Mach–Zehnder interferometers incorporating electrochromic molecules for controlled single-photon detection

Numerical simulation of all-optical logic functions at micrometer scale by using plasmonic Metal-Insulator-Metal (MIM) waveguides

A compact realization of Feynman Reversible and NOR logic gate using Plasmonic waveguide based MZI for all-optical signal processing

Strong interests on optical quantum based metrology, quantum information and particularly in quantum cryptography are continuously growing. The main limitations to the developments in these fields are due to non-ideal devices: both single photon sources and single photon detectors. In these field of applications, detectors require to be able to resolve the number of photons in a light pulse. Presently state of the art indicates that classical semiconductor light detectors (i.e. avalanche photodiode or single photon avalanche diode) are not able to discriminate the number of photon arriving at the same time. In the meanwhile, superconducting devices have shown the possibility to resolve single photon pulses. One of the most promising superconducting detectors is the Transition-Edge Sensor (TES): a microcalorimeter that takes advantage of the sharp transition (few millikelvin) from the superconducting to the normal phase; for this reason it is sometimes called Superconductive Phase Thermometer (SPT). In the ultraviolet (UV) to infrared (IR) wavelength range, the photons are absorbed directly by the superconductive thin film and the absorbed energy induces an increase of the TES resistance. Thanks to the applied bias voltage, which maintains the device in the transition region the photon absorption induces a decrease of the TES current, measured by a dc-SQUID amplifier, and the pulse integral of the bias power reduction corresponds to the absorbed energy. This means that TESs have the very interesting properties to be able to detect single photons with an intrinsic energy resolution, without filters or gratings, that limit the quantum efficiency. By contrary of classical detectors, if monochromatic light irradiates TESs, as usually happens in communication systems, they show the photon-number resolving (PNR) capability and due to the good signal to noise ratio TESs are almost free from dark counts. Moreover, in the superconducting detector family, TESs are the only true photon-number resolving detectors operating in the VIS-NIR range. Together with quantum information science, the PNR property results useful even for optical radiometry too. In the optical community, the candela - the International System (SI) unit for the luminous intensity - has not a common consensus whether its present definition fully satisfies the current and future needs of growing associated technology. Furthermore, actually there are substantial efforts directed toward a new definitions of four base SI units: the proposal wants to link the SI units to fundamental constants, leaving f.i. material artifact. Considering the recent advances in optical radiometry and in quantum technologies, for the candela world it means redefine its unit linking to the Planck constant and consequently expressing the luminous intensity unit in terms of photon number rather than optical power. This challenge has been accepted by several national metrology institutes to demonstrate the feasibility of redefining the candela. Inside this research project called `qu-candela', the TES PNR capability has been considered to build the bridge between the quantum and classical world of radiometry: i.e. the detector possibility to measure optical powers from one single photon per second to the lower limits of cryogenic radiometry, 104 photons per second. The theme of this work of thesis is to investigate both optical and electrical characterization of different kind of TESs based on a titanium/gold multilayer film, produced and developed at the National Institute of Metrological Research (INRIM) of Torino. Thanks to the proximity effect, the multilayer allows to lower with continuity the critical temperature from that of the Ti bulk (Tc ~ 390 mK) to those of interest: ~ 300 mK and ~ 100 mK. Detectors with higher Tc have shown a faster response pulse with a relaxing time constant of the order of 200 ns, while for the lower Tc sensors, the time constant is about 10 µs. By contrary to the response time, the detector intrinsic energy resolution is proportional to its film critical temperature. Our sensors work to discriminate incident photon from UV wavelengths to those typical of the telecommunications, 1310 nm and 1550 nm. Irradiating a TES with an active area of 10x10 µm^2 by incident photons of 0.79 eV (corresponds to a wavelength 1570 nm), the best energy resolution obtained has been 0.18 eV. Detectors with higher active area 20x20 µm^2 have a worse energy resolution, because it is also proportional to the material film heat capacity. In the meanwhile due to the same reason these kind of sensors present a bigger saturation energy. This has allowed to investigate on the TES capability to discriminate up to 29 incident photons simultaneously. Until now, such count represents the bigger amount of photons discriminated by single photon detectors, without reaching the device saturation, with a linear behaviour. From this count it has been estimated 12 photons on average, per pulse, at 9 kHz repetition rate; this results in a photon flux of about 105 photons/s, demonstrating the possibility of having a detector able to work from low flux regime to 1 photon/s to flux measurable by conventional semiconductor device (f.i. single photon avalanche detector SPAD). An innovative absolute calibration technique for PNR detector has been demonstrated. The absolute technique is based on the Klyshko's efficient solution to measure detection efficiency in photon counting rate and well know for common click-no-click detector. In fact, exploiting the recent developments in quantum state world, it is possible to work with quasi single photon state, by using a parametric down conversion heralded single photon source, and calibrate PNR detectors without requiring reference standards. The best detection efficiency, of ca. 50%, has been reached by coupling the smaller active area detectors with a 9 µm core optical fiber, single mode at telecom wavelengths

