High-visibility Interference Research Articles

Entanglement-induced collective many-body interference.

Entanglement and interference are both hallmark effects of quantum physics. Particularly rich dynamics arise when multiple (at least partially) indistinguishable particles are subjected to either of these phenomena. By combining both entanglement and many-particle interference, we propose an interferometric setting through which N-particle interference can be observed, while any interference of lower orders is strictly suppressed. We experimentally demonstrate this effect in a four-photon interferometer, where the interference is nonlocal, in principle, as only pairs of photons interfere at two separate and independent beam splitters. A joint detection of all four photons identifies a high-visibility interference pattern varying as a function of their collective four-particle phase, a genuine four-body property.

Sapphire Fiber Fabry-Perot Sensors With High Fringe Visibility

It is difficult for sapphire fiber Fabry-Perot (FP) sensors to generate high quality interference fringes because their highly multimode property. In this paper, we report a sapphire fiber FP strain sensing system with high fringe visibility, realized by the use of a self-made fiber collimator. The collimator is made by a quarter-period graded index optical fiber (GIOF) fused to a single mode fiber (SMF). In the contrast experiment, no effective signal was collected from a sensing system without the fiber collimator. While adding a fiber collimator into the sensing system, high visibility interference fringes were collected. The fringe visibility is 49.37% when the FP cavity length is 341.1 μm, and 20.09% when the cavity length is 1343.3 μm. The effective range of the cavity is over 1000 μm. Strain experiment at room temperature shows the measuring range is 10037 μϵ with relative error of 0.37% while the gauge length is 100 mm. High temperature experiment was also carried out and the result shows that the sensor can work at 1300 °C. This method is expected to be helpful for decreasing the difficulty of fabricating high fringe visibility sapphire fiber FP sensors, and increasing the measuring range.

Signal Pick-Up Technique for Sapphire Fiber External Fabry-Perot Interferometer Using Tapered Multimode Fibers

A tapered multimode fiber (MMF) to sufficiently and stably pick up the interference signal of a sapphire fiber external Fabry-Perot interferometer (EFPI) is proposed and investigated. The tapered MMF was fabricated following the adiabaticity criteria for the local tapering angle such that the excited fundamental mode in MMF barely couples to the higher-order modes, ensuring a high-visibility interference output from the sapphire fiber EFPI. Tapered MMF also guarantees a robust fusion splice between the leading single-mode fiber and the sapphire fiber. Experimental results show that using tapered MMF the excitation coefficient of fundamental mode can be improved by more than 3.5 times comparing with conventional technique using non-tapered MMF, and the fringe contrast of the interference spectrum can increase by 2.3 times. The response of the collected interference spectra from the sapphire fiber EFPI using tapered MMF was tested when ambient pressure and temperature was changed from 0.1 MPa to 6.1 MPa and 17 °C to 302°C respectively. The results validate that tapered MMF can provide a high stability against environmental pressure and temperature variations.

A high-performance ranging method with a long distance range and high accuracy

Distance measurement technology can greatly aid scientific research and engineering applications. The optical frequency-domain interferometry ranging method (OFDI-RM) has attracted a great deal of attention as it can achieve accuracy down to the micron and nanometer resolution. However, the main drawback of OFDI-RM is its short measuring range, which hinders its widespread use. In this paper, we present a high-performance OFDI-RM with a long distance range, high accuracy and high resolution. A high return-loss focus ranging probe is used to match the light intensity of reference and signal beams for acquiring high visibility interference fringes in the frequency domain at a distance of more than 100 mm. A novel signal processing method, which takes advantage of high-degree polynomial fitting and Hilbert transform (HT) to perfectly extract the harmonic component of the frequency-domain interference fringes, has also been demonstrated. The performance of OFDI-RM is verified by a linear motion measurement experiment. The measured range of OFDI-RM increased up to 170 mm, with an accuracy and resolution of up to 0.0040 mm–0.0005 mm/step, respectively.

Machine Learning-Assisted Measurement Device-Independent Quantum Key Distribution on Reference Frame Calibration.

In most of the realistic measurement device-independent quantum key distribution (MDI-QKD) systems, efficient, real-time feedback controls are required to maintain system stability when facing disturbance from either external environment or imperfect internal components. Traditionally, people either use a “scanning-and-transmitting” program or insert an extra device to make a phase reference frame calibration for a stable high-visibility interference, resulting in higher system complexity and lower transmission efficiency. In this work, we build a machine learning-assisted MDI-QKD system, where a machine learning model—the long short-term memory (LSTM) network—is for the first time to apply onto the MDI-QKD system for reference frame calibrations. In this machine learning-assisted MDI-QKD system, one can predict out the phase drift between the two users in advance, and actively perform real-time phase compensations, dramatically increasing the key transmission efficiency. Furthermore, we carry out corresponding experimental demonstration over 100 km and 250 km commercial standard single-mode fibers, verifying the effectiveness of the approach.

