An on-demand single-photon source is a fundamental building block in quantum science and technology. We experimentally demonstrate the proof of concept for a scheme to generate on-demand single photons via actively multiplexing several heralded photons probabilistically produced from pulsed spontaneous parametric down-conversions (SPDCs). By utilizing a four-photon-pair source, an active feed-forward technique, and an ultrafast single-photon router, we show a fourfold enhancement of the output photon rate. Simultaneously, we maintain the quality of the output single-photon states, confirmed by correlation measurements. We also experimentally verify, via Hong-Ou-Mandel interference, that the router does not affect the indistinguishability of the single photons. Furthermore, we give numerical simulations, which indicate that photons based on multiplexing of four SPDC sources can outperform the heralding based on highly advanced photon-number-resolving detectors. Our results show a route for on-demand single-photon generation and the practical realization of scalable linear optical quantum information processing.
We propose a new single photon source based on the principle of active multiplexing of heralded single photons which, unlike previously reported architecture, requires a limited amount of physical resources. We discuss both its feasibility and the purity and indistinguishability of single photons as function of the key parameters of a possible implementation.
The non-deterministic nature of photon sources is a key limitation for single photon quantum processors. Spatial multiplexing overcomes this by enhancing the heralded single photon yield without enhancing the output noise. Here the intrinsic statistical limit of an individual source is surpassed by spatially multiplexing two monolithic silicon correlated photon pair sources, demonstrating a 62.4% increase in the heralded single photon output without an increase in unwanted multi-pair generation. We further demonstrate the scalability of this scheme by multiplexing photons generated in two wave…
Heralded single-photon source (HSPS) with competitive single photon purity and indistinguishability has become an essential resource for photonic quantum information processing. Here, for the first time, we proposed a theoretical regime to enhance heralded single-photons generation by multiplexing the degree of the freedom of orbital angular momentum (OAM) of down-converted entangled photon pairs emitted from a nonlinear crystal. Experimentally, a proof-of-principle experiment has been performed through multiplexing three OAM modes. We achieve a 47$\%$ enhancement in single photon rate. A seco…
Efficient generation of single photons is one of the key challenges of building photonic quantum technology, such as quantum computers and long-distance quantum networks. Photon source multiplexing -- where successful pair generation is heralded by the detection of one of the photons, and its partner is routed to a single mode output -- has long been known to offer a concrete solution, with output probability tending toward unity as loss is reduced. Here, we present a temporally multiplexed integrated single photon source based on a silicon waveguide and a low-loss fibre switch and loop archit…
Single-photon sources based on optical parametric processes have been used extensively for quantum information applications due to their flexibility, room-temperature operation and potential for photonic integration. However, the intrinsically probabilistic nature of these sources is a major limitation for realizing large-scale quantum networks. Active feedforward switching of photons from multiple probabilistic sources is a promising approach that can be used to build a deterministic source. However, previous implementations of this approach that utilize spatial and/or temporal multiplexing s…
We present a simple technique to reduce the emission rate of higher-order photon events from pulsed spontaneous parametric down-conversion. The technique uses extra-cavity control over a mode locked ultrafast laser to simultaneously increase repetition rate and reduce the energy of each pulse from the pump beam. We apply our scheme to a photonic quantum gate, showing improvements in the non-classical interference visibility for 2-photon and 4-photon experiments, and in the quantum-gate fidelity and entangled state production in the 2-photon case.
Single photons are a vital resource for photonic quantum information processing. However, even state-of-the-art single photon sources based on photon-pair generation and heralding detection have only a low probability of delivering a single photon when one is requested. We analyse a scheme that uses a switched fibre delay loop to increase the delivery probability per time bin of single photons from heralded sources. We show that, for realistic experimental parameters, combining the output of up to 15 pulses can yield a performance improvement of a factor of 10. We consider the future performan…
We present a multiplexed single photon source that re-synchronises heralded single photons generated by four-wave mixing in photonic crystal fibre using a fibre delay loop controlled by a single switch. By combining the probabilities of generating heralded single photons on four consecutive pump pulses we obtain an enhancement of the per-bin single-photon delivery probability. Our source demonstrates a way in which single-photon source multiplexing may be carried out with minimised resource overhead while retaining the benefits of a robust and alignment-free platform.
