The wavelength of a single photon can be changed significantly without destroying its fundamental quantum character, researchers have reported. This finding has application in the developing field of quantum information science, which includes quantum computing, communication and cryptography. Quantum information science could be used to solve some problems much more rapidly than classical computer technology can. The single photons used in this work are generated “on demand,” in contrast to “heralded” schemes, where single photons are available conditionally on detection of another photon, whose arrival time is probabilistic, said Kartik Srinivasan and Xiao Tang at the National Institute of Standards and Technology (NIST). On-demand generation of single photons is extremely important for various quantum information systems, they noted. At present, most systems that can perform local quantum operations and storage work in the visible wavelengths to the edge of the near-infrared: from 400 to 900 nm. For quantum communication to take place between such systems over long distances, they said, the wavelength of single photons carrying quantum information must be in the telecommunication band: at 1300 nm, for example. This means that photons must be efficiently converted in wavelength for these systems to be used in a long-distance quantum network. The collaboration that evolved between the scientists’ laboratories at NIST was a result of their findings in the area of single-photon detection and wavelength conversion. Together, they focused at first on the sensitive spectroscopy of solid-state materials using newly developed upconversion detectors (as reported in Photonics Spectra, October 2009, page 28). The detection process involved changing the wavelength of the photons, and the solid-state source (quantum dot) emits single photons of nonclassical light, so there was an opportunity to demonstrate that the light emission’s quantum character was preserved after its wavelength was altered, said Matthew Rakher and Lijun Ma, the two lead experimental researchers working in Srinivasan’s and Tang’s laboratories, respectively. This happens because the upconversion device changes photons from one wavelength to another, while the nonwavelength quantum properties of the photon – such as polarization and phase – remain the same after converting. The energy and momentum conservation required in the conversion process guarantee this, they added. They then found that systems that naturally interact strongly with light of different wavelengths could be connected in the form of a hybrid system. They used a custom-built liquid helium cryostat that contained a sample of material with embedded InAs quantum dots and a specially designed near-field fiber optic probe (see Figure) to generate the single photons. At 6 K, the quantum dots were excited with a pulsed laser at a fixed repetition rate to generate single photons at 1300 nm, which were extracted efficiently through the fiber optic probe and passed out of the cryostat by fiber feed-throughs. In this schematic of the NIST experiment, triggered single photons at 1300 nm are created by exciting a semiconductor quantum dot (QD) with a fixed-repetition-rate pulsed laser. The photons are outcoupled into an optical fiber and interfaced with an upconversion system in which a strong pump laser near 1550 nm and a nonlinear crystal, periodically poled lithium niobate waveguide (PPLN WG), are used to convert the wavelength to 710 nm. The photon correlation setup is used to verify the single-photon nature of the upconverted light by splitting it into two paths for coincidence detection, since a single photon cannot simultaneously be detected in both paths. The technique referred to is time-correlated single photon counting (TCSPC). Courtesy of NIST. The efficiency of the fiber optic collection technique negated the need for less efficient traditional free-space optics, the scientists said. The collected photons in the fiber were then transported to the upconversion detector for detection, recording and signal processing. The detector consisted of a wavelength conversion device and a commercially available silicon-based single-photon counter. A specially designed periodically poled lithium niobate waveguide was used as the wavelength conversion device. The detection was accomplished by combining the 1300-nm single photons from the quantum dot with a strong pump beam near 1550 nm. In the experiment, the ratio of power at 1550 nm to 1300 nm was more than 14 orders of magnitude. The strong pump was created by seeding an erbium-doped fiber amplifier with a tunable 1550-nm diode laser. After combining with the strong pump in the wavelength conversion device, the single photons at 1300 nm were converted to 710 nm and then measured with a silicon single-photon counter. Such silicon-based single-photon counters are well-established for use in the visible and near-visible ranges but do not work in the telecommunication bands. The available options for detection of single photons in the telecom bands have problems, the researchers said. They feel that it is better to convert photons in the telecom wavelengths into the visible or near-visible range and then detect them using silicon-based detectors. The scientists found that the frequency upconversion process can be used for efficient and high-signal-to-noise spectroscopy down to the single-photon level. Their overall detection efficiencies were very close to the best attainable with commercially available InGaAs single-photon counters but with better signal to noise. They also showed that for time-resolved experiments, frequency upconversion outperforms InGaAs detection by at least an order of magnitude in the dynamic range. High-signal-to-noise detection is critically important in quantum information because it stores information carried by single photons, the researchers noted. Low efficiencies and high dark count rates have been shown theoretically to have detrimental effects on the probabilities of success for quantum operations. So for quantum information protocols to be used for applications in the real world, high-performance detection is a must. Srinivasan and Tang said that frequency upconversion is already used for detection in telecommunication bands in quantum cryptography systems with heralded single photons from nonlinear optical processes, and their work extends this for use with on-demand single photons from solid-state sources. Thus, solid-state sources can now be readily considered for these real-world systems as well. The frequency upconversion technology provides an effective way to convert flying quantum bits (qubits) to stationary qubits or reverse, which opens up the possibility of building advanced hybrid quantum systems composed of atoms or ions, quantum dots and photons, Rakher and Ma added. Such hybrid systems could ultimately be used in long-distance secure quantum communication and in engineering of scalable quantum networks, they said. They are now working on single-photon frequency conversion as a way to connect their quantum dot system to an atomic quantum memory for photon storage. This requires frequency conversion not only to match the absorption of the atomic memory, but also to shape the wave packet of the single photon to maximize the absorption.