Computational encryption, information-theoretic secrecy and quantum cryptography offer progressively stronger security against unauthorized decoding of messages contained in communication transmissions. However, these approaches do not ensure stealth—that the mere presence of message-bearing transmissions be undetectable. We characterize the ultimate limit of how much data can be reliably and covertly communicated over the lossy thermal-noise bosonic channel (which models various practical communication channels). We show that whenever there is some channel noise that cannot in principle be controlled by an otherwise arbitrarily powerful adversary—for example, thermal noise from blackbody radiation—the number of reliably transmissible covert bits is at most proportional to the square root of the number of orthogonal modes (the time-bandwidth product) available in the transmission interval. We demonstrate this in a proof-of-principle experiment. Our result paves the way to realizing communications that are kept covert from an all-powerful quantum adversary.

We have constructed and experimentally characterized what we believe to be the first fiber-based source of degenerate polarization-entangled photon pairs in the telecommunication band. Our source design utilizes bichromatic pump pulses and an optical-fiber Sagnac loop aligned to deterministically separate degenerate photon pairs at a central wavelength. The source exhibits 0.997+/-0.006 fidelity with a maximally entangled state, measured using quantum state tomography. When reconfigured to produce identical photon pairs, the source exhibits a Hong-Ou-Mandel interference visibility of 0.97+/-0.04.

Erasing quantum-mechanical distinguishability is of fundamental interest and also of practical importance, particularly in subject areas related to quantum information processing. We demonstrate a method applicable to optical systems in which single-mode filtering is used with only linear optical instruments to achieve quantum indistinguishability. Through "heralded" Hong-Ou-Mandel interference experiments we measure and quantify the improvement of indistinguishability between single photons generated via spontaneous four-wave mixing in optical fibers. The experimental results are in excellent agreement with predictions of a quantum-multimode theory we develop for such systems, without the need for any fitting parameter. PACS numbers: 42.50.Dv, Quantum indistinguishability is inextricably linked to several fundamental phenomena in quantum mechanics, including interference, entanglement, and decoherence [1-3]. For example, only when two photons are indistinguishable can they show strong second-order interference [4]. From an applied perspective, it forms the basis of quantum key distribution [5], quantum computing [6], quantum metrology [7], and many other important applications in modern quantum optics. In practice, however, the generation and manipulation of quantummechanically indistinguishable photons is quite challenging, primarily due to their coupling to external degrees of freedom.In this Letter, we experimentally investigate a pathway to erasing quantum distinguishability by making use of the Heisenberg uncertainty principle. This method, although designed specifically for optical systems, might be generalizable to other physical systems, including those of atoms and ions. It uses a filtering device that consists of only linear optical instruments, which in our present rendering is a temporal gate followed by a spectral filter. The gate's duration T and the filter's bandwidth B (in angular-Hertz) are chosen to satisfy BT < 1 so that any photon passing through the device loses its temporal (spectral) identity as required by the Heisenberg uncertainty principle. In this sense, the device behaves as a single-mode filter (SMF) that passes only a single electromagnetic mode of certain temporal profile while rejecting all other modes. Hence, applying such a SMF to distinguishable single photons can produce output photons that are indistinguishable from each other [8,9]. Our calculations show that for appropriate parameters very high levels of quantum indistinguishability can be achieved with use of the SMF, while paying a relatively low cost in terms of photon loss. This method is superior to using tight spectral or temporal filtering alone for similar purposes [10,11], where the photon loss is much higher. In fact, in Refs. [8,9] we have shown that the use of a SMF can significantly improve the performance of heralding-type single-photon sources made from optical fibers or crystalline waveguides [12][13][14][15].In our experiment, pairs of signal and idler photons are generated in two separate optical-fiber sp...

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