An interdisciplinary quantum communications and sensing research effort has been underway at the NASA Glenn Research Center since the summer of 2000. Researchers in the Communications Technology, Instrumentation and Controls, and Propulsion and Turbomachinery Divisions have been working together to study and develop techniques that use the principle of quantum entanglement (QE). This work is supported principally by the Nanotechnology Base R&T program at Glenn. As applied to communications and sensing, QE is an emerging technology that holds promise as a new and innovative way to communicate faster and farther, and to sense, measure, and image environmental properties in ways that are not possible with existing technology. Quantum entangled photons are inseparable as described by a wave function formalism. For two entangled photons, the term inseparable means that one cannot describe one photon without completely describing the other. This inseparability gives rise to what appears as spooky, or nonintuitive, behavior because of the quantum nature of the process. For example, two entangled photons of lower energy can be created simultaneously from a single photon of higher energy in a process called spontaneous parametric down-conversion. Our research is focused on the use of polarization-entangled photons generated by passing a high-energy (blue) photon through a nonlinear beta barium borate crystal to generate two red photons that have orthogonal, but entangled, polarization states. Although the actual polarization state of any one photon is not known until it is measured, the act of measuring the polarization of one photon completely determines the polarization state of its twin because of entanglement. This unique relationship between the photons provides extra information about the system. For example, entanglement makes it easy to distinguish entangled photons from other photons impinging on a detector. For many other applications, ranging from quantum computation and information to quantum sensing, the entanglement property is critical.
An interdisciplinary quantum communications and sensing research effort has been underway at the NASA Glenn Research Center since the summer of 2000. Researchers in the Communications Technology, Instrumentation and Controls, and Propulsion and Turbomachinery Divisions have been working together to study and develop techniques that use the principle of quantum entanglement (QE). This work is supported principally by the Nanotechnology Base R&T program at Glenn. As applied to communications and sensing, QE is an emerging technology that holds promise as a new and innovative way to communicate faster and farther, and to sense, measure, and image environmental properties in ways that are not possible with existing technology. Quantum entangled photons are inseparable as described by a wave function formalism. For two entangled photons, the term inseparable means that one cannot describe one photon without completely describing the other. This inseparability gives rise to what appears as spooky, or nonintuitive, behavior because of the quantum nature of the process. For example, two entangled photons of lower energy can be created simultaneously from a single photon of higher energy in a process called spontaneous parametric down-conversion. Our research is focused on the use of polarization-entangled photons generated by passing a high-energy (blue) photon through a nonlinear beta barium borate crystal to generate two red photons that have orthogonal, but entangled, polarization states. Although the actual polarization state of any one photon is not known until it is measured, the act of measuring the polarization of one photon completely determines the polarization state of its twin because of entanglement. This unique relationship between the photons provides extra information about the system. For example, entanglement makes it easy to distinguish entangled photons from other photons impinging on a detector. For many other applications, ranging from quantum computation and information to quantum sensing, the entanglement property is critical.
Entanglement has been an extremely active field of research both from a theoretical and an experimental point of view. As the understanding of the phenomena grows, so does the interest in using it to advance different technological fields: communica- tion, computing and sensing being just some examples. It is now well known that entangled photon pairs can be generated through a process known as spontaneous parametric down-conversion (SPDC). In this thesis I therefore integrate the SPDC generated photons in a variety of experiments each exploiting a different charac- teristic of this quantum effect. In particular, both chapter 3 and chapter 4 utilise the spatial degree of freedom of SPDC generated entangled photons to enhance the quantity of information that can be transmitted in quantum communication systems. In particular, in chapter 3 the information capacity of two of the best known spa- tial modal sets is analysed in the context of real life finite-aperture communication systems, while chapter 4 proposed a new approach for imparting information onto photons. On the other hand, both chapters 5, and 6 rely on the ability of SPDC to generate completely indistinguishable photon pairs, which, when made to interfere, bunch together in what is known as N00N state. In particular, chapter 5 demon- strates that the recently discovered “Coherent Perfect Absorption” process (CPA) can be used to coherently control and absorb two-photon N00N states and therefore can be employed in the generation of quantum gates, while in chapter 6, the quantum interference process that produces the N00N states is used to test the role of relativity in quantum mechanics through the construction of a “quantum gyroscope”.
