterministic gates generally require a strong interaction between the particles that are used to carry the physical qubits (1). Recently, controlled-not gates have been realized with the use of trapped ions and incorporated in elaborate quantum algorithms (24). So far, individually addressed two-qubit gates have not been demonstrated with neutral atoms. Promising results have been obtained on entangling neutral atoms with the use of cold controlled collisions in an optical lattice (5), but the single-qubit operations are difficult to perform in such a system.

Another approach is to bypass the requirement for a direct interaction between the qubits and use instead an interference effect and a measurement-induced state projection to create the desired operation (6, 7). An interesting recent development of this idea is the use of photon detection events to create entangled states of two atoms (810). This provides “conditional” quantum gates, where the success of the logical operation is heralded by appropriate detection events. These schemes can be extended to realize a full controlled-not gate, or a Bell-state measurement, or more generally to implement conditional unitary operations (8, 11). They could be implemented by using, for instance, trapped ions (12) or atoms in microscopic dipole traps. These proposals require the controlled emission of indistinguishable single photons by at least two identical emitters.

Various single-photon sources have been implemented using solid-state systems as well as atoms or ions. Solid-state systems such as single molecules, nitrogen-vacancy centers in diamond, and quantum dots allow high singlephoton rates (13). However, realizing truly identical sources is a major problem for such systems because of inhomogeneities in both the environment of the emitters and the emitters themselves. Another approach is provided by sources based on neutral atoms (14, 15) or ions (16) strongly coupled to a mode of a high-finesse optical cavity. Such sources are spectrally narrow, and the photons are emitted into a well-defined spatial mode, thus opening the way to coherent coupling of the quantum state of single atoms and single photons. However, the rate at which the system can emit photons is limited by the cavity and is often low in practice. Moreover, the need to achieve the strong coupling regime of cavity quantum electrodynamics remains a demanding experimental requirement.

We present a triggered single-photon source based on a single rubidium atom trapped at the focal point of a lens with a high numerical aperture of 0.7. We also show that we have full control of the optical transition by observing Rabi oscillations. Under these conditions, our system is equivalent to the textbook model formed by a two-level atom driven by monochromatic light pulses. Previous work has shown that by using holographic techniques one can create arrays of dipole traps, each containing a single atom, which can be addressed individually (17). The work presented here can therefore be directly scaled to two or more identical emitters.

We trapped the single rubidium-87 atom at the focus of the lens using a far-detuned optical dipole trap (810 nm), loaded from an optical molasses. The same lens was used to collect the fluorescence emitted by the atom (Fig. 1). The experimental apparatus is described in more detail by Schlosser et al. (18, 19). A crucial feature of our experiment is the existence of a “collisional blockade” mechanism (19) which allows only one atom at a time to be stored in the trap; if a second atom enters the trap, both are immediately ejected. In this regime, the atom statistics are sub-Poissonian and the trap contains either one or zero (and never two) atoms, with an average atom number of 0.5.

Fig. 1.

Schematic of the experiment. The same lens is used to focus the dipole trap and collect the fluorescence light. The fluorescence is separated by a dichroic mirror and imaged onto two photon-counting avalanche photodiodes (APD), placed after a beam-splitter (BS). The insert shows the relevant hyperfine levels and Zeeman sublevels of rubidium-87. The cycling transition is shown by the arrow. Also shown is the nearby F′ = 2 level responsible for the depumping.

The trapped atom was excited with 4-ns pulses of laser light, resonant with the S½, F = 2 → P3/2, F′ = 3 transition at 780.2 nm. The laser pulses were generated by frequency doubling pulses at 1560 nm, generated by using an electrooptic modulator to chop the output of a continuous-wave diode laser. A fiber amplifier was used to boost the peak power of the pulses before the doubling crystal. The repetition rate of the source was 5 MHz.

Fluorescence photons were produced by spontaneous emission from the upper state, which has a lifetime of 26 ns. The pulsed laser beam was right-circularly polarized (σ+-polarized) with respect to the quantization axis defined by a magnetic field applied during the excitation. The trapped atom was optically pumped into the F = 2, mF = +2 ground state by the first few laser pulses. It then cycled on the F = 2, mF = +2 → F′ = 3, mF′ = +3 transition, which forms a closed two-level system emitting σ+-polarized photons. Impurities in the polarization of the pulsed laser beam with respect to the quantization axis, together with the large bandwidth of the exciting pulse (250 MHz), resulted in off-resonant excitation to the F′ = 2 upper state, leading to possible de-excitation to the F = 1 ground state. To counteract this, we added a repumping laser resonant with the F = 1→ F′ = 2 transition. We checked that our two-level description was still valid in the presence of the repumper by analyzing the polarization of the emitted single photons (supporting online material text).

The overall detection and collection efficiency for the light emitted from the atom was measured to be 0.60 ± 0.04%. This was obtained by measuring the fluorescence rate of the atom for the same atomic transition driven by a continuous-wave probe beam and confirmed by a direct measurement of the transmission of our detection system (SOM text).

