After the preceding four introductory chapters, we are finally ready to discuss optical quantum information processing. While in principle quantum communication can be implemented with atoms or electrons, in practice the implementation of choice for long-distance quantum communication will almost certainly be optical. In this chapter, we will develop some of the key topics in quantum communication with single photons, and we will discuss continuous-variable quantum communication in Chapter 8. In some ways, quantum communication is the most technologically advanced part of quantum information processing. Most protocols in this chapter, such as teleportation and cryptography, have been convincingly demonstrated in the lab, and there are already several commercial organizations that sell cryptographic systems based on quantum mechanical principles. In particular, the promise of secure communication with quantum cryptography is one of the driving forces behind the development of single-photon sources and detectors. In this chapter, we first construct several optical representations of the qubit, including polarized photons and time-bin encoding. Since the single-photon implementation of a qubit is central to optical quantum information processing, this constitutes a large part of the chapter. In Section 5.2 we discuss quantum teleportation and entanglement swapping with single photons. We also present a method for entanglement distillation using entanglement swapping. In Section 5.3 we introduce decoherence-free subspaces, which can be used to reduce noise in the transmission of quantum information, and even establish communication channels in the absence of shared reference frames.
After the preceding four introductory chapters, we are finally ready to discuss optical quantum information processing. While in principle quantum communication can be implemented with atoms or electrons, in practice the implementation of choice for long-distance quantum communication will almost certainly be optical. In this chapter, we will develop some of the key topics in quantum communication with single photons, and we will discuss continuous-variable quantum communication in Chapter 8. In some ways, quantum communication is the most technologically advanced part of quantum information processing. Most protocols in this chapter, such as teleportation and cryptography, have been convincingly demonstrated in the lab, and there are already several commercial organizations that sell cryptographic systems based on quantum mechanical principles. In particular, the promise of secure communication with quantum cryptography is one of the driving forces behind the development of single-photon sources and detectors. In this chapter, we first construct several optical representations of the qubit, including polarized photons and time-bin encoding. Since the single-photon implementation of a qubit is central to optical quantum information processing, this constitutes a large part of the chapter. In Section 5.2 we discuss quantum teleportation and entanglement swapping with single photons. We also present a method for entanglement distillation using entanglement swapping. In Section 5.3 we introduce decoherence-free subspaces, which can be used to reduce noise in the transmission of quantum information, and even establish communication channels in the absence of shared reference frames.
Quantum mechanics allows a number of communications protocols that would not be possible classically, including provably-secure messages (quantum cryptography), increased channel capacity, and the ability to transmit an unknown quantum state (quantum teleportation). This tutorial will provide an introductory description of what appear to be the most promising applications of quantum communications, including quantum cryptography and quantum repeaters. The most commonly-used methods for quantum cryptography will be described, including the advantages and disadvantages of each in practical applications. The maximum range of current quantum cryptography systems is limited by the fact that optical amplifiers would destroy the quantum coherence of single photons and cannot be used in these kinds of systems. One potential solution to this problem is the transmission of single photons in free space, which would allow a global system for quantum cryptography based on a network of satellites and ground stations. Free-space quantum cryptography systems of that kind have now been demonstrated over relatively short distances. The range of quantum cryptography systems in optical fibers can also be extended using quantum repeaters that make use of entanglement purification and quantum teleportation. These same techniques could also be used to implement a “quantum Internet” for the transmission of qubits from one quantum computer to another. The practical challenges that will have to be met in order to implement quantum repeaters and quantum teleportation over large distances will be discussed.
Quantum information has been witnessing great science value and latent application since 1980's. The research work presented here consist of mainly two important parts: manipulations of multiphoton entanglement and atomic ensembles based quantum memory. In first part, the experimental technique multi-photon entanglement is further developed to study fundamental issues in quantum mechanics, remarkable applications to quantum communication and quantum computation. Specifically, we have demonstrated a bit-flip error-free transfer of quantum information, violation of Bell's inequality beyond Tsirelson's bound, teleportation of a two-qubit composite system, as well as the one-way computing by two-photon-four-qubit cluster state. To overcome un-scalability problem due to probabilistic feature in linear optical quantum information processing, we investigated in the second part the physics of atomic ensembles based quantum memory. We show that theoretically, entanglement between distant locations can be deterministically generated. The experimental work has thoroughly developed the necessary techniques and we have achieved deterministic single photon source, and interference of the photons from independent atomic ensembles, teleportation between photonic and atomic qubits, a novel way to create a robust entanglement between an atomic and a photonic qubit, and memory based entanglement swapping. We believe, the developed techniques here would dramatically facilitate progresses in many fields including global quantum communication, linear optical quantum computation and the foundations of quantum mechanics etc.
