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Quantum Communications and Quantum Imaging

Undetected photon interference measurements on a silicon chip

Chiara Michelini, Stefano Signorini, Valerio Pruneri, Lorenzo Pavesi

Published Aug 22, 2023• 2 citations
Publisher

Abstract

Interference measurements with undetected photons employ entangled photon pairs in order to overcome classical limitations in measurement sensitivity. With the undetected photon technique, one photon (signal) interacts with an object, and the signature of the interaction is stored in its entangled counterpart (idler): the interacting photon remains undetected, while the detection is performed on the photon that did not interact with the object. While the signal photon can be chosen in any spectral region suitable for the interaction, the idler photon is generated in a spectral region where detectors are efficient, scalable, and cheap. To date, the configurations proposed are in bulk setups. In this work, we propose a novel configuration on an integrated device, with the advantages of the reduced dimension, the lower cost and the robustness to alignment. In our experiment, we pumped a silicon-on-silica integrated circuit with a classical beam at a wavelength of 1.568 μm. Via intermodal spontaneous four-wave mixing, we generated highly non-degenerate time-energy entangled signal and idler photons at 1.99 μm and 1.29 μm, respectively. As we have integrated two identical sources of entangled photons in series, the photon pairs generated in the two sources are indistinguishable, and controlling the phase of the pump beam and the phase of the signal photons after the first nonlinear source, we observed interference patterns with 24% maximum visibility in the idler photon counts. We successfully measured the dephasing induced on the signal photons by measuring only the idler ones.

