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.
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.
We present recent results on the coherent control of an optical transition in a single rubidium atom, trapped in an optical tweezer. We excite the atom using resonant light pulses that are short (4 ns) compared with the lifetime of the excited state (26 ns). By varying the intensity of the laser pulses, we can observe an adjustable number of Rabi oscillations, followed by free decay once the light is switched off. To generate the pulses we have developed a novel laser system based on frequency doubling a telecoms laser diode at 1560 nm. By setting the laser intensity to make a pi-pulse, we use this coherent control to make a high quality triggered source of single photons. We obtain an average single photon rate of 9600 s-1 at the detector. Measurements of the second-order temporal correlation function show almost perfect antibunching at zero delay. In addition, we present preliminary results on the use of Raman transitions to couple the two hyperfine levels of the ground state of our trapped atom. This will allow us to prepare and control a qubit formed by two hyperfine sub-levels.
We present recent results on the coherent control of an optical transition in a single rubidium atom, trapped in an optical tweezer. We excite the atom using resonant light pulses that are short (4 ns) compared with the lifetime of the excited state (26 ns). By varying the intensity of the laser pulses, we can observe an adjustable number of Rabi oscillations, followed by free decay once the light is switched off. To generate the pulses we have developed a novel laser system based on frequency doubling a telecoms laser diode at 1560 nm. By setting the laser intensity to make a pi-pulse, we use this coherent control to make a high quality triggered source of single photons. We obtain an average single photon rate of 9600 s-1 at the detector. Measurements of the second-order temporal correlation function show almost perfect antibunching at zero delay. In addition, we present preliminary results on the use of Raman transitions to couple the two hyperfine levels of the ground state of our trapped atom. This will allow us to prepare and control a qubit formed by two hyperfine sub-levels.
Summary form only given. In quantum information processing, a qubit is realised as a superposition of atomic or ionic eigenstates, or as a superposition of the states of a photon. A coupling between these different types of qubits is necessary to perform quantum operations or quantum transmissions, e.g. in a quantum network where single atoms or ions are entangled with individual photons. To generate such an entanglement in a controlled way, a triggered source for single photons, a photon pistol, will be needed. Our scheme will lead to the emission of one photon into a single, predetermined mode of the radiation field in response to a trigger event. This photon is emitted from an atom strongly coupled to a high-finesse optical cavity, and the trigger is a classical light pulse. The device combines cavity-QED with an adiabatic transfer technique. We consider a single lambda-type three-level atom being placed inside an empty cavity.
A single atom strongly coupled to a high-finesse optical-cavity is a versatile tool for quantum information processing. Utilized as a single-photon source, it allows one to generate single photons very efficiently in a well defined spatio-temporal mode. In a first experiment, polarization-control over the photons is shown. A time-resolved two-photon interference experiment proves the indistinguishability of these photons - required in various quantum information processing schemes. Moreover, in a second experiment, entanglement between the polarization of the emitted photon and the population of the atomic Zeeman levels is created. Subsequent state mapping of the atomic state onto another photon results in a pair of polarization-entangled photons emitted one after the other from the cavity. Although these schemes are in principle possible in free space, the cavity boosts the efficiency by several orders of magnitude.
Single-atom-based single-photon source has several advantages, such as narrow bandwidth, wavelength matching with the absorption line of the same atomic ensemble, and insensitivity to the environment disturbing, and it is very important not only for basic researches in quantum optic field but also for applications in quantum information processing. In this paper, we report the generation of a 10-MHz-repetition-rate triggered single-photon source at 852 nm based on a trapped single cesium atom in a far-off-resonance microscopic optical dipole trap (FORT). To generate an optical dipole trap, a far-red-detuned 1064 nm laser beam is tightly focused by using a high numerical aperture lens, a typical trap depth is 2 mK and trap waist is 2.3 m. To obtain a maximum probability of pulsed excitation, the frequency of the pulsed laser should be resonant with the atomic energy levels and the trapped single atom must be excited with a -pulse. However, the interaction between the FORT laser and the atoms causes AC Stark shifts of the atomic energy levels. Thus, in order to demonstrate the resonant pulsed excitation, it is important to calculate and measure the shift of 6S1/2|Fg=4,mF=+4-6P3/2|Fe=5,mF=+5 cyclical transition in the FORT. For a two-level system, the probability of pulsed excitation can be described by Rabi oscillations with a characteristic Rabi frequency . With an optimized time sequence, we experimentally demonstrate the Rabi oscillation between the ground state and the excited state, and the peak power of -pulse laser is about 1.25 mW. We also measure the temporal envelope of single photons after a -pulse excitation. A gated pulsed excitation and cooling technique are used to reduce the possibility that atoms are heated by -pulse laser. The typical trapping lifetime of single cesium atom is extended from~108 ups to~2536 ms. The corresponding number of excitations is improved from 108 to 360000. The second-order intensity correlations of the emitted single-photon are characterized by implementing Hanbury Brown-Twiss setup. The statistics shows a strong anti-bunching with a value of 0.09 for the second-order correlation at zero delay. In the future, we will perform a Hong-Ou-Mandel two-photon interference experiment to analyze the indistinguishability of the single photons. We will also trap single atoms in a magic-wavelength optical dipole trap where the ground and the excited states have the same shift.
