Generation of large-scale entanglement with time-domain multiplexing

Quantum entanglement, correlations between multiple systems that cannot be described by classical physics, is a key to realization of large-scale quantum computation [1]. Since the early 2000s, many researches and approaches have been taken to realize large-scale quantum entanglement. However, the largest record was quantum entanglement between 14 parties, generated using ion trap system [2]. In optical systems, quantum entanglement can be easily generated by interfering non-classical lights called squeezed light [3]. Despite this simplicity, conventional method in optical system requires larger setup as the number of entangled parties increases and the experimental demonstrations were limited to generation of nine-partite entanglement [4]. In most physical system, not limited to optical system, large-scale generation of quantum entanglement has remained a challenging task and a huge obstacle toward realization of quantum computing.

In 2011, however, a solution to this obstacle was proposed [5]; Dr. Nicolas Menicucci from RMIT University proposed a time-domain multiplexing method. This method allows generation of large-scale quantum entanglement on optical system without having to increase the size of experimental setup. Although this proposal was a theoretical breakthrough that shows the possibility of feasible large-scale quantum entanglement generation, experimental demonstration was not achieved due to technological difficulties.

In 2013, our group, based on the above proposal, developed necessary technologies and demonstrated generation of large-scale quantum entanglements, overcoming the limitation in their size for the first time [6]. By applying time-domain multiplexing method, we can consider a continuous-wave squeezed light as multiple, independent, and temporally localized wave packets of squeezed lights and generate large-scale entanglement using only two squeezed-light sources. To generate such a large-scale quantum entanglement, we first generate multiple disjointed two-mode entanglements by interfering two squeezed-light wave packets, then, we delay one of the wave packet and interfere them with wave packets before and after. This results in an entanglement consists of multiple squeezed light wave packets that are entangled with temporally adjacent wave packets.

Also, in the process of making time-domain multiplexed entanglement, we also developed many technology such as a low-loss optical delay line. As the result, we generated quantum entanglement consists of more than 16,000 parties, 1,000 times more than that of the previous research. By further improve the stability of our system, in principle, it is possible to generate large-scale quantum entanglement without limitations on the number of the entangled parties.

Moreover, this particular entanglement we generated can be combined with quantum teleportation to realize time-domain multiplexed quantum computer. By using time-domain multiplexing, not only that we can realize large-scale quantum entanglement, but we can also realize teleportation-based quantum computer in a scalable fashion. As a next step, we are now pursuing realization of large-scale quantum computer based on the experimental system we developed for generation of time-domain multiplexed large-scale quantum entanglement.

Fig.1. Abstract comparison between optical quantum computer using the conventional method and time-domain multiplexing method.
Fig.2. Time-domain multiplexed quantum computer. Quantum teleportation plays an essential role in this type of quantum computer. The system for generation of large-scale quantum entanglement developed in this work can be considered as a part of time-domain multiplexed quantum computer.
Optical generation of non-classical states toward universal quantum computation

A quantum computer, which uses nature of quantum mechanics for computing, is a promising device to efficiently solve some problems beyond present classical computers [1]. Univesal quantum computing requires certain quantum gates as well as classical computers consist of NAND gates. In the field of continuous variable quantum information processing, it is known that we can build the quantum computer by exploiting only five optical-quantum gates in principle [2]. In our laboratory, four of them have been already implemented [3]. Our present challenge is to realize the last fifth gate. This is called “cubic phase gate”, a key block of universal quantum computing.

It is theoretically shown that the cubic phase gate can be realized by combining quantum teleportation technologies and one highly-nonclassical quantum state [4], represented by a superposition of zero- to three-photon-number states. Although some experiments of generating superpositions of photon-number states have been reported [5, 6], they were limited up to two-photon level. Furthermore, the generated states in their experiments undergo a nearly fifty percent loss in the process of generation and measurement. This is probably because they exploited the pulsed scheme so that they couldn’t get high measurement efficiency. Since the quantum teleportation circuit for the cubic phase gate requires high measurement efficiency, the systems of the previous experiments are not suitble for our purpose.

In 2013, based on the continuous wave scheme, we achieved generation of arbitrary superpositions of zero- to three-photon-number states with 80% high-efficiency [7]. In the same scheme we also generated the state required for the cubic phase gate, whose non-classicality has been verified [8]. With this scheme, together with quantum teleportation technologies, we are now trying to implement the cubic phase gate.

Fig. 1. Experimental schematic of non-classical superposition state generation. When we get simultaneous clicks of three APDs in the Trigger mode, we immediately know that we have the desired non-classical state in the Signal mode.
Fig. 2. Experimentally obtained density matricies and Wigner functions of photon-number superposition states (hbar = 1). The state (c) is the very one for the cubic phase gate.