RESUMEN
Improving imaging quality while reducing the sampling time simultaneously is a crucial challenge that limits the practical application of temporal ghost imaging (TGI). To improve the performance of TGI, various methods have been proposed and verified. However, a work analyzing in detail the influence of intensity accuracy and detection noise of TGI is still absent. Here, we establish an evaluation model to quantify the imaging quality of TGI and differential TGI (DTGI). Our model considers the intensity detection accuracy, threshold, and noise of the test path during image reconstruction and quantifies their influences by developing general imaging formulas of (D)TGI. We also simulate the imaging of (D)TGI numerically. The evaluation demonstrates that (D)TGI is relatively not sensitive to detection accuracy and thresholds of the test path, and image quality is degraded slightly even when those parameters turn much worse. (D)TGI is relatively robust to detection noise but will be unable to reconstruct the object when noise is too strong. DTGI does not show clear advantages over TGI. Our work develops an effective model to quantify the image quality with practical parameters and is significant to real applications of (D)TGI.
RESUMEN
Entanglement-based quantum communication offers an increased level of security in practical secret shared key distribution. One of the fundamental principles enabling this security-the fact that interfering with one photon will destroy entanglement and thus be detectable-is also the greatest obstacle. Random encounters of traveling photons, losses, and technical imperfections make noise an inevitable part of any quantum communication scheme, severely limiting distance, key rate, and environmental conditions in which quantum key distribution can be employed. Using photons entangled in their spatial degree of freedom, we show that the increased noise resistance of high-dimensional entanglement can indeed be harnessed for practical key distribution schemes. We perform quantum key distribution in eight entangled paths at various levels of environmental noise and show key rates that, even after error correction and privacy amplification, still exceed 1 bit per photon pair and furthermore certify a secure key at noise levels that would prohibit comparable qubit based schemes from working.
RESUMEN
Ghost imaging (GI) can reconstruct the image of an object by measuring the correlation function of two beams, none of which carries the structure information of the object independently. This powerful technology makes it possible to obtain high-quality imaging of the object even in the presence of noise. Here, we introduce the GI method into quantum device evaluation in the time domain. We realized a proof-of-principle experiment to evaluate the temporal detection efficiency of a gated-mode single-photon avalanche detector (SPAD). The experimental results show that high-quality evaluation of temporal characteristics of the SPAD can be realized by the method of temporal GI (TGI). Our work indicates that the TGI method is an effective tool to monitor the temporal characteristics of quantum devices in real time and will bring a new perspective to the security evaluation of quantum communication.
RESUMEN
Quantum key distribution (QKD) can generate secure key bits between remote users employing the features of quantum physics. However, a shared reference frame is necessary for QKD systems in most scenarios. A reference-frame-independent (RFI) scheme can tolerate the reference frame drifting between legitimate remote users, which is significant in the operation of relative moving terminals such as satellites and aircraft. We design and experimentally demonstrate an RFI-BB84-QKD system by joint encoding with the polarization and orbital angular momentum states of the photons. We use self-compensating fiber Sagnac interferometers to perform high-speed polarization modulation, and q-plates to passively manipulate the rotation-invariant photon states, which makes the system feasible for high-speed operation using off-the-shelf components.
RESUMEN
The tunable beam splitter (TBS) is a fundamental component used in optical experiments. A TBS can preserve the orbital angular momentum (OAM) states; in addition, the polarization states of photons are valuable for some particular experiments, such as high-dimensional quantum information processing. We use polarization beam splitters and half-wave plates to realize such a TBS under a compact structure, which can reduce the number of elements that require comparing with existing works. The experiments verify that the TBS has good performances in tunability, polarization, and OAM state preservation. A Sagnac interferometer is implemented with the proposed TBS to evaluate its practical usability, and the mean visibilities greater than 99.30% under varying polarization states demonstrate its potential for optical information processing.
RESUMEN
Phase manipulation is essential to quantum information processing, for which the orbital angular momentum (OAM) of photon is a promising high-dimensional resource. Dove prism (DP) is one of the most important elements to realize the nondestructive phase manipulation of OAM photons. DP usually changes the polarization of light and thus increases the manipulation error for a spin-OAM hybrid state. DP in a Sagnac interferometer also introduces a mode-dependent global phase to the OAM mode. In this work, we implemented a high-dimensional controlled-phase manipulation module (PMM), which can compensate the mode-dependent global phase and thus preserve the phase in the spin-OAM hybrid superposition state. The PMM is stable for free running and is suitable to realize the high-dimensional controlled-phase gate for spin-OAM hybrid states. Considering the Sagnac-based structure, the PMM is also suitable for classical communication with the spin-OAM hybrid light field.
RESUMEN
Improving the tolerance of channel noise is an important task for devising and implementing quantum key distribution (QKD) protocols. Quantum phase-flip error rejection (QPFER) code [Phys. Rev. Lett.92, 077902 (2004)PRLTAO0031-900710.1103/PhysRevLett.92.077902] has been introduced by Wang to increase the tolerable phase-flip noise of QKD implementations. However, an experiment that demonstrates its advantages is still missing. Here, we experimentally verify the QPFER code with the assistance of two photon quantum states generated by spontaneous parametric downconversion. The methods of parity check and postselection are introduced to the protocol for achieving the phase-flipping rejection. Comparing with the standard realization of the single photon polarization encoding BB84 scheme, the quantum error rate after decoding is obviously reduced when the probability of channel noise is less than 25%. The experiment results also show that QPFER protocol can reduce error rate, obtain a higher key rate, and be robust in the noisy channel when the noise level is in a proper region.
RESUMEN
Orbital angular momentum (OAM) is an important resource in high-dimensional quantum information processing, as its quantum number can be infinite. Dove prism (DP) is a most common tool to manipulate OAM light. However, the Dove prism changes the polarization of the photon states and decreases the sorting fidelity of the interferometer. In this work, we analyze the polarization-dependent effect of the DP on OAM light manipulation in the normal single-path Sagnac interferometers (SPSIs) with beam splitter (BS) and polarizing beam splitter (PBS). The results demonstrate that the BS SPSI is more sensitive to the input polarization and the specific parameters of the DP. We have also proposed and realized a modified BS SPSI, of which the sorting fidelity can be 100% in principle and is independent on the input polarization and the transmission matrix of the DP. The experiments demonstrate that the fidelity of the modified BS SPSI is about 5%~10% higher than that of the normal one. The modified BS SPSI is easy to implement (only two more half-wave plates are required) and is stable for free running at the scale of several hours. These merits make the structure suitable for applications in critical quantum information processing tasks, such as quantum cryptography.