Recent research has uncovered a remarkable ability to manipulate and control electromagnetic fields to produce effects such as perfect imaging and spatial cloaking [1,2]. To achieve spatial cloaking, the index of refraction is manipulated to flow light from a probe around an object in such a way that a "hole" in space is created, and it remains hidden [3][4][5][6][7][8][9][10][11][12][13][14]. Alternatively, it may be desirable to cloak the occurrence of an event over a finite time period, and the idea of temporal cloaking was proposed in which the dispersion of the material is manipulated in time to produce a "time hole" in the probe beam to hide the occurrence of the event from the observer [15]. This approach is based on accelerating and slowing down the front and rear parts, respectively, of the probe beam to create a well controlled temporal gap in which the event occurs so the probe beam is not modified in any way by the event. The probe beam is then restored to its original form by the reverse manipulation of the dispersion. Here we present an experimental demonstration of temporal cloaking by applying concepts from the time-space duality between diffraction and dispersive broadening [16]. We characterize the performance of our temporal cloak by detecting the spectral modification of a probe beam due to an optical interaction while the cloak is turned off and on and show that the event is observed when the cloak is turned off but becomes undetectable when the cloak is turned on. These results are a significant step toward the development of full spatio-temporal cloaking.
Interferometry using discrete energy levels in nuclear, atomic or molecular systems is the foundation for a wide range of physical phenomena and enables powerful techniques such as nuclear magnetic resonance, electron spin resonance, Ramsey-based spectroscopy and laser/maser technology. It also plays a unique role in quantum information processing as qubits are realized as energy superposition states of single quantum systems. Here, we demonstrate quantum interference of different energy states of single quanta of light in full analogy to energy levels of atoms or nuclear spins and implement a Ramsey interferometer with single photons. We experimentally generate energy superposition states of a single photon and manipulate them with unitary transformations to realize arbitrary projective measurements, which allows for the realization a high-visibility single-photon Ramsey interferometer. Our approach opens the path for frequency-encoded photonic qubits in quantum information processing and quantum communication. Main Text:The two-state model represents the most fundamental quantum system and can be applied to a wide variety of physical systems. Ramsey interferometry, magnetic resonance imaging, and electron-spin resonance spectroscopy are governed by similar 2-level system dynamics, which involves molecular-atomic levels, nuclear spin, and electronic spin, respectively. The coupling between energy levels is achieved using electromagnetic fields, which can be tailored at will and allows for many advanced techniques such as adiabatic elimination and stimulated Raman adiabatic passage in higher dimensional atomic system, or spin locking in NMR. Quantum interference involving systems in superposition of different energies is at the heart of fundamental and applied physics. For example, quantum coherence has been highly useful in increasing the accuracy of time measurement from the first idea of using NMR suggested by Rabi in 1945 (1) to the first atomic clock relying on the Ramsey interferometry (2,3), which has been recently extended by using trapped single ions (4). In addition, Ramsey interferometry on single Rydberg atoms has allowed the nondestructive measurement of the number of photon in a cavity (5) and single spin manipulation using the same techniques constitutes one of the most promising routes towards quantum processing (6-8). Matter-wave interferometers using collective energy levels of atoms in a BEC have also been demonstrated (9) and used to measure gravity down to record breaking precision (10). Nevertheless, a fundamental quantum system that has not been extensively studied in the context of discrete 2-level energy systems (i.e. frequency) is the single photon. Translating those studies to photonics system can be implemented by controlling light with light using nonlinear optics. For classical light the analogy between atomic/molecular optics and nonlinear optics is well known (11) and there are various cases where the complex dynamics of light propagation in a nonlinear medium can be si...
