Hybrid Quantum Teleportation

The laws of quantum mechanics enable optical communications with the ultimate capacity and quantum computers to solve certain problems with unprecedented speed. A key ingredient in such quantum information processing is quantum teleportation: the act of transferring quantum information from a sender to a spatially distant receiver by utilizing shared entanglement and classical communications. Especially, optical quantum teleportation is essential for various quantum communication protocols. Quantum logic gates based on optical quantum teleportation are one of the building blocks of optical quantum computers.

After its original proposal in 1993 [1], a research group in Austria succeeded in teleporting photonic quantum bits (qubits) in 1997 [2]. However, this scheme involved several features which hindered its applications to optical quantum information processing. One is its low transfer efficiency, estimated to be far below 1%. This is due to the probabilistic nature of entanglement generation and the joint measurement of two photons. Another is that this scheme required post-selection of successful events by measuring the output qubits after teleportation [3]. The transferred qubits are destroyed in this process, and thus cannot be used for further information processing. Various related experiments followed, most of which withhold the same disadvantages.

In 2013, we demonstrated deterministic quantum teleportation of photonic qubits for the first time [4]. That is, photonic qubits are always teleported in each attempt, in contrary to the former probabilistic scheme. In addition, it does not require post-selection of the successful events. The success of our experiment in overcoming previous limitations lies in a hybrid technique of photonic qubits and continuous-variable (CV) quantum teleportation [5,6,7]. The strength of CV teleportation, first demonstrated in 1998 [7], is its deterministic nature due to the on-demand availability of entangled waves and the complete joint measurement of two waves. It has long been used to teleport the amplitude and phase signals of optical waves, rather than photonic qubits. However, its application to photonic qubits had long been hindered by experimental incompatibilities: typical pulsed-laser-based qubits have a broad frequency bandwidth that is incompatible with the original continuous-wave-based CV teleporter, which works only on narrow frequency sidebands. We overcame this incompatibility by developing an innovative technology: a broadband CV teleporter [8] and a narrowband qubit compatible with that teleporter [9]. Furthermore, we discovered that qubit information can be faithfully transferred with the help of gain adjustment mechanism in CV teleportation [10].

This hybrid technique enabled the realization of completely deterministic quantum teleportation of photonic qubits without post-selection. The transfer accuracy (fidelity) ranged from 79 to 82 percent for four different qubits, all of which exceed the classical limit of teleportation. This is a decisive breakthrough in the field of optical teleportation 16 years after the first experimental realizations. We hope our work stimulates the further development of hybrid quantum information processing to overcome the current limitations in both the qubit and CV regimes.

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.
Loop-based photonic quantum computer

A photonic quantum computer has the advantage of room-temperature operation and applicability to optical communication, and thus is a promising candidate to realize a universal quantum computer. Thus far, there have been proof-of-principle demonstrations of elementary quantum logic gates and quantum error correction with small-scale optical circuits. However, there still remain several obstacles for large-scale optical quantum computing. One is that large-scale quantum computing requires large-scale optical circuits, thus requiring a large number of optical components and space. Another problem is that different optical circuits are necessary for different quantum computation since one optical circuit can perform only one specific quantum computation.

Recently, we proposed a new architecture for optical quantum computation which can efficiently perform arbitrarily large-scale quantum computation with minimal optical components [1]. In this architecture, a string of input and ancillary pulses in a single optical beam are sent to a nested loop circuit in Fig.1. All of these pulses are first stored in the outer loop, which acts as an optical memory to store many optical quantum states. In contrast, the inner loop is a quantum processor which sequentially processes the optical pulses and perform quantum computation. Here, quantum logic gates are programmably performed by dynamic control of system parameters, such as optical switches, beam splitter transmissivity, phase shifters, and amplifier gain.

This architecture can deal with any number of optical pulses without increase in optical circuit scale, and repeat an unlimited number of quantum operations by measuring each pulse within the coherence time of the light source, thus offering high scalability. Furthermore, it offers a universal gate set for both qubits and continuous variables once suitable ancillary pulses are provided. In addition, arbitrary quantum computation can be performed with the same optical circuit only by changing the program to control the system parameters. Our scheme will promote large-scale optical quantum computing and also greatly reduces resources and cost for the development of photonic quantum computers. We are now developing this loop-based photonic quantum computer while analyzing computational accuracy and implementation methods of quantum algorithms in this architecture.

Fig. 1. Loop-based architecture for quantum computing [1]. HD, Homodyne detector; Disp., Displacement operation; PS, Phase shifter; VBS, Variable Beam Splitter.