Since its original proposal by Bennett et al. [1], the concept of quantum teleportation has attracted a lot of attention and has even become one of the central elements for advanced and practical realizations of quantum information protocols. It is essential for long-distance quantum communication by means of quantum repeaters [2] and it has also been shown to be a useful tool for realizing universal quantum logic gates in a measurement-based fashion [3].Many proposals and models for quantum computation rely upon quantum teleportation, such as the efficient linear-optics quantum computing scheme by Knill, Laflamme, and Milburn [4] and the so-called one-way quantum computer using cluster states [5].Although much progress has been made in demonstrating quantum teleportation of photonic qubits [6][7][8][9][10][11], most of these schemes shared one fundamental restriction: an unambiguous two-qubit Bell-state measurement (BSM), as needed for teleporting a qubit using two-qubit entanglement, is always probabilistic when linear optics is employed, even if photon-numberresolving detectors (PNRDs) had been available [12,13]. There are two experiments avoiding this constraint, however, in these, either a qubit can no longer be teleported when it is coming independently from the outside [7] or an extra nonlinear element leads to extremely low measurement efficiencies of the order of 10 −10 [8]. A further experimental limitation, rendering these schemes fairly inefficient, is the probabilistic nature of the entangled resource states [13]. Efficient, near-deterministic quantum teleportation, however, is of great benefit in quantum communication in order to save quantum memories in a quantum repeater; and it is a necessity in teleportation-based quantum computation. An additional drawback of the previ-2 ous experiments, due to the lack of PNRDs, was the need for either destroying the teleported qubit [20] or attenuating the input qubit [10], thus further decreasing the success rate of teleportation.We overcome all the above limitations through a totally distinct approach: continuousvariable (CV) quantum teleportation of a photonic qubit. The strength of CV teleportation lies in the on-demand availability of the quadrature-entangled states and the completeness of a BSM in the quadrature bases using linear optics and homodyne detections [15]. So far, these tools have been employed to unconditionally teleport CV quantum states such as coherent states [16,21]. However, their application to qubits [18,22] has long been out of reach, since typical pulsed-laser-based qubits (like those in Refs. [6][7][8][9][10][11]) have a broad frequency bandwidth, incompatible with the original continuous-wave-based CV teleporter that only works on narrow sidebands [16,21]. We overcome this incompatibility by utilizing a very recent, advanced technology: a broadband CV teleporter [23] and a narrow-band time-bin qubit compatible with that teleporter [24]. Importantly, this qubit uses two temporal modes to represent a so-called dual-rail encoded q...

Abstract:We develop an experimental scheme based on a continuouswave (cw) laser for generating arbitrary superpositions of photon number states. In this experiment, we successfully generate superposition states of zero to three photons, namely advanced versions of superpositions of two and three coherent states. They are fully compatible with developed quantum teleportation and measurement-based quantum operations with cw lasers. Due to achieved high detection efficiency, we observe, without any loss correction, multiple areas of negativity of Wigner function, which confirm strongly nonclassical nature of the generated states. Haroche, "Quantum Zeno dynamics of a field in a cavity," Phys. Rev. A 86, 032120 (2012).

We present a general formalism to describe continuous-variable (CV) quantum teleportation of discrete-variable (DV) states with gain tuning, taking into account experimental imperfections. Here the teleportation output is given by independently transforming each density matrix element of the initial state. This formalism allows us to accurately model various teleportation experiments and to analyze the gain dependence of their respective figures of merit. We apply our formalism to the recent experiment of CV teleportation of qubits [S. Takeda et al., Nature 500, 315 (2013)] and investigate the optimal gain for the transfer fidelity. We also propose and model an experiment for CV teleportation of DV entanglement. It is shown that, provided the experimental losses are within a certain range, DV entanglement can be teleported for any non-zero squeezing by optimally tuning the gain.

We experimentally generate arbitrary time-bin qubits using continuous-wave light. The advantage unique to our qubit is its compatibility with deterministic continuous-variable quantum information processing. This compatibility comes from its optical coherence with continuous waves, well-defined spatio-temporal mode, and frequency spectrum within the operational bandwidth of the current continuous-variable technology. We also demonstrate an efficient scheme to characterize time-bin qubits via eight-port homodyne measurement. This enables the complete characterization of the qubits as two-mode states, as well as a flexible analysis equivalent to the conventional scheme based on a Mach-Zehnder interferometer and photon-detection.There are two complementary approaches in optical quantum information processing: discrete-variables (DV) and continuous-variables (CV). DV experiments are conducted by qubits represented by single-photon optical pulses [1][2][3][4][5][6]. However, due to inefficient generation and imperfect detection of qubits, most of the experiments are probabilistic and require post-selection [7,8]. In contrast, CV experiments rely on the wave nature of light. They can be performed deterministically via quadrature squeezing, highly-efficient homodyne detection and feedforward operations, at the expense of relatively low operation fidelities [9,10]. Recently, there has emerged a "hybrid" approach to combine both techniques to circumvent the current limitations [11]. Its major advantage is deterministic operation of qubits with CV techniques; one of the most striking examples is deterministic quantum teleportation of qubits with a CV teleporter, as is proposed in Refs. [12,13]. The recent experiment on CV teleportation of highly non-classical optical pulses [14] opens the way to this hybrid teleportation. However, typical qubits are generated by pulse-pumped spontaneous parametric down-conversion (SPDC) [1-6] and thus have no optical coherence with continuous-waves on which the CV teleporter is based. Furthermore, the bandwidth of these qubits is orders of magnitude wider than the operational bandwidth of the CV teleporter (only around 10 MHz).Here we overcome this incompatibility by generating a time-bin qubit using CW light. This qubit consists of two temporally-separated optical pulses, described as a superposition of a photon in one pulse |1, 0 and the other pulse |0, 1 : |ψ = c 0 |1, 0 + c 1 e iΦ |0, 1 . Thus far, such qubits have been prepared by pulsed lasers [3][4][5] and used as a key resource for various DV experiments over long distances (e.g., quantum cryptography [15,16], quantum teleportation [17], and entanglement swapping [18]). In contrast, our time-bin qubit is generated from a CW-pumped nondegenerate optical parametric oscillator (NOPO), which is a cavity-enhanced version of the SPDC. The NOPO cavity enhances the SPDC process only inside its resonant mode, thereby generating qubits in a well-defined and controlled spatio-temporal mode. The 6.2 MHz bandwidth of the resultant qubit is with...

Abstract. Hybrid quantum teleportation -continuous-variable teleportation of qubits -is a promising approach for deterministically teleporting photonic qubits. We propose how to implement it with current technology. Our theoretical model shows that faithful qubit transfer can be achieved for this teleportation by choosing an optimal gain for the teleporter's classical channel.