We propose an alignment-free photonic qubit transmission scheme and apply it in quantum key distribution schemes, in which the authorized parties do not have to calibrate their Cartesian reference frames in advance. The polarization states are labeled by time, and the receiver can obtain the original state in his own coordinate by postselecting a determinate time of arrival. The scheme does not resort to decoherence-free subspace based on multiple photon states, and it can be implemented with only passive linear optics, which make it efficient and practical with current technology. We demonstrate its application in quantum key distributions, and the scheme should find applications in long-distance quantum communication without frame alignment.
We propose an alignment-free photonic qubit transmission scheme and apply it in quantum key distribution schemes, in which the authorized parties do not have to calibrate their Cartesian reference frames in advance. The polarization states are labeled by time, and the receiver can obtain the original state in his own coordinate by postselecting a determinate time of arrival. The scheme does not resort to decoherence-free subspace based on multiple photon states, and it can be implemented with only passive linear optics, which make it efficient and practical with current technology. We demonstrate its application in quantum key distributions, and the scheme should find applications in long-distance quantum communication without frame alignment.
We propose an alignment-free two-party polarization-entanglement transmission scheme for entangled photons by using only linear-optical elements, requiring neither ancillary photons nor calibrated reference frames. The scheme is robust against both the random channel noise and the instability of reference frames. Furthermore, the success probabilities for entanglement transmission are, in principle, improved to unity when active polarization controllers are used. The distinct characters of a simple structure, being easy to implement, and the high fidelity and efficiency make our protocol very useful for long-distance quantum communications and distributed quantum networks in practical applications. As an example, we give its application in a reference-frame-alignment-free quantum key distribution protocol with entanglement.
We propose a scheme for encoding logical qubits in a subspace protected against collective rotations around the propagation axis using the polarization and transverse spatial degrees of freedom of single photons. This encoding allows for quantum key distribution without the need of a shared reference frame. We present methods to generate entangled states of two logical qubits using present day down-conversion sources and linear optics, and show that the application of these entangled logical states to quantum information schemes allows for alignment-free tests of Bell's inequalities, quantum dense coding, and quantum teleportation.
Quantum communication employs the counter-intuitive features of quantum physics for tasks that are impossible in the classical world. It is crucial for testing the foundations of quantum theory and promises to revolutionize information and communication technologies. However, to execute even the simplest quantum transmission, one must establish, and maintain, a shared reference frame. This introduces a considerable overhead in resources, particularly if the parties are in motion or rotating relative to each other. Here we experimentally show how to circumvent this problem with the transmission of quantum information encoded in rotationally invariant states of single photons. By developing a complete toolbox for the efficient encoding and decoding of quantum information in such photonic qubits, we demonstrate the feasibility of alignment-free quantum key-distribution, and perform proof-of-principle demonstrations of alignment-free entanglement distribution and Bell-inequality violation. The scheme should find applications in fundamental tests of quantum mechanics and satellite-based quantum communication. Quantum communication promises important advances in information and communication technology, yet it suffers from alignment sensitivity. Here, an alignment-free approach is demonstrated using liquid crystal devices, allowing for broader applications, including satellites.
In quantum key distribution (QKD), the users need to share the same reference frame. If their reference frames are inconsistent, the QKD system will not function properly. The most widely used method today is the active real-time calibration of both communication reference frames by using classical communication. In order to get rid of the real-time calibration operation of the reference frames, a QKD protocol independent of reference frame is proposed, called reference-frame-independent QKD (RFI-QKD). The RFI-QKD protocol is immune to the effects of slowly changing reference frame drift, requiring that only one set of bases should be aligned by Alice and Bob, and the remaining two sets of bases can slowly change in the channel. In the real QKD system, a set of basis vectors can always be found to maintain a stable alignment. However, some assumptions are made for the sources in most reported researches, i.e. with a trusted and fixed photon-number distribution (PND), which usually cannot be satisfied in practical implementations. Those unreasonable assumptions will inevitably compromise the security of practical QKD systems. To solve the problem, in this work, we present a passive light source monitoring (PLSM) scheme for RFI-QKD, which is accomplished by a passive monitoring module consisting of a beam splitter and two detectors on the source side. Through the PLSM module, we can have four monitoring events by using two local detectors and then precisely estimate the bounds of source distributions. Specifically, we take the three-intensity decoy-state-based RFI-QKD for example for illustraing the events. Compared with the original RFI-QKD, our PLSM method can passively monitor the PND and has many advantages, with light source fluctuations, finite-size effects and reference frame deflection angles taken into account.
When quantum information is encoded in optical polarization, practical protocols must contend with the fact that preparation and measurement reference frames may be misaligned, owing to physical orientations and birefringence effects. For long-distance transmissions, a common solution involves multiplexing strong classical reference signals, providing a basis for correction. Here we numerically investigate the performance of a polarization-frame alignment protocol utilizing single-photon-level signals and measurements, in the context of quantum key distribution (QKD) over channels with detector noise and imperfect sources. To the extent possible, the same states and measurement bases are used as for the QKD protocol we intend to support. Our results show that a small fraction of resources from the overall signal---a few hundred photon detections, in total---are required for good performance, restoring the state to better than 99% of its original quality, and we illustrate the protocol's successful operation and adaptability through experimental demonstrations in which it was used in the laboratory and the field.
Encoding quantum information in photonic time-bin states is typically considered impractical for moving free-space quantum communication due to the difficulties with phase stabilization of distant quantum time-bin interferometers and turbulence of free-space channels. We demonstrate a novel approach using reference frame independent time-bin quantum key distribution that completely avoids the need for active relative phase stabilization while simultaneously overcoming a highly multi-mode channel without any active mode filtering. This scheme enables passive, self-compensating time-bin quantum communication without any mode filtering, mode sorting, adaptive optics, active basis selection, or active phase alignment. We realize a proof-of-concept demonstration using hybrid polarization and time-bin entangled photons that demonstrates a sustained asymptotic secure key rate greater than 0.07 bits/coincidence over a 15 m multi-mode fiber optical channel and showing entanglement correlations over a moving 38.5 dB loss free-space channel, including system losses. The scheme simplifies the use of time-bin encoding and can be readily applied over various spatially multi-mode and fluctuating channels involving rapidly moving platforms, including airborne and satellite systems.