Microfiber Mach-Zehnder interferometer embedded in low index polymer

On-demand creation of entanglement between distant qubits is a necessary ingredient for distributed quantum computation. We propose an entanglement scheme that allows for single-shot deterministic entanglement creation by detecting a single photon passing through a Mach–Zehnder interferometer with one transmon qubit in each arm. The entanglement production essentially relies on the fact that superconducting microwave structures allow one to achieve strong coupling between the qubit and the photon. By detecting the photon via a photon counter, a parity measurement is implemented and the wave function of the two qubits is projected onto a maximally entangled state. Most importantly, the entanglement generation is heralded such that our protocol is not susceptible to photon loss due to the indivisible nature of single photons.

Single-photon sources based on quantum dots have been shown to exhibit almost ideal properties such as high brightness and purity in terms of clear anti-bunching as well as high two-photon interference visibilities of the emitted photons, making them promising candidates for different quantum information applications such as quantum computing, quantum communication and quantum teleportation. However, as most single-photon sources also quantum dots typically emit light at wavelengths of electronic transitions within the visible or the near infrared range. In order to establish quantum networks with remote building blocks, low-loss single photons at telecom wavelengths are preferable, though. Despite recent progress on emitters of telecom-photons, the most efficient single-photon sources still work at shorter wavelengths. On that matter, quantum frequency down-conversion, being a nonlinear optical process, has been used in recent years to alter the wavelength of single photons to the telecom wavelength range while conserving their nonclassical properties. Characteristics such as lifetime, first-order coherence, anti-bunching and entanglement have been shown to be conserved or even improved due to background suppression during the conversion process, while the conservation of indistinguishability was yet to be shown. Here we present our experimental results on quantum frequency down-conversion of single photons emitted by an InAs/GaAs quantum dot at 903.6 nm following a pulsed excitation of a p-shell exciton at 884 nm. The emitted fluorescence photons are mixed with a strong pump-field at 2155 nm inside a periodically poled lithium niobate ridge waveguide and converted to 1557 nm. Common issues of a large background due to Raman-scattered pump-light photons spectrally overlapping with the converted single photons could largely be avoided, as the pump-wavelength was chosen to be fairly longer than the target wavelength. Additional narrowband spectral filtering at the telecom regime as a result of the small conversion bandwidth and using a high-performance fiber-Bragg-grating solely left the detector dark counts as the only noise source in our setup. Therefore, we could achieve conversion efficiencies of more than 20 %. In order to test the indistinguishability, sequentially emitted photons were fed into a Mach-Zehnder interferometer and spatially as well as temporally overlapped at the output beam splitter. Cross-correlation measurements between both output-ports of the beam splitter exhibit two-photon interference contrasts of more than 40 % prior to and after the down-conversion step. Accordingly, we demonstrate that the process of quantum frequency conversion preserves photon indistinguishability and can be used to establish a versatile source of indistinguishable single photons at the telecom C-Band. Furthermore our scheme allows for converting photons in a wavelength band from 900 nm to 910 nm to the same telecom target wavelength. This enables us to test indistinguishability of frequency-converted photons, originally stemming from different sources with dinstinguishable wavelengths.

Beam propagation method (BPM) was used to study 2x2 Mach-Zehnder interferometer switch with electro-optical effects in titanium diffused lithium niobate based directional coupler was used to develop the design. This design is capable of de-multiplexing the wavelength1300nm. This project intends to design high performance NxN electro-optic switch. This optical device is widely used in optical network, especially in the optical link of fiber-to-the-home (FTTH). The design is carried out using BPM_CAD, which is a very powerful and user-friendly optics waveguides modeling method as it core element. Research on optical waveguide switching using directional coupler (DC) and Mach-Zehnder interferometer (MZI) has been going on and already created great interest among the researchers. There are different types of material being used in much different way apart from the most common electro-optic materials such as lithium niobate. Recently, the study was also confined to the use of silica on silicon technology considering that the cost of the technology. Other non-linear-optic materials such as polymers have been embedded into part of the silica waveguide.

