We report on the implementation of quantum entanglement distribution and quantum state teleportation over a 14.4-km urban dark-fiber link, which is partially underground, partially overhead, and patched in several stations. We characterize the link for its use as a quantum channel and realize its active polarization stabilization. Using a type-II cavity-enhanced SPDC photon pair source, a $^{40}$Ca$^{+}$ single-ion quantum memory, and quantum frequency conversion to the telecom C-band, we demonstrate photon-photon entanglement, ion-photon entanglement, and teleportation of a qubit state from the ion onto a remote telecom photon, all realized over the urban fiber link.

This White Paper describes the operating principle, technical details, and performance of the Menlo Systems ORS Ultrastable Laser System. It gives a general overview of applications for ultrastable lasers, with specific use cases of Menlo Systems’ ORS systems. A more detailed presentation of an application using a novel approach for clock comparisons [1] demonstrates the capabilities of a commercial ultrastable laser system to perform state-of-the-art measurements, and the potential for future field applications

Quantum technologies, spanning from sensing and metrology to simulations and computing, rely upon precise and low noise laser systems. Currently, we are witnessing a paradigm shift, where laboratory-based experiments are engineered such to develop reliable operating devices. The goal of providing continuous operation is key to enable their deployment for, e.g., PNT applications or cloud-based quantum computing services. Ultra-low noise laser systems are becoming integral part of these quantum devices due to their pivotal role in the effective functioning of the physics package. In fact, the performance are fundamentally linked to the noise properties of the driving laser fields, imposing the need of a careful choice of the appropriate sources, their spectral properties, and their stabilization. Here we present some of our recent ultra-stable laser system engineered for enabling several applications, we will describe ultra-stable comb and laser systems for quantum computing using neutral Yb, Sr, Rb, or Cs atoms, electric field sensing with Rydberg Rb atoms, and portable compact comb systems to enable uninterrupted operation of optical clocks in the field. A detailed noise analysis of the systems will be presented.

We present a rack-mounted ultra-low-noise laser system for Sr lattice clocks, with spectral purity transfer stability of 4×10 -18 @1s and 2×10 -19 @100s. We realize a system stability of 5×10 -16 @1s, representing state-of-the-art performance for rack-mounted commercial systems.

We present a transportable photonic microwave oscillator, porting optical stability to a 10 GHz microwave signal. The system is composed of two main sub-units: a compact 8 height-unit (HU) 194 THz ultra-stable laser with Hz linewidth showing a fractional frequency stability (modified Allan deviation) of 8.5 × 10 -15 at 1 s and a 3 HU ultra-low noise optical frequency comb used to phase-coherently divide the optical frequency reference's spectral purity down to a 10 GHz microwave signal. Characterizing the synthesized microwave by means of a commercial digital cross-correlator, a phase noise power spectral density of -70 dBc/Hz at 1 Hz and -160 dBc/Hz at 10 kHz Fourier frequency is measured, maintaining a white plateau at -165 dBc/Hz up to 10 MHz offset.

Quantum technologies are nowadays emerging as enabling tools for practical applications, such as quantum sensing, quantum computing and quantum metrology. Lasers play a central role in many of these technological platforms, e.g. for atomic clocks, ion-based or neutral atom-based quantum computers or atom interferometers. Here we present a complete laser system to cool, trap and control strontium atoms in an optical lattice or in tweezer arrays. A sub-Hz linewidth master laser, locked to a high-finesse optical cavity provides the frequency reference for an ultra-low noise comb. The rack-mounted laser system consists of all cooling, repumping, and clock lasers stabilized to the optical frequency comb. Each of the involved laser frequencies can therefore be tuned and mapped in the frequency domain with a high degree of stability. The system is controlled via a software interface, allowing to operate the cold-atom-based physics package autonomously. The system is tailored for the operation of 88Sr or 87Sr optical lattice clocks, or for quantum computing applications, but other sub-Hz lasers could be obtained by phase locking additional clock laser frequencies to the ultra-stable comb, enabling convenient and accurate optical frequency ratio measurements. The laser system architecture and the relevant characterization measurements will be presented, proposing some user-cases such as quantum computing and atom interferometry on strontium atoms. This represents a technological leap for quantum optics, allowing to explore further applications of quantum sensors outside a traditional lab.

We present a fully phase-locked comb-disciplined-laser system, conceived to operate the Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS-100). Via phase-locking to the comb, four ECDLs and a Ti:Sapphire laser inherit the stability of an ultra-stable laser.

Frequency comb synthesized microwaves have been so far realized with tabletop systems, operated in well-controlled environments. Here, we demonstrate state-of-the-art ultrastable microwave synthesis with a compact rack-mountable apparatus. We present absolute phase noise characterization of a 12 GHz signal using an ultrastable laser at $\sim{194}\;{\rm THz}$∼194THz and an Er:fiber comb divider, obtaining $ - {83}\;{\rm dBc/Hz}$-83dBc/Hz at 1 Hz and $ \lt - {166}\;{\rm dBc/Hz}$<-166dBc/Hz for offsets greater than 5 kHz. Employing semiconductor coating mirrors for the same type of transportable optical frequency reference, we show that $ - {105}\;{\rm dBc/Hz}$-105dBc/Hz at 1 Hz is supported by demonstrating a residual noise limit of division and detection process of $ - {115}\;{\rm dBc/Hz}$-115dBc/Hz at 1 Hz. This level of fidelity paves the way for the deployment of ultrastable photonic microwave oscillators and for operating transportable optical clocks.

