Control an optical “signal” propagating across some channel via a “control” at a gate. The optical signal for this may be as simple as a stream of photons, perhaps at some frequency, and in general the control is also some number of photons. Unfortunately, unlike electrons, photons have very weak non-linear interactions with one another. The concept shown here is only one such method of creating a strong non-linear interaction between photons. The hopeful solution proposed here is to utilize a tight concentration of optical fields in conjunction with guided surface plasmons along a conducting nanowire in order to achieve strong non-linear interactions between optical emitters. Two photons are sent through a nanowire towards an atom, where they collide, such that one photon (red) transfers its information to the other photon. Today the signal is an electrical current. For a quantum computer the signal can be an optical one, and it works using a single photon which is the smallest component of light. To work, the photons have to meet and “talk”, and the photons very rarely interact together” says Anders SØndberg SØrensen who is a Quantum Physicist at the Niels Bohr Institute at Copenhagen University. He explains that light does not function like in Star Wars, where the people fight with light sabres and can cross swords with the light. That is pure fiction and can't happen. When two rays of light meet and cross, the two lights go right through each other. That is called linear optics. What he wants to do with the light is non-linear optics. That means that the photons in the light collide with each other and can affect each other. This is very difficult to do in practice. Photons are so small that one could never hit one with the other. Unless one can control them -and it is this Anders SØrensen has developed a theory about.
Control an optical “signal” propagating across some channel via a “control” at a gate. The optical signal for this may be as simple as a stream of photons, perhaps at some frequency, and in general the control is also some number of photons. Unfortunately, unlike electrons, photons have very weak non-linear interactions with one another. The concept shown here is only one such method of creating a strong non-linear interaction between photons. The hopeful solution proposed here is to utilize a tight concentration of optical fields in conjunction with guided surface plasmons along a conducting nanowire in order to achieve strong non-linear interactions between optical emitters. Two photons are sent through a nanowire towards an atom, where they collide, such that one photon (red) transfers its information to the other photon. Today the signal is an electrical current. For a quantum computer the signal can be an optical one, and it works using a single photon which is the smallest component of light. To work, the photons have to meet and “talk”, and the photons very rarely interact together” says Anders SØndberg SØrensen who is a Quantum Physicist at the Niels Bohr Institute at Copenhagen University. He explains that light does not function like in Star Wars, where the people fight with light sabres and can cross swords with the light. That is pure fiction and can't happen. When two rays of light meet and cross, the two lights go right through each other. That is called linear optics. What he wants to do with the light is non-linear optics. That means that the photons in the light collide with each other and can affect each other. This is very difficult to do in practice. Photons are so small that one could never hit one with the other. Unless one can control them -and it is this Anders SØrensen has developed a theory about.
Abstract A symmetrical approach is suggested (Chang DE et al. Nat Phys 3:807, 2007) to realize a single-photon transistor, where the presence (or absence) of a single incident photon in a ‘gate’ field is sufficient to allow (prevent) the propagation of a subsequent ‘signal’ photon along the nanowire, on condition that the ‘gate’ field is symmetrically incident from both sides of an emitter simultaneously. We present a scheme for single-photon transistors based on the strong emitter-surface-plasmon interaction. In this scheme, coherent absorption of an incoming ‘gate’ photon incident along a nanotip by an emitter located near the tip of the nanotip results in a state flip in the emitter, which controls the subsequent propagation of a ‘signal’ photon in a nanowire perpendicular to the axis of the nanotip.
Quantum information processing systems and related technologies are likely to involve switching and amplification functions in ultrasmall objects such as nanotubes. In today's electronic devices the transistor performs these functions. A 'quantum age' equivalent of the conventional transistor would, ideally, use photons rather than electrons as information carriers because of their speed and robustness against decoherence. But robustness also stops them being easily controlled. Now a team from optETH and ETH in Zurich demonstrates the realization of a single-molecule optical transistor. In it, a single dye molecule coherently attenuates or amplifies a tightly focused laser beam, depending on the power of a second 'gating' beam. The transistor is the most fundamental building block in present-day technologies. For the purpose of quantum information processing schemes and for the development of a 'quantum computer', photons are attractive information carriers because of their speed and robustness against decoherence. However, their robustness also prevents them from being easily controlled; despite this, experiments now show the realization of a quantum optical transistor. The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. In the continuing development of increasingly powerful computers as well as alternative technologies based on the prospects of quantum information processing, switching and amplification functionalities are being sought in ultrasmall objects, such as nanotubes, molecules or atoms1,2,3,4,5,6,7,8,9. Among the possible choices of signal carriers, photons are particularly attractive because of their robustness against decoherence, but their control at the nanometre scale poses a significant challenge as conventional nonlinear materials become ineffective. To remedy this shortcoming, resonances in optical emitters can be exploited, and atomic ensembles have been successfully used to mediate weak light beams7. However, single-emitter manipulation of photonic signals has remained elusive and has only been studied in high-finesse microcavities10,11,12,13 or waveguides8,14. Here we demonstrate that a single dye molecule can operate as an optical transistor and coherently attenuate or amplify a tightly focused laser beam, depending on the power of a second ‘gating’ beam that controls the degree of population inversion. Such a quantum optical transistor has also the potential for manipulating non-classical light fields down to the single-photon level. We discuss some of the hurdles along the road towards practical implementations, and their possible solutions.