We demonstrate multiphoton interference using a resource-efficient frequency multiplexing scheme, suitable for quantum information applications that demand multiple indistinguishable and pure single photons. In our source, frequency-correlated photon pairs are generated over a wide range of frequencies by pulsed parametric down conversion. Indistinguishable single photons of a predetermined frequency are prepared using frequency-resolved detection of one photon to control an electro-optic frequency shift applied to its partner. Measured photon statistics show multiplexing increases the probabi…
It is a fundamental challenge in quantum optics to deterministically generate indistinguishable single photons through non-deterministic nonlinear optical processes, due to the intrinsic coupling of single- and multi-photon generation probabilities in these processes. Actively multiplexing photons generated in many temporal modes can decouple these probabilities, but key issues are to minimize resource requirements to allow scalability, and to ensure indistinguishability of the generated photons. We demonstrate the multiplexing of photons from four temporal modes solely using fiber-integrated …
Optical communication systems are able to send the information from one user to another in light beams that travel through the free space or optical fibers, therefore how to send larger amounts of information in smaller periods of time is a long term concern, one promising way is to use multiplexing of photon's different degrees of freedoms to parallel handle the large amounts of information in multiple channels independently. In this work, by combining the wavelength and time division multiplexing technologies, we prepare a multifrequency mode time bin entangled photon pair source at differen…
On-demand generation of indistinguishable single- and multi-photon states is a key technology for scaling up optical quantum information and communication applications. Nonlinear parametric photon-pair sources and heralded single-photon sources (HSPSs) had been the most standard resource of quantum information applications for decades. However, the intrinsic uncertainty of the produced number of photon pairs in such sources is a critical drawback that prevents on-demand photon-pair and heralded single-photon generation. Here we demonstrate large-scale time multiplexing of indistinguishable her…
Photonic qubits constitute a leading platform to disruptive quantum technologies due to their unique low-noise properties. The cost of the photonic approach is the non-deterministic nature of many of the processes, including single-photon generation, which arises from parametric sources and negligible interaction between photons. Active temporal multiplexing - repeating a generation process in time and rerouting to single modes using an optical switching network - is a promising approach to overcome this challenge and will likely be essential for large-scale applications with greatly reduced r…
We consider periodic single-photon sources with combined multiplexing in which the outputs of several time-multiplexed sources are spatially multiplexed. We give a full statistical description of such systems in order to optimize them with respect to maximal single-photon probability. We carry out the optimization for a particular scenario which can be realized in bulk optics and its expected performance is potentially the best at the present state of the art. We find that combined multiplexing outperforms purely spatially or time multiplexed sources for certain parameters only, and we charact…
Using spontaneous parametric down-conversion, photon pairs entangled in frequency and polarization were generated. After frequency resolving the photon pairs, the polarization correlations were measured on several polarization basis, and it was confirmed that the frequency resolved photon pairs were entangled in polarization, indicating the photon pairs can be used as a source of wavelength division multiplexing quantum key distribution.
We propose and experimentally demonstrate a novel approach to a heralded single photon source based on spectral multiplexing (SMUX) and feed-forward-based spectral manipulation of photons created by means of spontaneous parametric down-conversion in a periodically-poled LiNbO3 crystal. As a proof-of-principle, we show that our 3-mode SMUX increases the heralded single-photon rate compared to that of the individual modes without compromising the quality of the emitted single-photons. We project that by adding further modes, our approach can lead to a deterministic SPS.
Integrated arrays of Superconducting Nanowire Single Photon Detectors (SNSPDs) have shown capabilities such as Photon Number Resolution, single photon imaging and coincidences detection, and can be effectively used also in other different applications related to quantum optics. The growing complexity of such applications requires the use of multiplexing schemes for the simultaneous readout of different detectors. A simple multiplexing scheme can be realized by arranging a series of SNSPDs elements, shunted by appropriate resistances. The goal of this work is to investigate and optimize this sc…
Orthogonal temporal modes (TMs) form a field-orthogonal, continuous-variable degree of freedom that is in principle infinite dimensional, and create a promising resource for quantum information science and technology. The ideal quantum pulse gate (QPG) is a device that multiplexes and demultiplexes temporally orthogonal optical pulses that have the same carrier frequency, spatial mode, and polarization. The QPG is the chief enabling technology for usage of orthogonal temporal modes as a basis for high-dimensional quantum information storage and processing. The greatest hurdle for QPG implement…
We demonstrate the multiplexing of a classical coherent and a quantum state of light in a single telecommunciation fiber. For this purpose we make use of spontaneous parametric down conversion and quantum frequency conversion to generate photon pairs at 854 nm and the telecom O-band. The herald photon triggers a telecom C-band laser pulse. The telecom single photon and the laser pulse are combined and coupled to a standard telecom fiber. Low background time correlation of the classical and quantum signal behind the fiber shows successful telecommunication channel multiplexing.
