Optical frequency conversion, in which the color of light is changed, is a process that has numerous applications in physics and technology. For example, green laser pointers typically involve second harmonic generation, where a strong beam produced by a laser at 1064 nm is frequency doubled in a nonlinear crystal to produce a visible beam at 532 nm. Producing light at a new wavelength comes with a number of potentially important additional benefits for quantum and classical information processing. In particular, it can be used to connect quantum systems operating in different frequency regions. High performance quantum memories often accept photons in the visible spectrum. However, stable single photon generation in the telecommunications band, which is near-infrared, might be preferable because low-loss, long-distance transmission of light through optical fibers is possible at those wavelengths. Frequency conversion, if accomplished in a manner that preserves all other quantum properties of light, can enable interfacing between these different components of future quantum information processing systems. Detector technology can also benefit from frequency conversion. Commercially available silicon single photon counters operate in the visible wavelength region with high quantum efficiency, low dark count noise, and the ability to detect signals with low timing jitter and high repetition rate. On the other hand, single photon counters operating in the telecommunications-band typically do not have the same level of performance. Frequency conversion can be used to detect low levels of near-infrared light with visible wavelength detectors. In addition to allowing quantum information processing applications, frequency conversion may benefit basic research in optical spectroscopy of nanoscale systems as well as applications including optical remote sensing.
Working with colleagues at NIST’s Information Technology Laboratory (ITL), we are performing experiments on quantum frequency conversion of single photons for advanced detector technology and hybrid quantum systems. To start (Fig. 1), single photons at the telecommunications wavelength of 1300 nm are generated from a single semiconductor quantum dot and extracted into an optical fiber. They are then combined with a strong 1550 nm pump laser in a periodically-poled lithium niobate waveguide (PPLN WG). This PPLN WG ensures momentum conservation between the three optical waves involved in this process – the 1300 nm single photons, the 1550 nm pump beam, and the newly generated photons at 710 nm. The photon stream is split into two paths, with each path sent to a single photon counter that provides a record of the arrival times of the photons. The newly generated photons are then detected by visible wavelength single photon counters, for example, in a photon correlation setup used to measure the quantum properties of the optical field.
Fig. 1. Quantum frequency conversion experiments. (Left) Single photons are produced by a single semiconductor quantum dot and coupled into an optical fiber. (Center) The single photon stream is combined with a strong classical pump beam in a nonlinear crystal. (Right) Photon correlation measurements characterize the quantum nature of the frequency converted single photons using time-correlated single photon counting.
Fig. 2(a) characterizes our ability to measure single photons before and after frequency conversion, through a measurement of the quantum dot’s excited state lifetime. The dynamic range of the measurement is improved by a factor of 25, due to the detection benefits associated with working at visible wavelengths. In particular, the improvement comes from the faster trigger frequency and lower dark count rate at which the visible wavelength detector operates. Fig. 2(b) is a measurement of the intensity correlation function for the frequency converted light at 710 nm. The histogram of the difference in arrival times between the two photon paths shown in Fig. 2(b) indicates a strong suppression of events with zero time delay. This indicates that the light is dominantly composed of single photons, as one does not expect to detect a single photon in both paths at the same time. Such ‘photon antibunching’ is not seen with classical light, even attenuated down to low power levels, as the statistics of classical light and true single photons are markedly different.
Fig. 2. (a) Comparison of the excited state lifetime of a single quantum dot, measured using an InGaAs single photon counter detecting the original photons at 1300 nm (left), and using a Si single photon counter detecting the upconverted photons at 710 nm (right). (b) Photon correlation measurement of the upconverted photons, displaying the photon antibunching signature of a source dominantly composed of single photons.
These results experimentally indicate that photon statistics are preserved during the frequency conversion process. Our collaboration with ITL has further verified this through measurements of higher order correlation functions with coherent and thermal light sources. Recently, we have examined how frequency conversion using a pulsed 1550 nm laser beam can modify not only the wavelength of the single photons, but also the temporal profile of their wavepacket. Along with a parallel demonstration of direct electro-optic modulation of single photon wavepackets, this work provides an important resource for connecting quantum systems, where one seeks to not only match operating frequencies but also bandwidths.
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