Ville Ahtee

Realization of two Fourier-limited solid-state single-photon sources

We demonstrate two solid-state sources of indistinguishable single photons. High resolution laser spectroscopy and optical microscopy were combined at T = 1.4 K to identify individual molecules in two independent microscopes. The Stark effect was exploited to shift the transition frequency of a given molecule and thus obtain single photon sources with perfect spectral overlap. Our experimental arrangement sets the ground for the realization of various quantum interference and information processing experiments.

Precision spectroscopy of acetylene transitions using an optical frequency synthesizer.

An optical frequency synthesizer is used for saturation spectroscopy of acetylene near 1540 nm. In the synthesizer, a user-specified frequency is generated from an atomic time base by phase locking the second harmonic of a cw near-IR external-cavity diode laser (ECDL) to a Ti:sapphire frequency comb. By stepping the repetition rate of the frequency comb, the ECDL frequency is swept over an acetylene transition in a saturated absorption spectroscopy setup. Hence, a spectral lineshape is measured with an absolute frequency scale. Line-center frequencies determined by fitting theoretical line profiles to the measured data are in good agreement with values measured with the ECDL stabilized to acetylene by third-harmonic locking and with the values recommended by the International Committee for Weights and Measures (CIPM).

Molecules as sources for indistinguishable single photons

We report on the triggered generation of identical photons by solid-state single-photon sources in two separate cryogenic laser scanning microscopes. Organic fluorescent molecules were used as emitters and investigated by means of high resolution laser spectroscopy. Continuous-wave photon correlation measurements on individual molecules proved the isolation of single quantum systems. By using frequency selective pulsed excitation of the molecule and efficient spectral filtering of its emission, we produced triggered Fourier-limited single photons. In a further step, local electric fields were applied to match the emission wavelengths of two different molecules via Stark effect. Identical single photons are indispensable for the realization of various quantum information processing schemes proposed. The solid-state approach presented here paves the way to the integration of multiple bright sources of single photons on a single chip.

Fiber-based acetylene-stabilized laser

We describe a fiber-based acetylene-stabilized laser system in which light is coupled to free space only to pass through the acetylene cell. The system is constructed using standard components used in telecommunication networks, making the approach straightforward to implement. The Doppler-free spectrum of acetylene can be detected, even without a lock-in amplifier using simple collinear pump-probe geometry, when the length of the optical path in the spectroscopy arrangement is dithered to cancel interference that is inevitably present in a fiber-based setup. The laser was stabilized to five different transitions of 13 C2H2 at the (nu1 + nu3) band using the third-harmonic technique, and the absolute frequencies were measured with an optical frequency comb generator. All the results are in good agreement with the values recommended by the International Committee for Weights and Measures (CIPM). The method for canceling the interference presented here can conceivably be generalized to other types of stabilized lasers as well.

Comparison of absolute spectral irradiance responsivity measurement techniques using wavelength-tunable lasers

Independent methods for measuring the absolute spectral irradiance responsivity of detectors have been compared between the calibration facilities at two national metrology institutes, the Helsinki University of Technology (TKK), Finland, and the National Institute of Standards and Technology (NIST). The emphasis is on the comparison of two different techniques for generating a uniform irradiance at a reference plane using wavelength-tunable lasers. At TKKs Laser Scanning Facility (LSF) the irradiance is generated by raster scanning a single collimated laser beam, while at the NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources (SIRCUS), lasers are introduced into integrating spheres to generate a uniform irradiance at a reference plane. The laser-based irradiance responsivity results are compared to a traditional lamp-monochromator-based irradiance responsivity calibration obtained at the NIST Spectral Comparator Facility (SCF). A narrowband filter radiometer with a 24 nm bandwidth and an effective band-center wavelength of 801 nm was used as the artifact. The results of the comparison between the different facilities, reported for the first time in the near-infrared wavelength range, demonstrate agreement at the uncertainty level of less than 0.1%. This result has significant implications in radiation thermometry and in photometry as well as in radiometry.

Single-frequency synthesis at telecommunication wavelengths

We have developed an optical single-frequency synthesizer for telecommunications wavelengths. The synthesizer is based on a conventional Ti:S frequency comb generator and generates a single user-specified frequency within the 192-196 THz gain bandwidth of the erbium-doped fiber amplifier from an atomic time base. The synthesized frequency can be directly used, e.g., for spectroscopy, high precision characterization of optical components, or as a reference in calibration of acetylene-stabilized lasers.

Absolute frequency measurement of an iodine-stabilized 543-nm He-Ne laser with modulation-synchronized phase-locked loop for improved frequency counting

An iodine-stabilized 543-nm He-Ne laser is an important laser radiation for the realization of the definition of the meter. These secondary standards should be absolutely measured for traceability to the SI-second. Due to the low power of 543-nm He-Ne lasers, a method to improve the signal-to-noise ratio of this measurement is needed. We use a modulation-synchronized phase-locked loop to lock a slave laser without transferring modulation, thus improving the signal-to-noise ratio through reduced bandwidth and higher optical power. The absolute frequency value thus obtained is in good agreement with the CIPM value.

Realization of two independent Fourier-limited solid-state single-photon sources

We combined high resolution laser spectroscopy and microscopy to identify individual molecules in two independent microscopes. Then the Stark effect was exploited to tune the transition frequencies of the molecules and thus obtain indistinguishable single photons.