Integrated photonic circuits are key components for photonic quantum technologies and for the implementation of chip-based quantum devices. Future applications demand flexible architectures to overcome common limitations of many current devices, for instance the lack of tuneabilty or built-in quantum light sources. Here, we report on a dynamically reconfigurable integrated photonic circuit comprising integrated quantum dots (QDs), a Mach-Zehnder interferometer (MZI) and surface acoustic wave (SAW) transducers directly fabricated on a monolithic semiconductor platform. We demonstrate on-chip single photon generation by the QD and its sub-nanosecond dynamic on-chip control. Two independently applied SAWs piezo-optomechanically rotate the single photon in the MZI or spectrally modulate the QD emission wavelength. In the MZI, SAWs imprint a time-dependent optical phase and modulate the qubit rotation to the output superposition state. This enables dynamic single photon routing with frequencies exceeding one gigahertz. Finally, the combination of the dynamic single photon control and spectral tuning of the QD realizes wavelength multiplexing of the input photon state and demultiplexing it at the output. Our approach is scalable to multi-component integrated quantum photonic circuits and is compatible with hybrid photonic architectures and other key components for instance photonic resonators or on-chip detectors.

High-efficiency single photon detection is an interesting problem for many areas of physics, including low temperature measurement, quantum information science and particle physics. For optical photons, there are many examples of devices capable of detecting single photons with high efficiency. However reliable single photon detection of microwaves is very difficult, principally due to their low energy. In this paper, we present the theory of a cascade amplifier operating in the microwave regime that has an optimal quantum efficiency of 93%. The device uses a microwave photon to trigger the stimulated emission of a sequence of atoms where the energy transition is readily detectable. A detailed description of the detector's operation and some discussion of the potential limitations of the detector are presented.

Book Review of Quantum Processes, Systems and Information by Benjamin Schumacher & Michael Westmoreland This is a textbook aimed at advanced undergraduate students that brings together more traditional quantum mechanics topics and quantum information theory. The book is novel both in this focus and its presentation of the material. The first five chapters discuss the basics of quantum theory using three isomorphic two-level systems: a photon in a Mach–Zehnder interferometer, a spin 1⁄2 particle and a two-level atom. These chapters also develop basic quantum information concepts such as the entropy of a message, interpreting unitary time evolution in terms of information capacity and the difference between distinct and distinguishable states in terms of the basic decoding and distinguishability theorems. The text then continues (chapters 6–9) with two-particle states, including entanglement, hidden variables, the no-cloning theorem, density operators and open systems. The transition to continuous systems, the starting point for many quantum mechanics textbooks, is made only in chapter 10, and the following chapters include standard wave mechanics found in many texts. The final three chapters revert the focus back to quantum information processing.

We describe the simplest single–photon encoding schemes and introduce the concept of a quantum optical gate. These gates can be used to build up arbitrary entangled states of many initially separate single photons. Experimental realization of such a gate involves the development of nonlinear phase shift elements sensitice at the single quantum level. We report our experimental efforts using linear optical elements and non–classical sources where we are able to demonstrate interference and entanglement of initially separate single–photon pulses.