DOI
10.1117/12.2676372

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Interference measurements with undetected photons employ entangled photon pairs in order to overcome classical limitations in measurement sensitivity. With the undetected photon technique, one photon (signal) interacts with an object, and the signature of the interaction is stored in its entangled counterpart (idler): the interacting photon remains undetected, while the detection is performed on the photon that did not interact with the object. While the signal photon can be chosen in any spectral region suitable for the interaction, the idler photon is generated in a spectral region where detectors are efficient, scalable, and cheap. To date, the configurations proposed are in bulk setups. In this work, we propose a novel configuration on an integrated device, with the advantages of the reduced dimension, the lower cost and the robustness to alignment. In our experiment, we pumped a silicon-on-silica integrated circuit with a classical beam at a wavelength of 1.568 μm. Via intermodal spontaneous four-wave mixing, we generated highly non-degenerate time-energy entangled signal and idler photons at 1.99 μm and 1.29 μm, respectively. As we have integrated two identical sources of entangled photons in series, the photon pairs generated in the two sources are indistinguishable, and controlling the phase of the pump beam and the phase of the signal photons after the first nonlinear source, we observed interference patterns with 24% maximum visibility in the idler photon counts. We successfully measured the dephasing induced on the signal photons by measuring only the idler ones.
We experimentally demonstrate undetected photon phase sensing on a silicon photonic chip. We use a novel design, implementing an innovative integrated SU(1,1) interferometer based on transverse waveguide modes, that leverages intermodal spontaneous four-wave mixing to generate highly non-degenerate idler–signal photons. In our experiment, we retrieve the phase applied to the signal photons at wavelength 1972 nm by detecting only the correlated idler photons at 1291 nm, demonstrating the first on-chip phase sensing based on undetected photons. This result paves the way for novel generations of quantum refractometric sensors.
Quantum interference with undetected photons has gained significant attention in the past ten years due to its potential to decouple the probing and the detection of an active sensing system. The technique is based on correlated photon pairs: one photon interacts with the analyte, and the signature of the interaction is stored in its correlated counterpart. The probing photon remains undetected, while the other photon is measured. When the photons are highly non-degenerate, the spectral separation of the probing and detected photons offers significant advantages in terms of ease of operation and enhanced sensitivity, enabling more precise measurements and analysis with respect to other sensing techniques. This possibility enables a wide range of applications, particularly in fields such as imaging, spectroscopy, and optical coherence tomography, among others. This thesis describes experiments for imaging and sensing based on undetected photons, both in a free-space optical setup and in an integrated photonic circuit. The free-space configuration was used to perform undetected three-dimensional imaging reconstruction of diffusive targets. This validation of imaging with undetected photons paves the way to further development to in vivo biomedical imaging, covert imaging, and remote sensing. On the other hand, the integrated configuration overcomes the limitation of alignment tolerance and scalability of a free-space optical setup, enabling the development of integrated sensors based on undetected photons.
Silicon quantum photonics promises to take quantum optics to the large scale, where single photons carry the quantum information which can accelerate difficult computational problems, provide perfectly secure communication, or allow high-accuracy measurements. The silicon photonics platform offers miniature, high-yield, and high-performance photonics which are integrable with CMOS electronics and telecommunications-band optics. To date, only non-scalable photon-pair sources, based on spontaneous four-wave mixing (SFWM) in straight silicon waveguides have been shown to interfere [1]. Until now, single-photon experiments in silicon [2] have exclusively used an off-chip apparatus, such as a silica arrayed waveguide grating, to control and manipulate photon frequency. We present a silicon-on-insulator quantum photonic device (Fig. 1a) able to generate and analyse two maximally entangled qubits. We use this device to demonstrate high-visibility quantum interferencebetween resonant SFWM sources (analogous to [1]), phase-stable frequency-selection, and quantum state tomography. We varied the phase  to obtain the fringes shown in Fig. 1b, with a visibility of 96.8%. This high visibility indicates that the two microring sources are substantially identical. We setup the entangled state via the on-chip 17-dB extinction filters, and used quantum state tomography to analyse it (Fig. 1c). We measured a fidelity of 0.8. This is the first time an entangled state of photonic qubits has been both generated and analysed on-chip, and this generation has been done in a way which is scalable to larger photon number. These results represent a new step towards fully silicon-integrated quantum photonics.
In this talk, I will discuss two applications of nonlinear quantum photonics in silicon nanowires: correlated photon pair sources and a single-photon wavelength converter. Correlated photon pairs can be generated in a nonlinear silicon waveguide through the spontaneous four-wave mixing (SFWM) process, where two photons from a bright pump laser are annihilated, and a two photons are spontaneously created at different wavelengths (the signal and idler). However, indistinguishable photon pairs are of particular interest for two-photon interference experiments, meaning that the signal and idler should have the same wavelength and be entirely identical to each other. Indistinguishable pairs can be generated with the SFWM process. where two pump lasers create a degenerate signal and idler at the frequency mid-way between them. Since the two photons are indistinguishable, it is not trivial to divide them into separate waveguides so as to manipulate them individually. I will present recent results concerning a silicon quantum splitter, where degenerate pairs are created in a ring resonator which is pumped in both directions. The ring is coupled to a sagnac loop interferometer, which combines the two directions at a 50 : 50 coupler. Here, a quantum interference effect (reverse-Hong Ou Mandel interference) splits the photons to two output waveguides. This device is ultra-compact and the use of a sagnac loop means that the phase is automatically set correctly for the interference. I will also discuss progress on single photon wavelength conversion in silicon nanowires using the four-wave mixing Bragg scattering (FWMBS) process. Here, two intense pump beams are used to modulate a weak signal, causing it to shift to the idler by the frequency difference between the pumps. In principle, this can allow coherent, noiseless frequency conversion of single photon signals. Fig. 1(b) shows output spectra from a 6mm silicon nanowire when the frequency shift is varied 100 GHz to 400 GHz. Multiple peaks are visible, corresponding to up-conversion and down-conversion of the input signal.
Considerable research work is being devoted to the integration of quantum optical components on a silicon chip [1]. Many results have been achieved on the generation of quantum states of light in silicon photonic devices, but all the sources demonstrated so far need off-chip filtering stages to clean the generated photons before detection. This necessity for off-chip components hinders usability in a completely integrated platform. In this work [2] we demonstrate for the first time on-chip generation of photon pairs via a silicon microring resonator fully integrated with spectral filters for the pump on the same chip. The sample, shown in Fig. 1 a, is a reconfigurable silicon photonic integrated circuit based on Bragg reflectors and tunable ring resonators. The combination of these devices results in pump rejection by more than 95 dB in a single chip, reducing the pump intensity to less than the flux of the photon pairs generated by spontaneous four wave mixing (Fig 1 b).
Silicon micro-ring resonators have been demonstrated to be potential “on- chip” efficient sources of time-energy entangled photons. In these devices, photon pairs are generated by spontaneous four-wave-mixing (SFWM), an elastic scattering event involving two pump photons that results in the generation of an idler and a signal photon. The mass production of such integrated devices for quantum optic experiment requires the implementation of fast and reliable techniques to monitor the emitted two-photon state as well as the device performances. In this work we show that the joint spectral density of photon pairs that would be generated by SFWM in a silicon ring resonator can be measured with unprecedented resolution and speed, improving of several orders of magnitudes the results of traditional approaches based on coincidence measurements.