Light states composed of multiple entangled photons—such as cluster states—are essential for developing and scaling-up quantum computing networks. Photonic cluster states can be obtained from single-photon sources and entangling gates, but so far this has only been done with probabilistic sources constrained to intrinsically low efficiencies, and an increasing hardware overhead. Here, we report the resource-efficient generation of polarization-encoded, individually-addressable photons in linear cluster states occupying a single spatial mode. We employ a single entangling-gate in a fiber loop configuration to sequentially entangle an ever-growing stream of photons originating from the currently most efficient single-photon source technology—a semiconductor quantum dot. With this apparatus, we demonstrate the generation of linear cluster states up to four photons in a single-mode fiber. The reported architecture can be programmed for linear-cluster states of any number of photons, that are required for photonic one-way quantum computing schemes.

The experimental realization of many-body entangled states is one of the main goals of quantum technology as these states are a key resource for quantum computation and quantum sensing. However, increasing the number of photons in an entangled state has been proved to be a painstakingly hard task. This is a result of the non-deterministic emission of current photon sources and the distinguishability between photons from different sources. Moreover, the generation rate and the complexity of the optical setups hinder scalability. Here we present a new scheme that is compact, requires a very modest amount of components, and avoids the distinguishability issues by using only one single-photon source. States of any number of photons are generated with the same configuration, with no need for increasing the optical setup. The basic operation of this scheme is experimentally demonstrated and its sensitivity to imperfections is considered.The majority of quantum information protocols require entanglement between different subsystems of a quantum state [1]. For example, the demonstration of a useful quantum computer, will involve the controlled entanglement of probably thousands of quantum elements [2]. This has been proved so far to be a very hard task, especially with photons whose generation rates decrease exponentially with their numbers [3]. This task is simplified when several single photon sources are combined on an optical chip through integrated waveguides [4], but the required complete indistinguishability between all of the sources is hard to achieve [5][6][7][8][9][10]. The highest numbers of entangled photons were created by combining polarization entangled photon pairs from several parametric down-conversion (PDC) sources [3]. Nevertheless, in order to increase the number of entangled photons, more sources, entangling operations, and matching delay lines are required. In addition, extremely low state detection rates were observed, due to the probabilistic nature of PDC, where typical photon-pair generation probabilities are of few percents. Previously, we demonstrated an approach that simplified this setup, such that only a single entangled photon pair source and a single delay line are required, regardless of the size of the generated state [11]. This setup still suffers from low generation rates, as it also uses PDC.In this work we present a new scheme that generates multi-photon linear cluster states [12]. The scheme is using single photon sources [13], where on-demand operation is almost achieved [14][15][16][17], and a single delay line in a loop arrangement. Thus,the amount of resources is reduced to only one single-photon emitter and one entangling gate. Such delay loops were previously used in schemes where quantum information is time-bin encoded [18, 19]. The use of only a single source simplifies the efforts for indistinguishability, removing the need for fine and stable spectral tuning of the different sources [10,[20][21][22]. Nevertheless, consequent emissions still require indistinguishability...

Single-photon detectors are widely used in modern quantum optics experiments and applications. Like all detectors, it is important for these devices to be accurately calibrated. A single-photon detector is calibrated by determining its detection efficiency; the standard method to measure this quantity requires comparison to another detector. Here, we suggest a method to measure the detection efficiency of a single photon detector without requiring an external reference detector. Our method is valid for individual single-photon detectors as well as multiplexed detectors, which are known to be photon number resolving. The method exploits the photon-number correlations of a nonlinear source, as well as the nonlinear loss of a single photon detector that occurs when multiple photons are detected simultaneously. We have analytically modeled multiplexed detectors and used the results to experimentally demonstrate calibration of a single photon detector without the need for an external reference detector.

The sensitivity of classical and quantum sensing is impaired in a noisy environment. Thus, one of the main challenges facing sensing protocols is to reduce the noise while preserving the signal. State of the art quantum sensing protocols that rely on dynamical decoupling achieve this goal under the restriction of long noise correlation times. We implement a proof-of-principle experiment of a protocol to recover sensitivity by using an error correction for photonic systems that does not have this restriction. The protocol uses a protected entangled qubit to correct a single error. Our results show a recovery of about 87% of the sensitivity, independent of the noise rate.

A single photon has many physical degrees of freedom (DOF) that can carry the state of a high-dimensional quantum system. Nevertheless, only a single DOF is usually used in any specific demonstration. Furthermore, when more DOF are being used, they are analyzed and measured one at a time. We introduce a two-qubit information system, realized by two degrees of freedom of a single photon: polarization and time. The photon arrival time is divided into two time-bins representing a qubit, while its polarization state represents a second qubit. The time difference between the two time-bins is created without an interferometer at the picosecond scale, which is much smaller than the detector's response time. The two physically different DOF are analyzed simultaneously by photon bunching between the analyzed photon and an ancilla photon. Full twoqubit states encoded in single photons were reconstructed using quantum state tomography, both when the two DOF were entangled and when they were not, with fidelities higher than 96%.