The generation of non-classical states of light [1] is an important scientific challenge with many potential applications. For example, squeezed states of light [2] have generated much interest in the past decade, because measurements can be performed with noise levels that are lower than those available with classical light. A particularly novel non-classical source of light is a triggered single-photon source: a source that has the property to emit with a high degree of certainty one (and only one) photon at a user-specified time. Such a deterministic source of single photons can be important for quantum information processing [3], quantum cryptography [4] and certain quantum computation problems [5]. A single quantum emitter such as a molecule or an atom is a good potential source because it can emit only one photon at a time. The emitted light is antibunched [6, 7] and consists of single photons separated by random time intervals, which depend on the excited state lifetime and on the pumping rate. Thus triggered emission of single photons can be obtained with controlled excitation of a single emitter. The first demonstration of a single photons source [8] using a single molecule in a solid is based on a rapid adiabatic passage method [9] to excite the molecule. In the effective spin picture, the Bloch vector follows the driving magnetic field adiabatically if the laser-molecule frequency detuning is swept slowly through the resonance. Since the passage inverts the field, the system goes from the ground to the excited state. From this fluorescent state the molecule can emit only a single photon. The molecular resonance is swept by Stark effect at fixed laser frequency. This is done easily by applying a radio frequency electric field to the molecule. The results showed photon states with statistics that differ radically from those of coherent states. But this method requires cryogenic temperatures (2K) since narrow zero phonon lines are needed for the adiabatic following. At room temperature a source of triggered single photons based on the concept of pulsed optical excitation of a single highly fluorescent molecule has been demonstrated [10]. A short pulse of green laser light pumps the molecule from the ground state to a vibronically excited level of the first electronic excited state. After fast (ps) relaxation to the lowest electronic excited state, the molecule subsequently emits a single photon. A frequency-doubled mode-locked Nd-YAG laser is used to pump highly stable single terrylene molecules contained in a p-terphenyl molecular crystal where they are protected from exposure to diffusing quenchers (such as oxygen). The result shows that the photons are emitted after the laser pulses at times separated by the laser repetition time within the lifetime of terrylene in p-terphenyl. Since the pump pulse width is very short compared to the fluorescence lifetime, the probability of two (or more) photon emission per pulse is always zero. At the maximum laser power, 90% of the laser pulses lead to a single photon emission. The parameters of this source (repetition rate and single photon generation probability) were limited only by the laser system used; nevertheless, this performance already surpasses that of previous work. The high performance combined with the vanishing two photon emission probability and the simplicity of this source suggests that it can be considered for a variety of quantum optical experiments and for secure quantum cryptography [11]. Semiconductor quantum dots (QD) are other potential systems for single photon generation [12]. QDs are often referred to as “artificial atoms” because many of their optical properties, such as ultranarrow transitions, arise from discrete, atomic-like energy levels [13, 14]. To reinforce this analogy a study of the photon antibunching in this system was needed. QDs differ from atoms and molecules since the creation of multiple excitons is possible in QDs. Consequently, if two electron-hole pairs can radiatively recombine, the simultaneous emission of two photons from a QD is then possible. Thus different regimes of photon statistics were expected because the probability of multiple exciton creation depends on excitation intensity. The fluorescence intensity correlation function of a single CdSe quantum dot has been investigated [15] using a coincidence setup (start-stop experiment). A strong photon antibunching has been observed over a large range of intensities (0.1-100 kW/cm2). The lack of coincidence at zero time delay indicates a highly efficient Auger ionization process [16], which suppresses multi-photon emission in these colloidal quantum dots. This result shows that room temperature single photon sources can be achieved with colloidal nanoparticles.