Parametric single-photon sources are well suited for large-scale quantum networks due to their potential for photonic integration. Active multiplexing of photons can overcome the intrinsically probabilistic nature of these sources, resulting in near-deterministic operation. However, previous implementations using spatial and temporal multiplexing scale unfavorably due to rapidly increasing switching losses. Here, we break this limitation via frequency multiplexing in which switching losses remain fixed irrespective of the number of multiplexed modes. We use low-noise optical frequency conversion for efficient frequency switching and demonstrate multiplexing of three modes. We achieve a generation rate of 4.6 × 104 photons per second with an ultra-low g(2)(0) = 0.07 indicating high single-photon purity. Our scalable, all-fiber multiplexing system has a total loss of just 1.3 dB, such that the 4.8 dB multiplexing enhancement markedly overcomes switching loss. Our approach offers a promising path to creating a deterministic photon source on an integrated chip-based platform.
We observe strong modal coupling between the TE 00 and TM 00 modes in Si 3 N 4 ring resonators revealed by avoided crossings of the corresponding resonances. Such couplings result in significant shifts of the resonance frequencies over a wide range around the crossing points. This leads to an effective dispersion that is one order of magnitude larger than the intrinsic dispersion and creates broad windows of anomalous dispersion. We also observe the changes to frequency comb spectra generated in Si 3 N 4 microresonators due polarization mode and higher-order mode crossings and suggest approaches to avoid these effects. Alternatively, such polarization mode-crossings can be used as a novel tool for dispersion engineering in microresonators.Optical microresonators are important for a wide range of applications, such as parametric frequency combs [1-10], optomechanics [11,12], and in quantum optics as sources for photon-pairs [13][14][15][16][17][18][19] or squeezed states [20][21]. The microresonator resonances can in principle be precisely calculated using the dispersion of the resonating modes and the resonator length. However, modal coupling between different types of modes can significantly alter the shape and position of their resonances. Mode splitting occurs for strong coupling [22], and coupling between whole families of modes results in avoided crossings [23][24][25][26][27]. This can lead to dramatic localized changes in the effective dispersion near these crossing points, which in general affects any parametric interaction that relies on precise frequency matching of different resonances. In particular it can play an important role in the formation of parametric frequency combs [24][25][26][27][28][29][30][31]. While mode-crossings can be disruptive for comb generation by inhibiting soliton formation [25] and distorting the comb spectrum [27], they can also be beneficial, allowing for comb formation in resonators with normal group-velocity dispersion (GVD) [8,24] or aiding the generation of dark solitons in normal GVD resonators [29]. In the context of frequency comb generation, only modal interactions between different families of spatial modes have been considered thus far. However, in dielectric waveguides, even when the waveguide is 'single mode', there are typically at least two guided fundamental modes, the fundamental quasi transverse electric (TE 00 ) and the fundamental quasi transverse magnetic (TM 00 ) mode, which correspond approximately to the polarization of light in the waveguide.Here, we report on the observation of avoided crossings that result from the strong modal coupling between the TE 00 and TM 00 polarization modes in Si 3 N 4 microring resonators. Similarly, strong polarization mode coupling has been shown to be useful for polarization conversion based on silicon oxinitride technology [31]. Since such a mode interaction can even occur in single-mode waveguides, it is more fundamental than other forms of modal interactions (i.e., between higher-order spatial modes). The physical o...
In order to achieve efficient parametric frequency comb generation in microresonators, external control of coupling between the cavity and the bus waveguide is necessary. However, for passive monolithically integrated structures, the coupling gap is fixed and cannot be externally controlled, making tuning the coupling inherently challenging. We design a dual-cavity coupled microresonator structure in which tuning one ring resonance frequency induces a change in the overall cavity coupling condition. We demonstrate wide extinction tunability with high efficiency by engineering the ring coupling conditions. Additionally, we note a distinct dispersion tunability resulting from coupling two cavities of slightly different path lengths, and present a new method of modal dispersion engineering. Our fabricated devices consist of two coupled high quality factor silicon nitride microresonators, where the extinction ratio of the resonances can be controlled using integrated microheaters. Using this extinction tunability, we optimize comb generation efficiency as well as provide tunability for avoiding higher-order mode-crossings, known for degrading comb generation. The device is able to provide a 110-fold improvement in the comb generation efficiency. Finally, we demonstrate open eye diagrams using low-noise phase-locked comb lines as a wavelength-division multiplexing channel.
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