Time-resolved and antibunching experiments on single quantum dots at 1300nm
C. Zinoni, Blandine Alloing, Christelle Monat, Val Zwiller, Lianhe H. Li, Andrea Fiore, L. Lunghi, Annamaria Gerardino, Hugues de Riedmatten, Hugo Zbinden, Nicolas Gisin
We present time integrated and time-resolved photoluminescence (PL) measurements on a single InAs∕GaAs quantum dot (QD), embedded in a planar microcavity, emitting in the 1300nm telecom band. The results of both measurements clearly identify the exciton and biexciton transitions from a single QD. By optimizing the extraction efficiency of the QD PL into the single mode fibers and carefully tuning two InGaAs avalanche photodiodes, we were able to measure the second order correlation function with integration times comparable to those made with silicon based technology. These measurements demonstrate that our single QDs are efficient sources of triggered single photons for quantum key distribution in the O band.
We present time integrated and time-resolved photoluminescence (PL) measurements on a single InAs∕GaAs quantum dot (QD), embedded in a planar microcavity, emitting in the 1300nm telecom band. The results of both measurements clearly identify the exciton and biexciton transitions from a single QD. By optimizing the extraction efficiency of the QD PL into the single mode fibers and carefully tuning two InGaAs avalanche photodiodes, we were able to measure the second order correlation function with integration times comparable to those made with silicon based technology. These measurements demonstrate that our single QDs are efficient sources of triggered single photons for quantum key distribution in the O band.
This thesis deals with the optical characterization of single quantum dot devices emitting at 1300nm. Thanks to the development and optimization of the growth technique we were able to achieve at the same time emission at 1300nm and ultra low QD densities. Our single QD devices present clear and reproducible spectral signatures in which we can identify exciton, biexciton and charged exciton transitions. Quasi-resonant excitation at 70K demonstrates background free single exciton transitions, which is very promising for the realization of a single photon device operating at temperatures in easy reach of thermoelectric coolers. A time-correlated single photon counting setup was built and used to measure the radiative lifetimes of single exciton transitions. These measurements also present new evidence on a background emission superposed to the narrow spectral transitions. Demonstration of single photon emission at these wavelengths required building a setup to measure the correlations between fiber-coupled single photons in a 300ps time window, emitted from a nano-device in free space at cryogenic temperatures, and with the capability of maintaining the optical alignment on a micrometer scale for several hours. With such a setup we have demonstrated that our QDs can generate single photon states at 1300nm. We used this single photon source to characterize novel detectors based on superconducting nanowires and measured for the first time the intensity correlation function at 1300nm on single photons from a QD. These detectors show at least 2 orders of magnitude improvement on the signal to noise ratio as compared to InGaAs APDs; this is very important since, for a QKD system, the detector noise, amongst others, determines the maximum distance over which a secure key can be exchanged. It should be noted that, due to the di±culties in these measurements, to date there has been only one other clear demonstration of single photon emission at 1300nm.
We report on the experimental demonstration of triggered single-photon emission at the telecom O-band from In(Ga)As/GaAs quantum dots (QDs) grown by metal-organic vapor-phase epitaxy. Micro-photoluminescence excitation experiments allowed us to identify the p-shell excitonic states in agreement with high excitation photoluminescence on the ensemble of QDs. Hereby we drive an O-band-emitting GaAs-based QD into the p-shell states to get a triggered single photon source of high purity. Applying pulsed p-shell resonant excitation results in strong suppression of multiphoton events evidenced by the as measured value of the second-order correlation function at zero delay of 0.03 (and ~0.005 after background correction).
We present the first demonstration of telecom fiber-based quantum key distribution using single photons from a quantum dot in a pillar microcavity. The source offers both telecommunication wavelength operation at 1.3 microns and Purcell enhancement of the spontaneous emission rate. Several emission lines from the InAs/GaAs quantum dot are identified, including the exciton-biexciton cascade and charged excitonic emission. We show an order of magnitude increase in the collected intensity of the emission from a charged excitonic state when temperature tuned onto resonance with the HE11 mode of the pillar microcavity, as compared to the off-resonance intensity. Above- and below-GaAs-bandgap optical excitation was used and the effect of the excitation energy on the photoluminescence investigated. Exciting below the GaAs-bandgap offers significant improvement in the quality of the single photon emission and a reduction of the multi-photon probability to 0.1 times the value for Poissonian light was measured, before subtraction of detector dark counts, the lowest value recorded to date for a quantum dot source at a fibre wavelength. We observe also the first evidence of Purcell enhancement of the spontaneous emission rate for a single telecommunication wavelength quantum dot in a pillar microcavity. We have incorporated the source into a phase encoded interferometric scheme implementing the BB84 quantum cryptography protocol and distributed a key, secure from the pulse splitting attack, over standard telecommunication optical fibre. We show a transmission distance advantage over that possible with (length-optimized) uniform intensity weak coherent pulses at 1310 nm in the same system.