The results reported by Versteegh et al. 1 and Huber et al. 2 certainly constitute a step forward in pursuing quantum communication technology towards future larger scalability in arrays of sources. They realised bright entangled photon sources in the spectral region of ∼935 nm and ∼913 nm, respectively, that combine high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nano-scale. These properties are now achieved in single devices. In 1909 Guglielmo Marconi shared with Carl Ferdinand Brown the Nobel Prize in physics in recognition of their contribution to the development of wireless telegraphy, setting the start of long distance radio transmission based on discrete signal. Marconi then developed with others the radio transmission to a commercial success. Less than a century later, Bennett et al. 3 proposed to transmit not only bits but also additional information to communicate and transfer the inherent property of a quantum physical system interconnected with a sender and a receiver. According to Bennett 3 “An unknown quantum state ϕ can be disassembled into, then later reconstructed from, purely classical information and purely non-classical Einstein-Podolsky-Rosen (EPR) correlations. To do so the sender, Alice, and the receiver, Bob, must prearrange the sharing of an EPR-correlated pair of particles. Alice makes a joint measurement on her EPR particle and the unknown quantum system, and sends Bob the classical result of this measurement. Knowing this, Bob can convert the state of his EPR particle into an exact replica of the unknown state ϕ which Alice destroyed.” The work by Bennett et al. 3 immediately received huge interest 4. It set the basis of the last 20 years of a research endeavour known as quantum communication. Significant work has been carried out to prove the feasibility of this idea and now more practical solutions have been made available, as demonstrated by the emergence of recent quantum technologies. Quantum communication is strongly relying on transmission of both classical bits and quantum bit or qubits. “Classical” bits are units of information represented as zero or one, true or false. Qubits are superposition of zero and one states and can encode quantum information. Quantum teleportation provides a mechanism of transferring a qubit from one location to another relying on the quantum superposition of, for example, strongly correlated photons states, which allow by measuring one photon of the pair to simultaneously know the state of the sibling photon. This translates in a transfer of information without the limitation associated of the actual physical qubit transportation. In fact, the transfer of qubits requires that an entangled quantum state is created and the two parts of this state are shared between two locations. Teleportation requires that a quantum channel is established between the source and the destination (Alice and Bob) and a classical information link is used to transmit two classical bits with each qubit. Classical bits can only be transmitted slower than the speed of light. Conversely, quantum correlations may occur instantaneously. The quantum states of single atoms, photons and solid state qubits have been teleported so far (Bouwmeester et al. 5; Gottesman & Chuang 6; Riebe et al. 7; Chen et al. 8; Krauter et al. 9 just to name a few). Photons are regarded as the most robust source of entangled quantum states due to their weak interaction with the environment, therefore they have a long coherence time as opposite to solid state qubits. Quantum communication protocols like quantum teleportation and quantum repeaters require entangled states. These states have been practically achieved in bulk optical systems based on nonlinear optics effects in birefringent crystals since many years now (Ou & Mandel 10). These sources are not deterministic as they rely on the random decay of one higher energy pump photon in two photons by spontaneous parametric down-conversion. Also they are not ideal for integration in nanophotonics circuits and have limited scalability to achieve large numbers of quantum gates or quantum networks. Artificial atoms, like quantum dots (QDs), on the other hand constitute a deterministic source of single or entangled photon states. A QD is made of a semiconductor of lower band gap embedded in a semiconductor of higher band gap. The band gap offsets results in a three dimensional electronic confinement. An electron may then be excited optically or electrically to the conduction band, leaving a hole in the valence band. An exciton state may be formed by these electron–hole pairs trapped by the QD. Further excitation of the QD may lead to a biexciton state with two electrons and two holes. The radiative decay of the exciton leads to the emission of one photon. The biexciton-exciton radiative cascade results in the emission of two photons. The photons are not re-emitted until the exciton or biexciton are excited again. This delivers a determinist photon emission. Owing to asymmetries in the QD, there is actually a fine structure splitting (FSS) in the exciton state due to the different electron and hole spin states of the QD, leading to slightly different transition frequencies for the two photons generated with horizontal and vertical polarization. This requires temporal post selection of the emitting photons to create a perfect entangled photons state, limiting the efficiency of QDs for entanglement generation (loss of photons) and therefore their practical use in quantum communication protocols. Versteegh et al. 1 and Huber et al. 2 grew semiconductor QDs embedded in [111]-oriented tapered nanowire waveguides grown in the wurtzite phase, specifically InP nanowires containing single InAsP; the synthesis methods are slightly different, in Versteegh et al. 1 thicker QDs are grown (diameter ∼25-30 nm). Although theoretically previously predicted, entanglement was not yet been demonstrated for a nanowire QD. Versteegh et al. 1 demonstrate a bright and coherent source of strongly entangled photon pairs from a position controlled nanowire QD with a fidelity as high as 0.859 ± 0.006 and concurrence of 0.80 ± 0.02 with a FSS value of 1.2 μeV, deduced from the polarization-dependence measurement of the energy emission difference from the exciton and biexcition states. There are two main advantage in a nanowire embedded QD for photon pairs generation: the high refractive index of the nanowire allows guiding the light originated from the QD and the tapered end allows an efficient light extraction with broad spectral operation; in QD embedded in nanowires grown along [111] direction, the FSS should vanish, thus providing a nearly ideal entanglement via biexcition-exciton radiative cascade decay without the need of any post-selection. In Versteegh et al. 1 the created two-photon quantum states provide different photon cross-correlation from what observed previously from QDs, revealing that the two-photon quantum state from the nanowire is closer to (|HV> + |VH>)/ than to the commonly measured state (|HH> + |VV>)/. The entangled photon states produced are