Aharonov-Bohm effect in graphene-based Fabry-Pérot quantum Hall interferometers.

Interferometers probe the wave-nature and exchange statistics of indistinguishable particles-for example, electrons in the chiral one-dimensional edge channels of the quantum Hall effect (QHE). Quantum point contacts can split and recombine these channels, enabling interference of charged particles. Such quantum Hall interferometers (QHIs) can unveil the exchange statistics of anyonic quasi-particles in the fractional quantum Hall effect (FQHE). Here, we present a fabrication technique for QHIs in van der Waals (vdW) materials and realize a tunable, graphene-based Fabry-Pérot (FP) QHI. The graphite-encapsulated architecture allows observation of FQHE at a magnetic field of 3T and precise partitioning of integer and fractional edge modes. We measure pure Aharonov-Bohm interference in the integer QHE, a major technical challenge in small FP interferometers, and find that edge modes exhibit high-visibility interference due to large velocities. Our results establish vdW heterostructures as a versatile alternative to GaAs-based interferometers for future experiments targeting anyonic quasi-particles.

Robust entanglement distribution via telecom fibre assisted by an asynchronous counter-propagating laser light

Distributing entangled photon pairs over noisy channels is an important task for various quantum information protocols. Encoding an entangled state in a decoherence-free subspace (DFS) formed by multiple photons is a promising way to circumvent the phase fluctuations and polarisation rotations in optical fibres. Recently, it has been shown that the use of a counter-propagating coherent light as an ancillary photon enables us to faithfully distribute entangled photon with success probability proportional to the transmittance of the optical fibres. Several proof-of-principle experiments have been demonstrated, in which entangled photon pairs from a sender side and the ancillary photon from a receiver side originate from the same laser source. In addition, bulk optics have been used to mimic the noises in optical fibres. Here, we demonstrate a DFS-based entanglement distribution over 1 km optical fibre using DFS formed by using fully independent light sources at the telecom band, and obtain a high-fidelity entangled state. This shows that the DFS-based scheme protects the entanglement against collective noise in 1 km optical fibre. In the experiment, we utilise an interference between asynchronous photons from continuous wave pumped spontaneous parametric down conversion (SPDC) and mode-locked coherent light pulse. After performing spectral and temporal filtering, the SPDC photons and light pulse are spectrally indistinguishable. This property allows us to observe high-visibility interference without performing active synchronisation between fully independent sources.

Design of polarization-insensitive high-visibility silicon-on-insulator quantum interferometer

We based on integrated silicon-on-insulator platforms design the key components of an on-chip interferometer, beam splitter and directional coupler included, valid in high-visibility interference at telecommunication wavelengths. Special attention is given to the equal-proportion beam splitting and directional coupling, which is achieved by carefully designing the geometric dimension of multi-mode interferometer structure. The proposed interferometer facilitates low loss, broad operating bandwidth, anticipated large tolerance on size variation induced in fabrication procedures, based on a particular wafer with silicon layer thickness of 320 nm. The most highlight property of polarization-insensitive, enables the path-selective qubits generation for bi-polarization that further makes possible quantum key distribution using high dimensional protocols. We numerically demonstrate interference at 1550 nm with visibilities of 99.50% and 93.99% for transverse-electric and transverse-magnetic polarization, respectively, revealing that the proposed interferometer structure is well capable of on-chip optical control especially in quantum optics regime.

Bounding the outcome of a two-photon interference measurement using weak coherent states.

The interference of two photons at a beam splitter is at the core of many quantum photonic technologies, such as quantum key distribution or linear-optics quantum computing. Observing high-visibility interference is challenging because of the difficulty of realizing indistinguishable single-photon sources. Here, we perform a two-photon interference experiment using phase-randomized weak coherent states with different mean photon numbers. We place a tight upper bound on the expected coincidences for the case when the incident wavepackets contain single photons, allowing us to observe the Hong-Ou-Mandel effect. We find that the interference visibility is at least as large as 0.995-0.013+0.005.