For a two-level atom and exactly resonant square light pulses of fixed duration T, the probability that an atom in the ground state will be transferred to the excited state is sin2T/2), where the Rabi frequency Ω is proportional to the square root of the power. Therefore, the excited state population and hence the fluorescence rate oscillates as the intensity is increased. To observe these Rabi oscillations, we illuminated the trapped atom with the laser pulses during 1 ms. We kept the length of each laser pulse fixed at 4 ns, with a repetition rate of 5 MHz, and measured the total fluorescence rate as a function of the laser power. The Rabi oscillations are clearly visible on our results (Fig. 2). From the height of the first peak and the calibrated detection efficiency measured previously, we derived a maximum excitation efficiency per pulse of 95 ± 5%.

Fig. 2.

Total count rate (squares) as a function of the average power of the pulsed beam, for a fixed pulse length of 4 ns and a repetition rate of 5 MHz. The solid line is a theoretical curve based on a simple two-level model that includes spontaneous emission and intensity fluctuations. The inserts show the time spectra for the laser intensities corresponding to π, 2π, and 3π pulses. arb. un., arbitrary units.

The reduction in the contrast of the oscillations at high laser power is mostly due to fluctuations of the pulsed laser peak power. This is shown by the theoretical curve in Fig. 2, based on a simple two-level model. This model shows that the 10% relative intensity fluctuations that we measured on the laser beam are enough to smear out the oscillations as observed.

The behavior of the atom in the time domain can be studied by using time-resolved photon-counting techniques to record the arrival times of the photons relative to the excitation pulses, thus constructing a time spectrum. By adjusting the laser pulse intensity, we observe an adjustable number of Rabi oscillations during the duration of the pulse, followed by the free decay of the atom once the laser has been turned off. The effects of pulses close to π, 2π, and 3π are displayed as inserts in Fig. 2 and show the quality of the coherent control achieved on a single atom.

To use this system as a single photon source, the laser power was set to realize a π pulse. To maximize the number of single photons emitted before the atom was heated out of the trap, we used the following sequence. First, we detected the presence of an atom in the dipole trap in real time using its fluorescence from the molasses light. Then, the magnetic field was switched on and we triggered an experimental sequence that alternated 115-μs periods of pulsed excitation with 885-μs periods of cooling by the molasses light (Fig. 3). The repumping laser remained on throughout, and the trap lifetime during the sequence was measured to be 34 ms. After 100 excitation/cooling cycles, the magnetic field was switched off and the molasses was turned back on until a new atom was recaptured and the process began again. On average, three atoms were captured per second under these conditions. The average count rate during the excitation was 9600 s–1, with a peak rate of 29,000 s–1 (corresponding to twice the first peak in Fig. 3).

Fig. 3.

Fluorescence signal measured by one of the two photodiodes during the experimental sequence, averaged over 22,958 cycles. Peaks are observed corresponding to the 115-μsperiodsof pulsed excitation, separated by periods of lower fluorescence induced by the molasses light during the 885 μs of cooling. The exponential decay of the signal is due to the lifetime of the atom in the trap, which is 34 ms under these conditions. (Insert) A close-up of the signal clearly shows the alternating excitation and cooling periods.

To characterize the statistics of the emitted light, we measured the second-order temporal correlation function, using a Hanbury Brown and Twiss type set-up. To do this, we used the beam splitter in the imaging system (Fig. 1), which sent the fluorescence light to two photon-counting avalanche photodiodes that were connected to a high-resolution time-to-digital conversion counting card in a start-stop configuration (resolution of about 1 ns). The card was gated so that only photons scattered during the 115 μs periods of pulsed excitation were counted, and the number of coincidence events was measured as a function of delay. The histogram obtained after 4 hours of continuous operation is displayed in Fig. 4 and shows a series of spikes separated by the period of the excitation pulses (200 ns). The 1/e half width of the peaks is 27 ± 3 ns, in agreement with the lifetime of the upper state. No background correction was done on the displayed data. The small flat background is attributed to coincidences between a fluorescence photon, and an event coming either from stray laser light (about 175 counts/s), or dark counts of the avalanche photodiodes (about 150 counts/s). When we correct for these events, the integrated residual area around zero delay is 3.4 ± 1.2% of the area of the other peaks.

Fig. 4.

Histogram of the time delays in the start-stop experiment. The histogram has been binned 4 times, leading to a 4.7-ns time resolution. No correction for background has been made. The absence of a peak at zero delay shows that the source is emitting single photons. During the 4-hour experimental run, 43,895 sequences were completed, which corresponds to a total of 505 s of excitation. A total of 4.83 × 106 photons were detected by the two photodiodes.

We calculated (20) that under our experimental conditions, the probability to emit exactly one photon per pulse is 0.981, whereas the probability to emit two photons is 0.019. These two-photon events would show up in the correlation curve as coincidences close to zero delay (still with no coincidences at exactly zero delay). From our calculation, the value for the ratio of the area around zero delay to the area of the other peaks is 3.7%, in excellent agreement with the experimental results.

Finally, we discuss the coherence properties of the emitted photons, necessary for entanglement protocols based on the interference between two emitted photons, either from the same atom or from different atoms. As our collection optics are diffraction limited, the outgoing photons should be in a single spatial mode of the electromagnetic field. As far as temporal coherence is concerned, the main limiting factor appears to be the motion of the atom in the trap, which can be controlled by optimized cooling sequences. We then anticipate that our source should be Fourier limited by the lifetime of the excited state. We are now working to characterize the coherence of our single-photon source and to use it to observe multiple atom interference effects.

Supporting Online Material

www.sciencemag.org/cgi/content/full/309/5733/454/DC1

SOM Text

References and Notes

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