Summary form only given. Optical logic down to the single photon level holds the promise of data processing with a better energy efficiency than electronic devices [1]. In addition, preservation of quantum coherence in such logical components would enable optical quantum logical gates [2-8]. Optical logic requires optical non-linearities to allow for photon-photon interactions. Non-linearities usually appear for large intensities, but discrete transitions in a well coupled single two-level system allow for giant non-linearities operating at the single photon level.This is achieved by engineering what has been coined a "one-dimensional atom" [2], wherein a light emitter is predominantly coupled to a single propagating spatial mode. This favored coupling with a traveling mode is obtained either in a resonant optical cavity [3,4] or with non-resonant field confinement such as in single mode waveguides [5,6] or through the interaction with a focused optical beam [7,8] . Here we take advantage of the large coupling efficiency and the broadband operation of a photonic wire containing a semiconductor quantum dot (QD) [9] to implement a transistor using two different optical QD transitions. Our system is made of a single InAs QD embedded in a GaAs tapered photonic wire (fig. 1a). This system, in which the QD is efficiently coupled to a single guided mode, has been exploited to realize ultrabright single-photon sources [9]. We exploit here its broad operation bandwidth (>100 nm around 950 nm) to efficiently address two different transitions of the QD with two different laser beams to implement a two-color giant non-linearity: a weak probe laser has its reflectivity controlled by a few photons of the control laser.
Constructing quantum computers and other quantum devices requires the ability to leverage quantum properties such as superposition and entanglement – but these effects are fragile and therefore hard to maintain. Recently, scientists at Ecole Normale Superieure in Paris demonstrated a novel method for controlling the quantum properties of light by probing a superconducting circuit in a cavity with microwave photons to control the energy levels that photon quanta can occupy. [17] When two atoms are placed in a small chamber enclosed by mirrors, they can simultaneously absorb a single photon. [16] Optical quantum technologies are based on the interactions of atoms and photons at the single-particle level, and so require sources of single photons that are highly indistinguishable – that is, as identical as possible. Current single-photon sources using semiconductor quantum dots inserted into photonic structures produce photons that are ultrabright but have limited indistinguishability due to charge noise, which results in a fluctuating electric field. [14] A method to produce significant amounts of semiconducting nanoparticles for light-emitting displays, sensors, solar panels and biomedical applications has gained momentum with a demonstration by researchers at the Department of Energy's Oak Ridge National Laboratory. [13] A source of single photons that meets three important criteria for use in quantum-information systems has been unveiled in China by an international team of physicists. Based on a quantum dot, the device is an efficient source of photons that emerge as solo particles that are indistinguishable from each other. The researchers are now trying to use the source to create a quantum computer based on boson sampling. [11] With the help of a semiconductor quantum dot, physicists at the University of Basel have developed a new type of light source that emits single photons. For the first time, the researchers have managed to create a stream of identical photons. [10] Optical photons would be ideal carriers to transfer quantum information over large distances. Researchers envisage a network where information is processed in certain nodes and transferred between them via photons. [9] While physicists are continually looking for ways to unify the theory of relativity, which describes large-scale phenomena, with quantum theory, which describes small-scale phenomena, computer scientists are searching for technologies to build the quantum computer using Quantum Information. In August 2013, the achievement of fully deterministic quantum teleportation, using a hybrid technique, was reported. On 29 May 2014, scientists announced a reliable way of transferring data by quantum teleportation. Quantum teleportation of data had been done before but with highly unreliable methods. The accelerating electrons explain not only the Maxwell Equations and the Special Relativity, but the Heisenberg Uncertainty Relation, the Wave-Particle Duality and the electron's spin also, building the Bridge between the Classical and Quantum Theories. The Planck Distribution Law of the electromagnetic oscillators explains the electron/proton mass rate and the Weak and Strong Interactions by the diffraction patterns. The Weak Interaction changes the diffraction patterns by moving the electric charge from one side to the other side of the diffraction pattern, which violates the CP and Time reversal symmetry. The diffraction patterns and the locality of the self-maintaining electromagnetic potential explains also the Quantum Entanglement, giving it as a natural part of the Relativistic Quantum Theory and making possible to build the Quantum Computer with the help of Quantum Information.