Single photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by non-resonant and resonant photoluminescence experiments in single QDs [1,2]. Up to now, all the measurements reported in the literature focus on the incoherent photoluminescence signal for achieving single photon emission. In this regime, the linewidth of the photoluminescence spectrum is limited to the so-called radiative limit. In this paper, we report on the resonant emission (RE) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity [2]. We present an original type of single photon source based on the coherent laser light scattering by a single QD [3]. Besides the antibunching effect showing the nonclassical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. Figure (a,b,c) shows high-resolution measurements by Fourier transform spectroscopy of the RE where the data are fitted to the first-order correlation function g(1)(τ) of the RE of a two-level system. The inverse Fourier transform of the g(1)(τ) function allows a direct comparison of the different RE emission profiles in figure (d,e,f). By reducing the resonant excitation power, the Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in an ultra-coherent character [3]. References [1] P. Michler et al, Science 290, 2282 (2000). [2] H. S. Nguyen et al, Phys. Rev. Lett. (accepted, in press). See arXiv 1109.4304. [3] H. S. Nguyen et al, Appl. Phys. Lett. 99, 261904 (2011).
We report a 'plug and play' single-photon source based on single quantum dots with an integrated optical fibre output. We attach an optical fibre to the sample surface in order to measure single-dot photoluminescence, which avoids the light coupling between quantum dots and free space. The excitation light (a HeNe Laser) is coupled into the optical fibre to excite the quantum dots, and the emitted photoluminescence is collected with same optical fibre. To verify the emission of single photons, correlation measurements of the luminescence were performed using a Hanbury Brown and Twiss setup with a 50/50 beamsplitter and two single-photoncounting avalanche photodiodes. Clear antibunching with the second-order correlation function at zero time delay less than 0.1 indicates single-photon emission. The devices are stable, and reproducible for an arbitrarily long time, which is very promising for real implementations of quantum key distribution and linear optical quantum computation.
Single photon emission is one of the most striking optical properties of semiconductor quantum dots (QDs), and the corresponding photon anti-bunching effect was observed by non-resonant and resonant photoluminescence experiments in single QDs [1,2,3,4]. Other experiments demonstrated the possibility to use single QDs for the generation of indistinguishable single photons [3]. An important general issue is to reach the so-called fundamental radiative limit. Several strategies have been followed in that sense such as the shortening of population lifetime induced by the Purcell effect [5] or the reduction of pure dephasing by resonant excitation [2,3,4]. Up to now, all the measurements reported in the literature focus on the incoherent photoluminescence signal for achieving single photon emission. In this regime, the linewidth of the photoluminescence spectrum is always limited to the radiative limit. In this paper, we report on the resonant emission (RE) of the exciton in single InAs/GaAs QDs embedded in a planar microcavity, in an orthogonal excitation-detection geometry [4]. We present an original type of single photon source based on the coherent laser light scattering by a single QD [6]. Besides the antibunching effect showing the non-classical nature of the emitted light, we observe that the QD emission spectrum is determined by the spectrum of the resonant excitation laser. We present high-resolution measurements by Fourier transform spectroscopy of the RE where the data are fitted to the first-order correlation function g(1)(τ) of the RE of a two-level system. The inverse Fourier transform of the g(1)(τ) function allows a direct comparison of the different RE emission profiles. We show that, by reducing the resonant excitation power, the so-called Mollow triplet shrinks to a single narrow Lorentzian line, while the relative intensity of this narrow peak increases and tends to overwhelm the RE spectrum. This implies that single QDs can produce single photons with a coherence time that is not limited anymore by the QD electronic properties, resulting in an ultra-coherent character [6]. In conclusion, the resulting " ultra-coherent single photon source " promises high degrees of indistinguishability of the emitted photons which is a crucial requirement for quantum information applications. [1] P. Michler et al., Science 290, 2282 (2000). [2] A. Muller et al., Phys. Rev. Lett. 99, 187402 (2007). [3] S. Ates et al., Phys. Rev. Lett. 103, 167402 (2009). [4] H. S. Nguyen et al., Phys. Rev. Lett. 108, 057401 (2012). [5] C. Santori et al., Nature 419, 594 (2002). [6] H. S. Nguyen et al., Appl. Phys. Lett. 99, 261904 (2011).