We report the implementation of a high-rate source of single- and two-photon states. By combining the advantages of short pulses and cavities, heralding rates as high as 200 kHz have been obtained for the single photons, as well as 250 Hz for the two-photon states. In this setup, homodyne measurements are conditioned by the heralding of the quantum states thanks to the introduction of a low-loss optical delay line in the heralded states path. This enables the detection of most of the heralded events, and fidelities reaching 68.5% (91% with correction for detection efficiency) and 50.4% (85% with correction) were obtained for the single- and two-photon states, respectively. Such high rates and fidelities in the generation of elementary Fock states may open the path for the production of complex quantum states.
We report the implementation of a high-rate source of single- and two-photon states. By combining the advantages of short pulses and cavities, heralding rates as high as 200 kHz have been obtained for the single photons, as well as 250 Hz for the two-photon states. In this setup, homodyne measurements are conditioned by the heralding of the quantum states thanks to the introduction of a low-loss optical delay line in the heralded states path. This enables the detection of most of the heralded events, and fidelities reaching 68.5% (91% with correction for detection efficiency) and 50.4% (85% with correction) were obtained for the single- and two-photon states, respectively. Such high rates and fidelities in the generation of elementary Fock states may open the path for the production of complex quantum states.
Heralded single photons are prepared at a rate of approximately 100 kHz via conditional measurements on polarization-nondegenerate biphotons produced in a periodically poled potassium-titanyl phosphate crystal. The single-photon Fock state is characterized using high-frequency pulsed optical homodyne tomography with a fidelity of (57.6+/-0.1)%. The state preparation and detection rates allowed us to perform on-the-fly alignment of the apparatus based on real-time analysis of the quadrature measurement statistics.
Multi-photon Fock states have diverse applications such as optical quantum information processing. For the implementation of quantum information processing, Fock states should be generated within the telecommunication wavelength band, particularly in the C-band (1530-1565 nm). This is because mature optical communication technologies can be leveraged for transmission, manipulation, and detection. Additionally, to achieve high-speed quantum information processing, Fock states should be generated in optical pulses with as short a duration as possible, as this allows embedding lots of information in the time domain. In this paper, we successfully generate picosecond pulsed multi-photon Fock states (single-photon and two-photon states) in the C-band with Wigner negativities for the first time, which are verified by pulsed homodyne tomography. In our experimental setup, we utilize a single-pixel superconducting nanostrip photon-number-resolving detector (SNSPD), which is expected to facilitate the high-rate generation of various quantum states. This capability stems from the high temporal resolution of SNSPDs (several tens of picoseconds in our case and also in general) allowing us to increase the repetition frequency of pulsed light from the conventional MHz range to the GHz range, although in this experiment the repetition frequency is limited to 10 MHz due to the bandwidth of the homodyne detector. Consequently, our experimental setup is anticipated to serve as a prototype of a high-speed optical quantum state generator for ultrafast quantum information processing at telecommunication wavelength.
Abstract : Single photons sources are desired for many potential quantum information applications. One common method to produce single photons is based on a heralding protocol, which involves generating a pair of photons in an optical nonlinear medium and then detecting one photon in the pair to announce the existence of the other photon of the pair. Compared to other schemes, such a method offers several significant advantages. A major challenge to overcome which is inherent to this method, however, is that the photon pairs are created stochastically in the optical nonlinear medium. Thus, in order to ensure high purity levels for the single photons generated via heralding, sources of this type must be operated at low photon-pair production rates in order to keep the probability for simultaneous creation of multiple photon-pairs to an acceptably low level. A direct result of this restriction is a low heralding rate of single photons. A potential solution for achieving high-rate, high-purity, single-photon generation combines several heralding single-photon sources which are actively multiplexed to act as one source. The overall goal of this project is to develop methods to perform such multiplexing based on the use of a low loss high speed fiber optical switch.
Two important challenges in quantum photonics are to generate useful states with high fidelity, and to detect them and verify their properties. Particularly valuable states are single photons and entangled photon pairs in well-defined optical modes, as they can be used in many quantum information protocols or used to build up more complex states. For sources, we employ integrated nonlinear optics (waveguides in lithium niobite and potassium titanyl phosphate) to maximize brightness and go beyond what is possible in bulk optics, showing simultaneously high state fidelity, heralding efficiency, and spectral purity across three experiments: first we show record heralding efficiency in a fully-fibered heralded single-photon source, and use it to probe the tradeoff between spectral purity and heralding efficiency in non-engineered sources. With an engineered source, we then herald up to 50 photons in a nonclassical state. The last source is for polarization-entangled photon pairs, with brightness of 3.5 million pairs/s·mW, fidelity to a Bell state of 96%, heralding efficiency of 43%, and HOM interference visibility of 82%. Once a complex state is constructed, it must also be verified. For this we employ a time-multiplexed detector consisting of a fibre loop and a single-photon detector. Surprisingly, we are able to extract information even in the saturation regime of the detector. We use the click statistics of the time-multiplexed detector to verify the non-classicality of quantum light, and we use its extremely high dynamic range (123 dB) to measure a macroscopic power level with a single-photon detector. Eliminating calibrated attenuators with this approach will allow direct standardization of quantum and classical optical power levels.
The preparation of single photons in a pure quantum state is a subject of great interest in physics, enabling the control of light at an unprecedented level. The ease with which certain degrees of freedom of photon states, such as polarisation, can be manipulated, along with the inherent resilience of photons to decoherence, makes them well suited for use as qubits. Recent rapid developments in the transmission and processing of quantum information, as well as the likely technological impact of potential real-world applications such as quantum cryptography and quantum computation, mean that the demand for high performance single photon sources is likely to increase in the near future. One approach to producing single photon states, which are known to be in a well-defined spatio-temporal mode without destructively measuring them, is to take advantage of nonlinear optics. Nonlinear processes can be used to realise frequency conversion by generating a single, correlated pair of photons from an intense pump laser source. The detection of one of the photons from a pair can then be used to indicate the presence of the other photon in the pair, a procedure known as heralding. This thesis describes the development of a source of heralded single photons at 1550 nm, generated directly in the core of a photonic crystal fibre (PCF). By taking advantage of low loss fibre components for the required spectral filtering of the generated photon state, a heralding fidelity of 52% was achieved. The source was designed to be used with a picosecond pulsed fibre laser, making it relatively low cost and maintenance free. With 148 mW of average pump power a heralded output photon rate of 6.4 × 104 s-1 was observed, demonstrating the brightness of the source. The purity of the generated single photons was established by measuring non-classical interference, with a visibility of 70%, between the photons output from this source and a source based on a PPLN waveguide. The fabrication of a series of birefringent PCFs for the generation of spectrally pure state photons at 1550 nm is also discussed. These PCFs will be useful for incorporation in the next generation of high performance, fibre-based photon sources.