Quantum information processing uses the unique properties of quantum mechanics to go beyond the limitations of classical systems. Ultrafast optical pulses that enable unique quantum states, such as entangled photon pairs and sources of scalable single photons, can enable advanced capabilities such as increased capacity, coding, and truly secure communications. Fundamental to some emerging schemes for linear optical quantum computation is a source of single-photon wavepackets capable of high-visibility interference in scalable networks. A research group at Oxford University (Oxford, UK) has developed a technique for generating this type of source using femtosecond-pumped waveguides and parametric down conversion (PDC), which could potentially improve applications such as quantum cryptography that exploit the interferences of multiple pure single-photon states in linear optical communication networks.
"Ultrafast laser pulses with temporal, spectral, and spatial properties that are tailored for quantum information processing applications may be used for generating photon pairs by means of spontaneous PDC," explains Ian Walmsley, head of the research group at Clarendon Laboratories at the University of Oxford. "Our waveguide source is pumped by femtosecond pulses, combines precision timing with spatial and spectral control, and exhibits extraordinary source brightness," says Walmsley. The source uses micro-structured waveguides, ultrashort timing pulses, and a novel type-II phase-matching configuration. At the heart of this approach are the photonic wavepackets that exhibit well-defined photon number and modal character. Well-defined modes also allow efficient fiber coupling.
The KTP waveguide is highlighted in the oval pullout shown here. The waveguide approach delivers greater control of photon emission count and modes.
"We have designed a type-II PDC interaction in a periodically poled KTP [potassium titanyl phosphate] waveguide that leads to easily separable photon pairs by means of their polarization," says Walmsley. This phase-matching configuration yields a horizontally polarized UV photon that spontaneously decays into two IR photons, which are horizontally and vertically polarized. In their system, the output of a mode-locked titanium sapphire laser with 100-fs pulsewidths and an 87-MHz repetition rate is directed into a 2-mm-long ß-barium borate crystal. The resulting UV pulses are centered at 400.5 nm with a power of 15 µW. This is focused with a 10X microscope objective into a 12-mm-long periodically poled z-cut KTP waveguide with an 8.7-µW period. A polarizing beamsplitter separates the IR photon pairs that result; the horizontal polarization is coupled into a multimode fiber for detection with a fiber-coupled avalanche photodiode. The vertical polarization is used as the trigger signal and a small percentage of the laser power is used for time gating. Using AND gates the researchers can synchronize the time-gated and non-gated signals to within 3 ns. The team demonstrated a preparation efficiency of single photons of nearly 85%, which means they can determine the presence of a single photon in a well-defined spatio-temporal mode with 85% fidelity.
According to Alan Lee Migdall, a physicist at the National Institute of Standards and Technology's Optical Technology Division (Gaithersburg, MD), "Single photon sources are critical for quantum information in general and quantum cryptography and computing both. There's been an effort to make these things as efficient and bright as possible, and this is the latest salvo in that area. [Walmsley's approach] is good because the problem with things in bulk is that it puts out light over a wide range of angles and the waveguide effort gives you a chance to engineer the mode during production." Migdall adds that just a few years ago, single photon sources generated about 10 photon pair coincidences per milliwatt of pump power per second, compared to Walmsley's system, which produces approximately 850,000 coincidences per milliwatt (see oemagazine, August 2004).
Further development of the single photon source could include modal engineering of the photon states and mode matching into single-mode fibers. The researchers don't see this as a problem given that the waveguide exhibits accurate modal control. The source's precise timing combined with high brightness should enable source scalability to larger systems using multi-waveguide chips, which will be important for future systems. Walmsley's group views their source as a fundamental building block for quantum information applications offering compatibility with all-fiber systems.