The light-matter quantum interface that can create quantum-correlations or entanglement between a photon and one atomic collective excitation is a fundamental building block for a quantum repeater. The intrinsic limit is that the probability of preparing such non-classical atom-photon correlations has to be kept low in order to suppress multi-excitation. To enhance this probability without introducing multi-excitation errors, a promising scheme is to apply multimode memories into the interface. Significant progresses have been made in temporal, spectral, and spatial multiplexing memories, but …
Multi-photon entangled states of light are key to advancing quantum communication, computation, and metrology. Current methods for building such states are based on stitching together photons from probabilistic sources. The probability of $N$ such sources firing simultaneously decreases exponentially with $N$, imposing severe limitations on the practically achievable number of coincident photons. We tackle this challenge with a quantum interference buffer (QIB), which combines three functionalities: firstly, it stores polarization qubits, enabling the use of polarization-entangled states as re…
Light carrying orbital angular momentum constitutes an important resource for both classical and quantum information technologies. Its inherently unbounded nature can be exploited to generate high-dimensional quantum states or for channel multiplexing in classical and quantum communication in order to significantly boost the data capacity and the secret key rate, respectively. While the big potentials of light owning orbital angular momentum have been widely ascertained, its technological deployment is still limited by the difficulties deriving from the fabrication of integrated and scalable p…
We have realized experimentally a non-degenerate polarization-entangled photon pair distribution in a commercial telecom dense wave-division multiplexing device (DWDM) with 8 channels. A promising point of this work is that the entangled photon pair is obtained via spontaneously parametric down conversion in a single type-II periodically poled KTiOPO4 crystal without usual post-selection. Another promising advantage is that we can actively control and switch the distribution of the photon pair between different channel pairs in DWDM at will. There is no crosstalk between the different channel …
We propose a practical quantum cryptographic scheme which combines high information capacity, such as provided by high-dimensional quantum entanglement, with the simplicity of a two-dimensional Clauser-Horne-Shimony-Holt (CHSH) Bell test for security verification. By applying a state combining entanglement in a two-dimensional degree of freedom, such as photon polarization, with high-dimensional correlations in another degree of freedom, such as photon orbital angular momentum (OAM) or path, the scheme provides a considerably simplified route towards security verification in quantum key distri…
Detectors that can resolve photon number are needed in many quantum information technologies. In order to be useful in quantum information processing, such detectors should be simple, easy to use, and be scalable to resolve any number of photons, as the application may require great portability such as in quantum cryptography. Here we describe the construction of a time-multiplexed detector, which uses a pair of standard avalanche photodiodes operated in Geiger mode. The detection technique is analysed theoretically and tested experimentally using a pulsed source of weak coherent light.
The cascade-emitted biphotons generated from the alkali metal atomic ensembles are an excellent entanglement resource which enables long-distance quantum communication. The communication of quantum information between distant locations can be realized by utilizing the low-loss telecom bandwidth in the upper transition of the cascaded photons in a fiber-based quantum network. Meanwhile, the infrared photon from the lower transition of this highly directional and frequency-correlated biphoton can be created under the four-wave mixing process and can be stored locally as a collective spin wave. H…
We develop a statistical theory describing the operation of multiplexed single-photon sources equipped with photon-number-resolving detectors that includes the potential use of different input mean photon numbers in each of the multiplexed units. This theory accounts for all relevant loss mechanisms and allows for the maximization of the single-photon probabilities under realistic conditions by optimizing the different input mean photon numbers unit-wise and the detection strategy that can be defined in terms of actual detected photon numbers. We apply this novel description to analyze periodi…
We report on a device that integrates eight superconducting transmon qubits in lambda/4 superconducting coplanar waveguide resonators fed from a common feedline. Using this multiplexing architecture, each resonator and qubit can be addressed individually thus reducing the required hardware resources and allowing their individual characterisation by spectroscopic methods. The measured device parameters agree with the designed values and the resonators and qubits exhibit excellent coherence properties and strong coupling, with the qubit relaxation rate dominated by the Purcell effect when brough…
Future multi-photon applications of quantum optics and quantum information science require quantum memories that simultaneously store many photon states, each encoded into a different optical mode, and enable one to select the mapping between any input and a specific retrieved mode during storage. Here we show, with the example of a quantum repeater, how to employ spectrally-multiplexed states and memories with fixed storage times that allow such mapping between spectral modes. Furthermore, using a Ti:Tm:LiNbO3 waveguide cooled to 3 Kelvin, a phase modulator, and a spectral filter, we demonstr…
Heralded single photon source (HSPS) is an important way in generating genuine single photon, having advantages of experimental simplicity and versatility. However, HSPS intrinsically suffers from the trade-off between the heralded single photon rate and the single photon purity. To overcome this, one can apply multiplexing technology in different degrees of freedom to enhance the performance of HSPS. Here, by employing spectral multiplexing and active feed-forward spectral manipulating, we demonstrate a HSPS at 1.5 μm telecom-band. Our experimental results show that the spectral multiplexing …