Summary form only given. The manipulation of the phase of classical and quantum light is a major key to quantum photonics [1]. Integrated optics has several advantages over bulk optical systems, most notably in robustness, handling and size. Writing three-dimensional waveguides circuits into transparent materials using an ultrafast laser is thereby a new emerging technology [2; 3] with possible applications in quantum communication [4], and quantum computing [5]. In our work, we experimentally demonstrate the high-precision manipulation of the optical phase of classical and quantum light, using a generalized directional coupler.The behavior of light in directional coupler consisting of two waveguides can be described by the Hamiltonian H=T+V1+V2, where T is the kinetic energy and Vi is the potential energy of waveguide i = 1 and i = 2. The wavefunction of the system is constructed as a superposition of the single waveguide modes Ψ=a|1+b|2, and the Schrödinger equation of the coupler reads as HΨ=ksmΨ, where ksm denotes the propagation constant of the supermode. The propagation constants of the supermodes can be calculated straightforwardly as ksm1,2=k0±κ(1∓ σ)/(1-σ2) , where k0 is the propagation constant of the individual waveguides, κ=2|V2|1=1|V1|2 represents the coupling constant, and σ=1|=2|1is the direct overlap of the individual waveguide modes. For σ → 0 the well known distribution of ksm1,2 is preserved. As ksm = (ksm1+ksm2)/2>k0, in a generalized directional coupler a phase delay Δφ is introduced that readswith lC = π/κ as the coupling length. The phase delay depends only on σ, which itself is a function of the spacing between the waveguides. As the spacing can be controlled in a very accurate manner, this approach allows the highly precise manipulation of the optical phase along a directional coupler. In order to measure this effect, we set up a tuned Mach-Zehnder (MZ) interferometer with identical lengths of the interferometer arms. A third waveguide was applied to one of the arms, forming an additional directional coupler in which the light completely couples forth and back with no intensity losses in the relevant interferometer arm (Fig. 1(a)). However, as the change in the propagation constant in this arm results in a phase delay, at the output of the MZ the ratio of intensities (for classical light) and the count rates (for single photons) changes. To obtain an all-reflecting coupler (ARC) for different coupling constants, the respective length of a coupler has to be adjusted and the associated phase delays are shown in Fig. 1(b). Due to decreasing σ for larger distances between the waveguides, the accumulated phase difference shrinks, even if the length of the coupler increases. Normalizing the phase difference by length allows the direct calculation of the change of the propagation constant caused by the ARC (Fig. 1(c)).In conclusion, we demonstrated a highly precise method for the manipulation of the optical phase in integrated waveguide structures by employing generalized directional coupling. The propagation constants of the supermodes in the structures are controlled by the spacing between the waveguides in the ARC. Our approach holds for classical as well as quantum light, and has the potential to find its use in a multitude of integrated-optical devices.

Photon subtraction (PS) is an important operation for optic quantum information processing. Conventional PS is implemented using a single linear beam splitter (BS) and photon detector. However, in this study, we show that the PS effect can be enhanced using two beam splitters and an optional phase modulator. This can be considered PS with an extended version of the well-known Mach–Zehnder (MZ) interferometer. By tuning the transmittance of the two beam splitters and phase modulator, the probability of success can be considerably improved over that of the original PS scheme with a single BS and photon detector. Moreover, if applied to a single-photon input, our proposed scheme can even implement deterministic PS, which is almost impossible for the original scheme with a single BS and photon detector. Owing to the higher probability of success, applying the PSMZ method to the entanglement enhancement of a very weak two-mode squeezed vacuum state is straightforward. Our result is helpful for improving the yield of output entanglement.

Since early 1990s, Mach–Zehnder interferometer has been used to investigate the interference of biphoton wave packets. Due to subpicosecond time coherence of biphoton generated by spontaneous parametric downconversion process, some physical processes are ignored in the interferometer, most likely the biphoton time‐domain interference. Here, the two‐photon interference phenomenon based on the Mach–Zehnder interferometer is theoretically studied, where the correlated photon pairs are produced by the four‐wave mixing in atomic system. In particular, the quantum interference effect to effectively control the coherent time of two‐photon by adjusting the input delay is used. In the damped Rabi oscillation regime, two‐photon bunching and antibunching effects are observed. In addition, in the group‐delay regime, the interference between biphoton precursor, slow‐light wave packets and also in between the precursor and the slow‐light wave packets is observed, which had never been reported before. These results may have potential applications in the fields of biphoton shaping and quantum information processing.

We have been investigating advanced students' learning of quantum mechanics\nconcepts and have developed interactive tutorials which strive to help students\nlearn these concepts. Two such tutorials, focused on the Mach-Zehnder\ninterferometer (MZI) and the double-slit experiment (DSE), help students learn\nhow to use the concept of "which-path" information to reason about the presence\nor absence of interference in these two experiments in different situations.\nAfter working on a pretest that asked students to predict interference in the\nMZI with single photons and polarizers of various orientations placed in one or\nboth paths of the MZI, students worked on the MZI tutorial which, among other\nthings, guided them to reason in terms of which-path information in order to\npredict interference in similar situations. We investigated the extent to which\nstudents were able to use reasoning related to which-path information learned\nin the MZI tutorial to answer analogous questions on the DSE (before working on\nthe DSE tutorial). After students worked on the DSE pretest they worked on a\nDSE tutorial in which they learned to use the concept of which-path information\nto answer questions about interference in the DSE with single particles with\nmass sent through the two slits and a monochromatic lamp placed between the\nslits and the screen. We investigated if this additional exposure to the\nconcept of which-path information promoted improved learning and performance in\nthe DSE questions with single photons and polarizers placed after one or both\nslits. We find evidence that both tutorials promoted which-path information\nreasoning and helped students use this reasoning appropriately in contexts\ndifferent from the ones in which they had learned it.\n