We consider a scheme of quantum teleportation where a receiver has multiple (N ) output ports and obtains the teleported state by merely selecting one of the N ports according to the outcome of the sender's measurement. We demonstrate that such teleportation is possible by showing an explicit protocol where N pairs of maximally entangled qubits are employed. The optimal measurement performed by a sender is the square-root measurement, and a perfect teleportation fidelity is asymptotically achieved for a large N limit. Such asymptotic teleportation can be utilized as a universal programmable processor. On the other hand, programmable processors (in short, processors) [2,3] are devices to manipulate a state via program states. Suppose that we wish to apply operation ε to an input state |χ in such that |χ in → ε(|χ in ). To do this by using a processor, we first generate the program state |ε , in which ε is stored. A processor then performs a fixed operation G and accomplishes the desired task such that G(|χ in ⊗ |ε ) = ε(|χ in ) ⊗ |ε ′ , just like a general-purpose computer executes a program stored in memory. In this way, a programmable processor provides the scheme of storing and retrieving operations. If a processor can deal with arbitrary ε, it is called a universal (programmable) processor. It was shown that a faithful [the output state is exactly ε(|χ in )] and deterministic (with a unit success probability) universal processor cannot be realized by a finite dimensional system [2]. The standard teleportation scheme provides a probabilistic universal processor [2], but the success probability becomes extremely small if the dimension of an input state is large; the obstacle is that Bob's unitary transformation in the teleportation scheme generally does not commute with ε [2, 4].Let us then consider the teleportation scheme proposed by Knill, Laflamme, and Milburn (KLM) [5] (and its deterministic version [6]), which is a technique to enable linear-optics quantum computation. In the KLM scheme, Bob has multiple (N ) output ports and obtains the teleported state by selecting one of the N ports according to the outcome of Alice's measurement (see Fig. 1). To complete the teleportation, however, Bob further needs to apply a unitary transformation (phase shift) to the state of the selected port, as well as the standard teleportation scheme. If the KLM scheme is successfully modified such that the unitary transformation is unnecessary (i.e., the state of one of the N ports becomes the teleported state as it is), the teleportation scheme can provide a universal processor. Suppose that Bob applies ε to every port (denoted by ε ⊗N ; see Fig. 1) in advance of the teleportation (this corresponds to the operation for storing ε). The teleportation procedure then results in the state processed by ε, regardless of which port is selected. This is because the operation of selecting a port (without any additional unitary transformation) always commutes with ε ⊗N , i.e., selecting a port after applying ε ⊗N causes the same resul...

The scheme of quantum teleportation, where Bob has multiple (N) output ports and obtains the teleported state by simply selecting one of the N ports, is thoroughly studied. We consider both deterministic version and probabilistic version of the teleportation scheme aiming to teleport an unknown state of a qubit. Moreover, we consider two cases for each version: (i) the state employed for the teleportation is fixed to a maximally entangled state, and (ii) the state is also optimized as well as Alice's measurement. We analytically determine the optimal protocols for all the four cases, and show the corresponding optimal fidelity or optimal success probability. All these protocols can achieve the perfect teleportation in the asymptotic limit of $N\to\infty$. The entanglement properties of the teleportation scheme are also discussed.Comment: 14 pages, 4 figure

We theoretically present a method to realize a deterministic photon-photon √ SWAP gate using a three-level lambda system interacting with single photons in reflection geometry. The lambda system is used completely passively as a temporary memory for a photonic qubit; the initial state of the lambda system may be arbitrary, and active control by auxiliary fields is unnecessary throughout the gate operations. These distinct merits make this entangling gate suitable for deterministic and scalable quantum computation.PACS numbers: 42.50. Ex, 03.67.Bg, 42.50.Dv Single photons are promising candidates for implementing qubits in quantum computation due to their long coherence times. Furthermore, one-qubit gates such as Hadamard and NOT gates can be readily realized using linear optical elements [1]. Photonic qubits have the disadvantage that it is difficult to realize two-qubit controlled gates such as controlled-NOT gates due to the weak mutual interaction between photons [2]. This problem has been partially overcome by linear optics quantum computation, which enables probabilistic controlled gates that successfully operate depending on the measurement results of ancillary photons [3,4].In the quest for realizing deterministic controlled gates in quantum optics, a measurable nonlinear phase shift between single photons has been demonstrated using a cavity quantum electrodynamics system in the bad-cavity regime [5]. This system has the characteristic that radiation from the atom is nearly completely forwarded to a one-dimensional field that is determined by the radiation pattern of the cavity. Such one-dimensional configurations can be realized by a variety of physical systems, including a leaky resonator interacting with an atom or a quantum dot [6,7], a single emitter near a surface plasmon [8], and a superconducting qubit near a transmission line or a resonator [9,10]. Since the incident light inevitably interferes with the radiation from the system due to the reduced dimensionality, the effective light-matter interaction can be drastically enhanced under this configuration. Utilizing this property, several quantum devices have been proposed to date, such as controlled logic gates [11,12], quantumstate converters [13,14,15] and entanglers of photonic or material qubits [15,16,17]. These devices perform their tasks with the help of active quantum control of the material part (such as initialization [11,12,13,14,15,16,17], single-qubit rotation [11,12], and classical pumping [13,14,15]) and by measurements [16,17].In the present study, we theoretically point out a unique potential of a three-level lambda system coupled to a one-dimensional photon field in the reflection geometry. (A lambda system is hereafter referred to as an "atom", although it can be implemented by other physical systems such as semiconductor quantum dots and superconducting Josephson junctions [18,19,20].) We show that a deterministic photon-photon √ SWAP gate can be realized by using the atom completely passively as a temporary memory for phot...