Characterization of coherent quantum frequency combs using electro-optic phase modulation

We demonstrate a two-photon interference experiment for phase coherent biphoton frequency combs (BFCs), created through spectral amplitude filtering of biphotons with a continuous broadband spectrum. By using an electro-optic phase modulator, we project the BFC lines into sidebands that overlap in frequency. The resulting high-visibility interference patterns provide an approach to verify frequency-bin entanglement even with slow single-photon detectors; we show interference patterns with visibilities that surpass the classical threshold for qubit and qutrit states. Additionally, we show that with entangled qutrits, two-photon interference occurs even with projections onto different final frequency states. Finally, we show the versatility of this scheme for weak-light measurements by performing a series of two-dimensional experiments at different signal-idler frequency offsets to measure the dispersion of a single-mode fiber.

Distribution of high-dimensional entanglement via an intra-city free-space link

Quantum entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Increasing the dimensionality of entanglement has been shown to improve robustness and channel capacities in secure quantum communications. Here we report on the distribution of genuine high-dimensional entanglement via a 1.2-km-long free-space link across Vienna. We exploit hyperentanglement, that is, simultaneous entanglement in polarization and energy-time bases, to encode quantum information, and observe high-visibility interference for successive correlation measurements in each degree of freedom. These visibilities impose lower bounds on entanglement in each subspace individually and certify four-dimensional entanglement for the hyperentangled system. The high-fidelity transmission of high-dimensional entanglement under real-world atmospheric link conditions represents an important step towards long-distance quantum communications with more complex quantum systems and the implementation of advanced quantum experiments with satellite links.

High visibility Hong-Ou-Mandel interference via a time-resolved coincidence measurement.

A high visibility Hong-Ou-Mandel (HOM) interference between two independently prepared photons plays an important role in various photonic quantum information processing. In a standard HOM experiment using photons generated by pulse-pumped spontaneous parametric down conversion (SPDC), larger detection time windows than the coherence time of photons have been employed for measuring the HOM visibility and/or drawing the HOM dip. If large amounts of stray photons continuously exist within the detection time windows, employing small detection time windows is favorable for reducing the effect of background noises. Especially, such a setup is helpful for the HOM experiment using continuous wave (cw)-pumped SPDC and the time-resolved coincidence measurement. Here we argue that the method for determining the HOM visibility used in the previous cw experiments tends to suffer from distortion arising from biased contribution of the background noises. We then present a new method with unbiased treatment of the cw backgrounds. By using this method, we experimentally demonstrate a high visibility HOM interference of two heralded telecom photons independently generated by SPDC with employing cw pump light. An observed HOM visibility is 0.87 ± 0.04, which is as high as those observed by using pulse-pumped SPDC photons.

Mutual injection coupling and phase locking of two multiwavelength fiber lasers

Passive phase locking of two multiwavelength fiber lasers has been demonstrated by mutual injection coupling and spatial filtering. Since the fiber loop mirrors are employed as two component lasers’ rear reflectors, multiple wavelengths operate simultaneously owing to their broadband reflection and the compound cavity’s mode selection effect. A single-mode filtering fiber is utilized to collect the expected spatial mode’s energy and feed them back into the coupled laser cavity, and stable and high visibility interference fringes have been obtained by the self-phasing process. A contrastive experiment has been made to investigate its coherent output properties when the rear mirrors are replaced by fiber Bragg gratings. Compared with the ordinary coherent combining of fiber lasers lasing at a single-wavelength, higher output power and efficiency have been obtained, and the cost is that the output beams’ coherence and stability decrease slightly.

All-fiber multiplexed source of high-purity single photons

Single-photon sources based on spontaneous photon-pair generation have enabled pioneering experiments in quantum physics. However, next-generation photonic quantum technologies require higher generation probabilities of photons in well-controlled pure states capable of high-visibility interference. We have harnessed bespoke fiber technology to develop virtually alignment-free sources that deliver high-purity heralded single photons in telecoms single-mode fiber. The resulting access to low-loss optical delay enabled us to actively route the heralded output from two almost identical sources to enhance the delivery probability of single photons relative to one individual source. Our results indicate how the scale of photonic quantum technologies might be increased via guided-wave multiplexing of high-purity photons.

Coherent and dynamic beam splitting based on light storage in cold atoms.

We demonstrate a coherent and dynamic beam splitter based on light storage in cold atoms. An input weak laser pulse is first stored in a cold atom ensemble via electromagnetically-induced transparency (EIT). A set of counter-propagating control fields, applied at a later time, retrieves the stored pulse into two output spatial modes. The high visibility interference between the two output pulses clearly demonstrates that the beam splitting process is coherent. Furthermore, by manipulating the control lasers, it is possible to dynamically control the storage time, the power splitting ratio, the relative phase, and the optical frequencies of the output pulses. With further improvements, the active beam splitter demonstrated in this work might have applications in photonic photonic quantum information and in all-optical information processing.

