In research of quantum computation and quantum communication, the Einstein-Podolsky-Rosen (EPR) pair (Fig.1) is one of the central concepts. In particular, nonlocal entangled electron pairs attract much attention because of wide potential application in quantum devices. However, the generation and manipulation of nonlocal entangled electrons in solid-state systems is still a challenge, since electrons interact with the macroscopic Fermi sea around them and it is hard to control a particular pair.

One promising approach to achieve nonlocal entanglement of an electron pair is to split the naturally entangled Cooper pairs in superconductors. Both theoretical and experimental investigations have been conducted with three-terminal devices consisting of a superconductor coupled to two leads. Recently we proposed a method to detect nonlocal spin entanglement based on the interference of Josephson current. For this purpose, we adopt the system in Fig. 2. In this setup, two spin-entangled electrons tunneling through the two split paths induce a novel Josephson current. This novel current exhibits a period of h/e responding to the magnetic flux, in contrast to the well-known period of h/2e for SQUID (Fig. 3). We also show that the nonlocal spin entanglement provides a quantum mechanical functionale for switching on and off this split tunneling process. The switch is controlled by a pilot Josephson junction and can be used to directly measure the magnitude of the novel Josephson current.

Further investigations on manipulation of entangled states of electron pair on demand are in progress.

Fig 1: Schematic picture for an electron pair in nonlocal spin-entangled state.

Fig 2: Schematic rendition of a Cooper-pair splitter in which the superconductors are connected via quantum dots controlled by gate voltage.

Fig 3: Oscillations of the maximal Josephson current with the magnetic flux with(red) and without(blue) split Cooper pairs.