A central challenge in developing quantum computers and long-range quantum networks lies in the distribution of entanglement across many individually controllable qubits 1 . Colour centres in diamond have emerged as leading solid-state 'artificial atom' qubits 2,3 , enabling on-demand remote entanglement 4 , coherent control of over 10 ancillae qubits with minute-long coherence times 5 , and memory-enhanced quantum communication 6 . A critical next step is to integrate large numbers of artificial atoms with photonic architectures to enable large-scale quantum information processing systems. To date, these efforts have been stymied by qubit inhomogeneities, low device yield, and complex device requirements. Here, we introduce a process for the high-yield heterogeneous integration of 'quantum micro-chiplets' (QMCs) -diamond waveguide arrays containing highly coherent colour centreswith an aluminium nitride (AlN) photonic integrated circuit (PIC). Our process enables the development of a 72-channel defect-free array of germanium-vacancy (GeV) and silicon-vacancy (SiV) colour centres in a PIC. Photoluminescence spectroscopy reveals long-term stable and narrow average optical linewidths of 54 MHz (146 MHz) for GeV (SiV) emitters, close to the lifetime-limited linewidth of 32 MHz (93 MHz). Additionally, inhomogeneities in the individual qubits can be compensated in situ with integrated tuning of the optical frequencies over 100 GHz. The ability to assemble large numbers of nearly indistinguishable artificial atoms into phase-stable PICs provides an architecture toward multiplexed quantum repeaters 7,8 and general-purpose quantum computers [9][10][11] . Main textArtificial atom qubits in diamond combine minute-scale quantum memory times 5 with efficient spin-photon interfaces 2 , making them attractive for processing and distributing quantum information 1,3 . However, the low device yield of functional qubit systems presents a critical barrier to large-scale quantum information processing (QIP). Furthermore, although individual diamond cavity systems coupled to artificial atoms can now achieve excellent performance, the lack of active chip-integrated photonic components and wafer-scale single crystal diamond currently prohibit scaling to large-scale QIP applications [8][9][10][11] . A promising method to alleviate these constraints is heterogeneous integration (HI), which is increasingly used in advanced microelectronics to assemble separately fabricated sub-components into a single, multifunctional chip. HI approaches have also recently been used to integrate PICs with quantum devices, including quantum dot single-photon sources 12,13 , superconducting nanowire single-photon detectors 14 , and nitrogen-vacancy (NV) centre diamond waveguides 15 . However, these demonstrations assembled components one-by-one, which presents a formidable scaling challenge. The diamond 'quantum micro-chiplet (QMC)' introduced here significantly improves HI assembly yield and accuracy to enable a 72-channel defect-free waveguide-coupled art...

The past decade has seen great advances in developing color centers in diamond for sensing, quantum information processing, and tests of quantum foundations. Increasingly, the success of these applications as well as fundamental investigations of light-matter interaction depend on improved control of optical interactions with color centers -from better fluorescence collection to efficient and precise coupling with confined single optical modes. Wide ranging research efforts have been undertaken to address these demands through advanced nanofabrication of diamond. This review will cover recent advances in diamond nano-and microphotonic structures for efficient light collection, color center to nanocavity coupling, hybrid integration of diamond devices with other material systems, and the wide range of fabrication methods that have enabled these complex photonic diamond systems. CONTENTS