Atom–nanowire coupling system is a promising platform for optical quantum information processing. Unlike the previous designing of optical switch and transistor requiring a dedicated multi-level emitter and high fineness microcavity, a new proposal is put forward which contains a single two-level atom asymmetrically coupled with two nanowires. Single-emitter manipulation of photonic signals for bilateral coherent incident is clear now, since we specify atomic saturation nonlinearity into three contributions which brings us a new approach to realizing light-controlled-light at weak light and single-atom levels. An efficient optically controllable switch based on self-matching-induced-block and a concise optical transistor are proposed. Our findings show potential applications in full-optical devices.
Single photons are the smallest excitation states of the elctromagnetic field. They can be experimentally created but there is no apparatus that performs this task with 100% efficiency. Many techniques and theorems of optical quantum information theory use single photons. In order to utilize them for information processing, it is reasonable to know if they had been successfully produced. In other words: One needs heralded single photons. How to achieve this, is the main result of this diploma thesis. The task has been to improve the efficiency of already existing single photon sources. An apparatus had to be found which postprocesses the output of such sources in order to either herald the photons or at least to improve the efficiency. This apparatus is explicitly allowed to use other optical sources, for example coherent states (laser). The situation has been formally represented by the assumption that single photon sources produce a mixture of vacuum and the single photon state. It is known that experimental set-ups also create higher photon contributions. These contributions are considered to be small in corresponding publications. For some cases they even seem to vanish at all. One can distinguish two regimes concerning the techniques which have been used in this work: linear and nonlinear optics. Linear optical tools, for example the beam splitter, do not absorb photons in the ideal case, but it is not easy to implement useful transformations by their means. Absorption and weak coupling are drawbacks which arise mainly in nonlinear optical media. This work shows that the interactions two-photon absorption and cross phase modulation can be used in schemes, which enable the heralding of single photons. Already there are some linear optical quantum gates which manipulate optical states by means of single photons. Photons do not interact with each other directly, in contrast to electrons for example. Therefore quantum gates which act on more than one mode are interesting. The effective interaction of these gates is not quitecomparable to unitary dynamics. Moreover, these gates do not function in each case. Measurements required in auxiliary modes yield different results for each single case and in order to obtain the desired effect the cases have to be selected according to the measurement results. It has been investigated, how such gates perform if they have statistical mixtures as a resource instead of perfect single-photon states. Yet, no improvement of the single-photon contribution has been accomplished by means of these apparatuses. Hereby it has not been disproved that it is possible to achieve this goal using other apparatuses and techniques of linear optics. Yet, it has been shown that an enhancement is possible but then a non-vanishing two-photon contribution is appended. One of the most basic quantum gates is the conditional sign flip. The optical 8 interaction cross phase modulation acts in quite the same way considering only single photons. This effect enables the heralding of single photons in the ideal case. Within this work it has been shown that a Mach-Zehnder interferometer, a cross phase modulating medium and a photo-detector can be employed for this purpose. One could use any source of light as input to the interferometer but a strong coherent state (laser) may be preferred for the purpose of enhancing the interaction. The absorption of two-photon states is another non-linear effect. Astonishingly this effect serves to generate a heralded single-photon state given two imperfect single-photon sources. First the photon adding tool is employed which consists of a beam splitter and a photo-detector. By means of that and the statistical mixtures of two single-photon sources one creates a mixture which contains vacuum, the single-photon state and additionally the two-photon Fock state. This mixture propagates through a Mach-Zehnder interferometer. The interferometer is adjusted such that it is transparent for incoming light. In addition to the interferometer alone a two-photon absorbing medium is placed in one of its arms. For this reason there is a chance that the two-photon contribution entering the interferometer is absorbed within this medium. We cannot exclude this unfortunate case. A detector is placed in one of the two output modes. The interferometer is adjusted such that the single photon state does not leave in the mode in which the detector is placed, but always in the other mode. The detector can only click if the two-photon state propagates through the interferometer and if it is not destroyed in the absorbing medium. If the detector then receives one photon it is obvious that another one must leave in the other mode. The whole phenomenon is very similar to the concept of null measurement. Although not a main result, it has been shown here that the so-called Hong-Ou-Mandel effect can be generalized. It has been proven that the beam splitter transforms Fock states with equal Fock number into superpositions of Fock numbers which are even in each mode. Another effect on special odd Fock states is conjectured.