modified by the nanowire shape and small anisotropy, justifying different photons states compared to the ones measured in QDs. The photons pairs produced by the QDs experience a variation of their polarization due to birefringence while propagating along the nanowire. Figure 1 shows SEM image of (a) symmetric and (b) asymmetric nanowires. The nanowire elongation in (b) is an extreme example that leads to geometric birefringence responsible for the rotation of the quantum state originated from the QDs. In Huber et al. 2 a FSS value of 18 μeV was determined, possibly responsible for a lower fidelity of the generated entangled states. In this case they did not observe evident sign of lateral elongation in scanning electron microscopy images showing the cross-section of the hexagonal nanowires cores, a small elongation ratio was confirmed unable to induce a significant level of FSS as the one observed. By inspecting the cross-section of the nanowires dislocation of the QD insertion with respect to the nanowire axis was observed. Wave function calculations of the electrons and holes for on and off-centred QDs show that off-centred QDs exhibit orbitals with lower symmetry (C2v) compared to centred QDs (C3v). This lower symmetry can be responsible only in minor percentage for the FSS. The authors conclude that the large FSS observed can be attributed to the compositional anisotropy rather than the geometrical asymmetry. This new nano-scale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes, allowing producing arrays of such sources on a chip. A further step may certainly be identified in having an electrically rather than as optical pump of the entangled photon pairs in the nanowire. Also photons extraction efficiency of 90% is predicted by redesigning the nanowires. Some work needs still to be done however to surpass the present sources based on spontaneous parametric down-conversion. The nanowire photon states entanglement fidelity is not as high as conventional sources and the single-photon coherence of the emitted photon pairs is not yet Fourier-transform limited. By developing a thermal annealing process to get rid of the compositional inhomogeneity in the nanowires a decrease of the FSS to sub-μeV level for any desired target wavelength could be achieved. By two-photon resonant excitation techniques, cooling of the quantum dot sample to 300mK and Purcell effect radiative decay enhancement, photons indistinguashibility could be achieved and possibly surpass in the future conventional sources.
Photons are very promising resources for the whole field of quantum information processing. Bulk optics are currently not suitable for the realization of complex quantum optical schemes due to several drawbacks. The ongoing miniaturization of photonic structures due to the availability of sophisticated nanofabrication has provided huge opportunities for physical research of novel phenomena in nanophotonic systems and quantum technological applications. These include quantum information processing such as quantum computing, quantum communication, quantum metrology, and quantum sensing. Quantum communication is already quite advanced with several quantum key distribution (QKD) systems commercially available offering absolute security. Photonic quantum technologies are enabling breakthroughs in experimental tests of quantum physics and are close to practical applications of quantum technologies in imaging, secure communications, ultrasensitive metrology and quantum computing. There are several advantages to encoding a quantum bit onto a photon: the preparation and manipulation of single photons is easy, and the photonic qubit is a natural candidate for quantum imaging and communications. Furthermore, the qubit can be encoded in various degrees of freedom such as on the position, phase, time-bin, energy, angular momentum, polarization of the photon—or a combination of these. This flexibility leads to enormous opportunities for scientific exploration and in practical applications. This special issue on “Photonic Quantum Technologies” presents selected papers on photonic quantum systems from leading groups in the world. The articles represent a flavor of the cutting-edge research that is going on in this area. The studied material systems range from molecules to semiconductor quantum emitters. A major challenge is the design of telecom photon emitters with high efficiency, good yield, spectral purity, stability and the possibility of mass manufacture. We believe that this Special Issue will provide an overview of recent exciting developments in the field of photonic quantum technologies, serving to keep researchers up-to-date with the latest progress and spurring on new concepts in this field.
Quantum communication and quantum computation are novel methods of information transfer and information processing, all fundamentally based on the principles of quantum physics. The performances outdo their classical counterparts in many aspects [1,2]. In almost all quantum communication and quantum computation schemes, quantum entanglement [3] plays a decisive role. In essence, an entangled system can carry all information (e.g., on their polarization properties) only in their correlations, while no individual subsystem carries any information. This leads to correlations that are much stronger than classically allowed [89, 100], which is a powerful resource for information processing. It is therefore important to be able to generate, manipulate, and distribute entanglement as accurately and as efficiently as possible.
Quantum optical sensing is a technique that utilizes quantum mechanics to enhance, sensitivity and precision to a level considered unparalleled so far. This chapter attempts to introduce the reader to the basics of this fast-growing field along with various technologies and applications. It begins with an explanation of quantum mechanics, superposition, entanglement, and quantum coherence that allow sensors to go beyond classical performance. One of the central technologies is quantum entanglement – the phenomenon whereby several particles become instantaneous spouses and are measured with utmost precision, for instance, distance, time, and magnetic fields. Entanglement Interferometry is entanglement applied in the detection of gravitational waves and high-resolution imaging. Squeezed light, the other area of interest, is a method that removes some of the fluctuations in one parameter of light, to increase the interferometer sensor, to measure small movements and low-frequency gravitational waves. Single photon emitting devices and detectors enable high-sensitivity measurements in applications such as bioimaging and quantum-securing communication. The chapter also illustrates the usage of this technology in practice in healthcare, environment, and navigation, like the examination of biomarkers and pollutants. Last is the discussion of issues such as quantum coherence and issues of scalability and development of QTs, as well as the proposed further development and more potential advances for QTs as a medium for mass use.