Entanglement and superposition are foundations for the emerging field of quantum commu‐ nication and information processing. These two fundamental features of quantum mechanics have made quantum key distribution unconditionally secure (Scarani et al., 2009; Weedbrook et al., 2010) compared to communication based on classical key distribution. Currently, implementation of an optical quantum communication is mainly based on discrete and continuous quantum variables. They are usually generated through nonlinear interaction processes in χ(2) (Kwiat et al., 1995) and χ(3) (Lee et al., 2006,2009) media. Discrete-variable qubit based implementations using polarization (Liang et al., 2006, 2007; Chen et al. 2007, 2008; Sharping et al., 2006) and time-bin (Brendel et al., 1999; Tittel et al., 1998, 1999) entanglement have difficulty to obtain unconditional-ness, and also usually have low optical data-rate because of post-selection technique with low probability of success in a low efficient single photon detector at telecom-band (Liang et al., 2005, 2006, 2007). Continuous-variable imple‐ mentations using quadrature entanglement (Yonezawa et al., 2004; Bowen et al., 2003; Silberhorn et al., 2002) and polarization squeezing (Korolkova et al., 2002) can have high efficiency and high optical data-rate because of available high speed and efficient homodyne detection. However, the quality of quadrature entanglement is very sensitive to loss, which is imperfect for implementing any entanglement based quantum protocols over long distance. Continuous-variable protocols that do not rely on entanglement, for instance, coherent-state based quantum communication (Yuen, 2004; Corndorf et al., 2003; Barbosa et al., 2003; Grosshans et al., 2002, 2003; Qi et al., 2007; Wilde Qi et al., 2008), are perfect for long distance optical communication. Several experimental approaches were taken to resolve transmission loss for long distance optical communication by using coherent light source. Optical wave
Quantum information processing and quantum communication are novel protocols that originate from the very fundamental and philosophical questions on superposition and entanglement raised since the early days of quantum mechanics. Strikingly, these new protocols offer capabilities beyond communication task possible with classical physics. One very important example is the secure key exchange based on the transmission of individual quantum signals between communication partners. The big vision and frontier in the field of quantum communication research is the development of a Quantum Internet, which establishes entanglement between many different users and devices. Extending today’s internet, the Quantum Internet will readily transfer quantum bits, rather than classical bits, between users near and far and over multiple different channels and could be used for secure communications, quantum computer networks and metrological applications. I will discuss recent advances on implementations and tools useful for generating and distributing photonic quantum entanglement over robust channels including. Time-bin encoding and reference-frame-free protocols. I will also present an overview of the upcoming Canadian quantum communication satellite QEYSSAT.
Encoding and processing quantum information in the time-of-arrival of photons offer significant advantages for quantum information science and technology. These advantages include ease of experimental realization, robustness over photon state transmission, and compatibility with existing telecommunication infrastructure. Additionally, time-of-arrival encoding has the potential for high-rate quantum communication and holds promise for the future development of quantum internet. This review explores the generation, processing, and applications of time-encoded quantum states, focusing on both single-photon states, energy–time entanglement, and time-bin entanglement. We summarize the nonlinear optics platforms and advanced laser and modulation techniques utilized for photon sources that enable quantum information encoding onto the photons' time-of-arrival. We also highlight advanced quantum state processing methods in the time domain, including the Franson interferometry, optical switch-based schemes, and state-of-the-art measurement and detection schemes that allow for high-speed and multi-dimensional quantum operations. Finally, we review the mainstream implementations mainly including the quantum communication demonstrations and outline future directions for developing practical quantum networks leveraging time-encoded photon states.
We present two types of photon sources designed for secure quantum communication, e.g. for quantum cryptography. Both types are based on the creation of photon pairs by spontaneous parametric downconversion in nonlinear crystals. The first is a heralded single photon source and the second is a source of polarization-entangled photon pairs. For the heralded single photon source the detection of one of the photons of a downconversion pair is used as a trigger to announce the presence of the other: the single photon. The source is characterized by a highly sub-Poisson photon number statistics making it very suitable for use in quantum cryptography protocols using single photonic qubits to create correlated information between a sender and a receiver. The entanglement source instead uses the inherent non-classical correlations between entangled qubits. We also present a hybrid-encoding where the sender uses polarization to encode information while the receiver uses time-bins. Both sources create photons with highly non-degenerate wavelengths of 810 nm and 1550 nm, taking advantage of the efficient detectors at near-infrared and the low transmission loss of optical fibers at telecommunication wavelengths.