Maximizing the information transmission rate through quantum channels is essential for practical implementation of quantum communication. Time-division multiplexing is an approach for which the ultimate rate requires the ability to manipulate and detect single photons on ultrafast timescales while preserving their quantum correlations. Here we demonstrate the demultiplexing of a train of pulsed single photons using time-to-frequency conversion while preserving their polarization entanglement with a partner photon. Our technique converts a pulse train with 2.69 ps spacing to a frequency comb wi…
In a quantum network, coherent emitters can be entangled over large distances using photonic channels. In solid-state devices, the required efficient light-emitter interface can be implemented by confining the light in nanophotonic structures. However, fluctuating charges and magnetic moments at the nearby interface then lead to spectral instability of the emitters. Here we avoid this limitation when enhancing the photon emission up to 70(12)-fold using a Fabry-Perot resonator with an embedded 19 micrometer thin crystalline membrane, in which we observe around 100 individual erbium emitters. I…
The orbital angular momentum, OAM, of photons offers a suitable support to carry the quantum data of multiple users. We present two novel optical setups that send the information of n quantum communication parties through the same free-space optical link. Those qubits can be sent simultaneously and share path, wavelength and polarization without interference, increasing the communication capacity of the system. The first solution, a qubit combiner, merges n channels into the same link, which transmits n independent photons. The second solution, the OAM multiplexer, uses CNOT gates to transfer …
Entanglement is a key resource in quantum information science and associated emerging technologies. Photonic systems offer a large range of exploitable entanglement degrees of freedom such as frequency, time, polarization, and spatial modes. Hyperentangled photons exploit multiple degrees of freedom simultaneously to enhance the performance of quantum information protocols. Here, we report a fully guided-wave approach for generating polarization and energy-time hyperentangled photons at telecom wavelengths. Moreover, by demultiplexing the broadband emission spectrum of the source into five sta…
We present a protocol for generating on-demand, indistinguishable single photons on a silicon photonic integrated chip. The source is a time-multiplexed spontaneous parametric down-conversion element that allows optimization of single-photon versus multiphoton emission while realizing high output rate and indistinguishability. We minimize both the scaling of active elements and the scaling of active element loss with multiplexing. We then discuss detection strategies and data processing to further optimize the procedure. We simulate an improvement in single-photon-generation efficiency over pr…
We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission at 625 MHz clock rate through a 97 km field-installed fiber using practical clock synchronization based on wavelength-division multiplexing (WDM). We succeeded in over-one-hour stable key generation at a high sifted key rate of 2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure key rate was estimated to be 0.78-0.82 kbps from the transmission data with the decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse.
Many existing schemes for linear-optical quantum computing (LOQC) depend on multiplexing (MUX), which uses dynamic routing to enable near-deterministic gates and sources to be constructed using heralded, probabilistic primitives. MUXing accounts for the overwhelming majority of active switching demands in current LOQC architectures. In this manuscript, we introduce relative multiplexing (RMUX), a general-purpose optimization which can dramatically reduce the active switching requirements for MUX in LOQC, and thereby reduce hardware complexity and energy consumption, as well as relaxing demands…
Distributing entangled pairs is a fundamental operation required for many quantum information science and technology tasks. In a general entanglement distribution scheme, a photonic pulse is used to entangle a pair of remote quantum memories. Most applications require multiple entangled pairs between remote users, which in turn necessitates several photonic pulses (single photons) being sent through the channel connecting those users. Here we present an entanglement distribution scheme using only a single photonic pulse to entangle an arbitrary number of remote quantum memories. As a consequen…
We demonstrate and verify quantum detector tomography of a superconducting nanowire single-photon detector (SNSPD) in a multiplexing scheme which permits measurement of up to 71000 photons per input pulse. We reconstruct the positive operator valued measure (POVM) of this device in the low photon-number regime, and use the extracted parameters to show the POVMs spanning the whole dynamic range of the device. We verify this by finding the mean photon number of a bright state. Our work shows that a reliable quantum description of large-scale SNSPD devices is possible, and should be applicable to…
In this paper, we study the feasibility of conducting quantum key distribution (QKD) together with classical communication through the same optical fiber by employing dense-wavelength-division-multiplexing (DWDM) technology at telecom wavelength. The impact of the classical channels to the quantum channel has been investigated for both QKD based on single photon detection and QKD based on homodyne detection. Our studies show that the latter can tolerate a much higher level of contamination from the classical channels than the former. This is because the local oscillator used in the homodyne de…
Quantum cryptography is set to become a key technology for future secure communications. However, to get maximum benefit in communication networks, transmission links will need to be shared among several quantum keys for several independent users. Such links will enable switching in quantum network nodes of the quantum keys to their respective destinations. In this paper we present an experimental demonstration of a photonic integrated silicon chip quantum key distribution protocols based on space division multiplexing (SDM), through multicore fiber technology. Parallel and independent quantum…