A micro-integrated laser module has been developed for the deployment in a compact, transportable 171Yb+ optical clock. With this laser module, the clock laser system demonstrated a Modified Allan Deviation of less than 1.5×10-15 for 1 s ≤ 100 s when compared against a superior reference laser. © 2020 The Author(s)

We report the development of a field-deployable frequency comb operated as clockwork for transportable Sr-clocks. While referencing it at ~194.4THz, we characterize spectral lines at ~214.6THz. Prospects on complete Sr-lattice laser systems will be outlined.

We report the development of a complete turn-key 5 × 10-16 ultra-stable laser system for operating neutral strontium atoms for optical lattice clocks or for quantum computing.

Methods for long-distance time and frequency transfer over optical fibres have demonstrated excellent performances and are evolving rapidly. CLONETS is a new European Union-funded coordination and support action intended to accelerate the transfer of these technologies to industry and to strengthen the coordination between research infrastructures and research and education network providers, in order to prepare the deployment of this technology for a sustainable, pan-European fibre network, providing high-performance clock services to European research infrastructures and supporting wider services to industry and society.

Dynamic development of long-distance methods of time and frequency signals transmission over optical fibers is giving the opportunity to create optical network with dedicated clock services. In order to prepare the deployment of this technology for a sustainable, pan-European fibre network, providing high-performance clock services to European research infrastructures and supporting wider services to industry and society, the CLONETS project, funded by the European Commission in H2020 program, is a coordination and support action intended to accelerate the transfer of these technologies to industry and to strengthen the coordination between research infrastructures and research and education network providers.. The optical network will cover a European area, and will be providing highest available performance for research infrastructures, commercial entities and other organisations.

Precision measurements represent a cornerstone in fundamental science. The capability of observing and quantifying subtle phenomena and events allows new discoveries and it confirms or confutes the theories describing our understanding of nature. The optical frequency comb, providing hundreds of thousands of phase-locked and evenly spaced laser lines, is one of the most fascinating enabling optical technologies and it is the result of a continuous pursuit for precision. Within the two decades from its inception, it has become a key instrument in many laboratories and has revolutionized numerous fields, spanning from time, frequency and length metrology to attosecond physics, gas-sensing and molecular fingerprinting. In this letter we summarize some steps of an exciting journey started 20 years ago, with a certain focus on the authors’ contribution, finally leading to the demonstration of frequency measurements at the 20th decimal digit, and we show some prospective developments.

CLONETS is a new, European Union funded project which aims to prepare the transition toward a permanent, pan-European, optical fiber-based network providing time and frequency comparisons and distribution at the highest performance levels for research infrastructures, as well as support to a wide range of services for industry and society. The project started in January 2017 and is scheduled for 30 months. The project consortium is formed by 19 organizations from 7 European countries. This paper provides information about the project in progress and briefly describes its results achieved during the first year. INTRODUCTION The project CLONETS (Clock Network Services) is motivated by recent progress in time and frequency (T&F) metrology and novel applications in fundamental physics, geodesy, telecommunication, industry and society that require time and frequency reference signals with significantly higher performance than is currently mediated by satellite techniques. Very high performance time and frequency reference signals are also moving from radio signal broadcasting to transport over optical fiber networks as the development of relevant technologies for T&F transfer on optical links progresses rapidly. However, widespread utilization of such signals is currently hampered by a lack of sustainable and reliable infrastructure. Nevertheless optical fiber links for T&F are being actively developed and operated by several countries in Europe, including some cross-border links. CLONETS aims to prepare the conditions in which these links may be combined and completed to form a pan-European network, with a sustainable organisation allowing it to operate as a long term service to research infrastructures, industry and society. CLONETS receives funding from the European Union's Horizon 2020 research and innovation programme (2014-2020). The project brings together a diversified group of actors: National Metrology Institutes (NMIs), academic research groups, National Research and Education Network providers (NRENs), an internet exchange and small and medium-sized high-technology companies, who concentrate a wide range of expertise and activities in this area. The proposed network is intended to be open to the participation of all relevant entities in Europe.

Time and frequency transfer based on optical fiber links techniques have demonstrated excellent performances. CLONETS is EU project intended to accelerate transfer of related technologies, in order to prepare deployment of technology for sustainable network providing high-performance clock services.

We report on a transportable photonic microwave synthesizer comprising an ultrastable laser and a ultra-low noise frequency comb. The last acts as a frequency divider, transferring the spectral purity from the optical to microwave domain.

We demonstrate a frequency comb-based time transfer technique on a 159 km long installed fiber link. Timing information is superimposed onto the optical pulse train of an ITU-channel-filtered mode-locked laser using an intensity modulation scheme. The environmentally induced optical path length fluctuations are compensated using a round-trip phase noise cancellation technique. When the fiber link is stabilized, a time deviation of 300 fs at 5 s and an accuracy at the 100 ps level are achieved.

We present a technique for simultaneous optical and microwave frequency transfer as well as time transfer using a mode-locked laser. We demonstrate a time deviation of 300 fs at 10 s averaging time on an installed 158 km fiber link.

We demonstrate an amplitude-to-phase (AM-PM) conversion coefficient for a balanced optical-microwave phase detector (BOM-PD) of 0.001 rad, corresponding to AM-PM induced phase noise 60 dB below the single-sideband relative intensity noise of the laser. This enables us to generate 8 GHz microwave signals from a commercial Er-fibre comb with a single-sideband residual phase noise of -131 dBc Hz(-1) at 1 Hz offset frequency and -148 dBc Hz(-1) at 1 kHz offset frequency.