Quantum key distribution without detector vulnerabilities using optically seeded lasers

Security in quantum cryptography is continuously challenged by inventive attacks targeting the real components of a cryptographic setup, and duly restored by new counter-measures to foil them. Due to their high sensitivity and complex design, detectors are the most frequently attacked components. Recently it was shown that two-photon interference from independent light sources can be exploited to avoid the use of detectors at the two ends of the communication channel. This new form of detection-safe quantum cryptography, called Measurement-Device-Independent Quantum Key Distribution (MDI-QKD), has been experimentally demonstrated, but with modest delivered key rates. Here we introduce a novel pulsed laser seeding technique to obtain high-visibility interference from gain-switched lasers and thereby perform quantum cryptography without detector vulnerabilities with unprecedented bit rates, in excess of 1 Mb/s. This represents a 2 to 6 orders of magnitude improvement over existing implementations and for the first time promotes the new scheme as a practical resource for quantum secure communications.

Fast and highly resolved capture of the joint spectral density of photon pairs

Controlling the spatial and spectral–temporal properties of photon pairs produced in artificially structured materials is fundamental to the realization of numerous photonic quantum information applications. Tailoring the joint spectral properties of photon pairs is of particular importance for applications relying on time–energy entanglement, high-visibility interference, and heralding. Yet measuring the joint spectral properties is a time-consuming task requiring coincidence counting, typically resulting in low-resolution spectra with a poor signal-to-noise ratio. In this work we capture the joint spectral correlations of photon pairs that would be produced in optical fibers with unprecedented speed, resolution, and signal-to-noise ratio, using a scheme based on stimulated four-wave mixing. We also illustrate that this technique can be used in engineering joint spectral correlations, making it a powerful tool for studying quantum states.

Time-domain spectroscopy in the mid-infrared.

When coupled to characteristic, fingerprint vibrational and rotational motions of molecules, an electromagnetic field with an appropriate frequency and waveform offers a highly sensitive, highly informative probe, enabling chemically specific studies on a broad class of systems in physics, chemistry, biology, geosciences, and medicine. The frequencies of these signature molecular modes, however, lie in a region where accurate spectroscopic measurements are extremely difficult because of the lack of efficient detectors and spectrometers. Here, we show that, with a combination of advanced ultrafast technologies and nonlinear-optical waveform characterization, time-domain techniques can be advantageously extended to the metrology of fundamental molecular motions in the mid-infrared. In our scheme, the spectral modulation of ultrashort mid-infrared pulses, induced by rovibrational motions of molecules, gives rise to interfering coherent dark waveforms in the time domain. These high-visibility interference patterns can be read out by cross-correlation frequency-resolved gating of the field in the visible generated through ultrabroadband four-wave mixing in a gas phase.

Observation of quasiperiodic dynamics in a one-dimensional quantum walk of single photons in space

We realize the quasi-periodic dynamics of a quantum walker over 2.5 quasi-periods by realizing the walker as a single photon passing through a quantum-walk optical-interferometer network. We introduce fully controllable polarization-independent phase shifters in each optical path to realize arbitrary site-dependent phase shifts, and employ large clear-aperture beam displacers, while maintaining high-visibility interference, to enable 10 quantum-walk steps to be reached. By varying the half-wave-plate setting, we control the quantum-coin bias thereby observing a transition from quasi-periodic dynamics to ballistic diffusion.

Off-axis digital holographic camera for quantitative phase microscopy.

We propose and experimentally demonstrate a digital holographic camera which can be attached to the camera port of a conventional microscope for obtaining digital holograms in a self-reference configuration, under short coherence illumination and in a single shot. A thick holographic grating filters the beam containing the sample information in two dimensions through diffraction. The filtered beam creates the reference arm of the interferometer. The spatial filtering method, based on the high angular selectivity of the thick grating, reduces the alignment sensitivity to angular displacements compared with pinhole based Fourier filtering. The addition of a thin holographic grating alters the coherence plane tilt introduced by the thick grating so as to create high-visibility interference over the entire field of view. The acquired full-field off-axis holograms are processed to retrieve the amplitude and phase information of the sample. The system produces phase images of cheek cells qualitatively similar to phase images extracted with a standard commercial DHM.