In the early development of quantum information processing (QIP), a communication protocol called quantum teleportation was discovered (1) that involves 1

Abstract:We theoretically and experimentally investigate the spectral tunability and purity of photon pairs generated from spontaneous parametric down conversion in periodically poled KTiOPO 4 crystal with group-velocity matching condition. The numerical simulation predicts that the purity of joint spectral intensity (P JSI ) and the purity of joint spectral amplitude (P JSA ) can be kept higher than 0.98 and 0.81, respectively, when the wavelength is tuned from 1460 nm to 1675 nm, which covers the S-, C-, L-, and U-band in telecommunication wavelengths. We also directly measured the joint spectral intensity at 1565 nm, 1584 nm and 1565 nm, yielding P JSI of 0.989, 0.983 and 0.958, respectively. Such a photon source is useful for quantum information and communication systems.

The progress in quantum operations of continuous-variable (CV) schemes can be reduced to that in CV quantum teleportation. The fidelity of quantum teleportation of an optical setup is limited by the finite degree of quantum correlation which can be prepared with a pair of finitely squeezed states. Reports of improvement of squeezing level have appeared recently, and we adopted the improved methods in our experimental system of quantum teleportation. As a result, we teleported a coherent state with a fidelity F = 0.83 ± 0.01, which is better than any other figures reported to date. In this paper, we introduce a measure ns, the number of teleportations expected to be carried out sequentially. Our result corresponds to ns=5.0±0.4. It suggests that our improvement would enable us to proceed toward more advanced quantum operations involving multi-step quantum operations.Quantum teleportation is a quantum operation in which one can send an arbitrary quantum state to another party by making use of the quantum correlation called quantum entanglement and classical channels between the sender and the receiver. More than that, progress in the operations of quantum states in continuous-variable (CV) schemes can be reduced to those in CV quantum teleportation [1], which can be regarded as one of the most fundamental quantum operations [2,3,4,5]. Measurement-based quantum computation with cluster states [6,7], and a cubic phase gate [8] which involves four-time sequential teleportation [9] can be taken as examples. Therefore quantum teleportation of high quality is essential to build advanced quantum operations which involve multi-step quantum operations.The optical scheme of CV quantum teleportation was proposed by [10], in which squeezed states are used as a source of entanglement. Squeezed states can be generated optically via parametric down-conversion [11].In order to evaluate quantum teleportation, we introduce the fidelity as F = Ψ in |ρ out |Ψ in for a coherent state input. It can be derived aswhere r is the squeezing parameter of an entanglement resource and n is the number of sequential teleportations [12].Eq.(1) shows that fidelity cannot reach F = 1 due to the finite squeezing level, or finite degree of quantum entanglement, the so-called Einstein-PodolskyRosen (EPR) correlation [13]. With r = 0, that is without entanglement, and n = 1, the fidelity is F = 1/2 which can be taken as the classical limit [14,15,16].We define a figure n s as another measure than fidelity here. It signifies how many times teleportations are expected to be achieved sequentially. n s is defined so that it satisfies 1/2 ≡ 1/(1 + n s e −2r eff ). Note that F =1/2 is the threshold of the success of teleportation. r eff is the effective squeezing parameter defined with an experimental fidelity as F exp = 1/(1 + e −2r eff ). Hence, n s can be expressed asThis figure has more explicit meaning than the fidelity when advanced quantum operations are considered. From eq.(1), higher fidelity is expected with higher squeezing level. In [17], the fir...

We demonstrate a Hong-Ou-Mandel interference between two independent, intrinsically pure, heralded single photons from spontaneous parametric down-conversion (SPDC) at telecommunication wavelength. A visibility of 85.5 ± 8.3% was achieved without using any bandpass filter. Thanks to the group-velocity-matched SPDC and superconducting nanowire single-photon detectors (SNSPDs), the fourfold coincidence counts are one order higher than that in the previous experiments. The combination of bright single-photon sources and SNSPDs is a crucial step for future practical quantum infocommunication systems at telecommunication wavelength.