Nonclassical states are essential for optics-based quantum information processing, but their fragility limits their utility for practical scenarios in which loss and noise inevitably degrade, if not destroy, nonclassicality. Exploiting nonclassical states in quantum metrology yields sensitivity advantages over all classical schemes delivering the same energy per measurement interval to the sample being probed. These enhancements, almost without exception, are severely diminished by quantum decoherence. Here, we experimentally demonstrate an entanglement-enhanced sensing system that is resilient to quantum decoherence. We employ entanglement to realize a 20% signal-to-noise ratio improvement over the optimum classical scheme in an entanglement-breaking environment plagued by 14 dB of loss and a noise background 75 dB stronger than the returned probe light. Our result suggests that advantageous quantumsensing technology could be developed for practical situations. DOI: 10.1103/PhysRevLett.114.110506 PACS numbers: 03.67.-a, 03.65.Ta, 42.50.Dv Quantum information processing (QIP) exploits fundamental quantum-mechanical properties to realize capabilities beyond the reach of classical physics. Nonclassical states are essential for optics-based QIP, providing the bases for quantum teleportation [1-3], device-independent quantum key distribution [4], quantum computing [5,6], and quantum metrology [7]. Nonclassical states can increase the signal-to-noise ratios (SNRs) of quantummetrology systems. Indeed, squeezed states have been employed to beat the classical-state limits in optical-phase tracking [8,9], biological sensing [10], and gravitational wave detection [11,12]. Squeezed states, however, are vulnerable to loss: a 10 dB SNR enhancement without loss degrades to 1 dB in a system with 6 dB of loss. Under ideal conditions, N00N states, which are superposition states of N photons in one mode and vacuum in another mode, and vice versa, yield SNR improvements comparable to those of squeezed states [13][14][15][16], but noise injection can easily render N00N states impotent in this regard [17,18]. Consequently, quantum decoherence, arising from environmental loss and noise, largely prevents any quantum-sensing performance advantage, casting doubt on the utility of QIP systems for practical situations.Quantum illumination (QI) is a radically different paradigm that utilizes nonclassical states to achieve an appreciable performance enhancement in the presence of quantum decoherence. QI can defeat eavesdropping on a communication link [19][20][21][22], and boost the SNR of a sensing system [23][24][25][26][27][28][29]. QI systems are comprised of (1) a source that emits entangled signal and idler beams; (2) an interaction in which the signal beam (used as a probe) is subjected to environmental loss, modulation, and noise en route from the source to the receiver; and (3) a receiver that makes a joint measurement on the returned signal beam and the idler beam, which has been stored in a quantum memory, to extract informat...

We demonstrate a photonic circuit with integrated long-lived quantum memories. Precharacterized quantum nodes-diamond microwaveguides containing single, stable, negatively charged nitrogenvacancy centers-are deterministically integrated into low-loss silicon nitride waveguides. These quantum nodes efficiently couple into the single-mode waveguides with >1 Mcps collected into the waveguide, have narrow single-scan linewidths below 400 MHz, and exhibit long electron spin coherence times up to 120 μs. Our system facilitates the assembly of multiple quantum nodes with preselected properties into a photonic integrated circuit with near unity yield, paving the way towards the scalable fabrication of quantum information processors. DOI: 10.1103/PhysRevX.5.031009 Subject Areas: Photonics, Quantum InformationAdvanced quantum information systems, such as quantum computers [1] and quantum repeaters [2], require multiple entangled quantum memories that can be individually controlled [3]. Over the past decade, there has been rapid theoretical and experimental progress in developing such entangled networks using stationary quantum bits (qubits) connected via photons [4][5][6]. Photonic integrated circuits (PICs) could provide a compact, phase-stable, and scalable architecture for such quantum networks. However, the realization of this promise requires near-unity-yield fabrication of high-quality solid-state quantum memories efficiently coupled to low-loss single-mode waveguides.A promising solid-state quantum memory with secondscale spin coherence times is the negatively charged nitrogen-vacancy (NV) center in diamond [7,8]. Its electronic spin state can be optically initialized, manipulated, measured [9,10], and mapped onto nearby auxiliary nuclear memories [11], which allows for quantum error correction [12]. Quantum network protocols based on these unique qualities have been proposed [13,14], and entanglement generation and state teleportation between two spatially separated quantum memories have been demonstrated [15,16]. Translating such entanglement techniques into an on-chip architecture promises scalability, but can only succeed if an efficient photonic architecture can be fabricated with a high yield. So far, waveguide patterning in diamond is challenging, preventing low-loss waveguides in the optical domain around 637 nm, the zero phonon line (ZPL) of the NV. This is in contrast to silicon nitride (SiN)-based photonics, which relies on well-developed fabrication processes [17], is CMOS compatible [18], and has a large band gap (∼5 eV) and high index of refraction (n ¼ 2.1), which makes it ideal for routing the visible emission of NVs.A second challenge in creating a scalable solid-state quantum network architecture is the inherently low yield production of high-quality quantum nodes due to the stochastic process of NV creation. The number of fabrication attempts necessary to create a monolithic network in which all nodes contain NV quantum memories with the desired spectral and spin properties scales exponentially...

Efficient collection of the broadband fluorescence from the diamond nitrogen vacancy (NV) center is essential for a range of applications in sensing, on-demand single photon generation, and quantum information processing. Here, we introduce a circular "bullseye" diamond grating which enables a collected photon rate of (2.7 ± 0.09) × 10(6) counts per second from a single NV with a spin coherence time of 1.7 ± 0.1 ms. Back-focal-plane studies indicate efficient redistribution of the NV photoluminescence into low-NA modes by the bullseye grating.

Single photon sources are of paramount importance in quantum communication, quantum computation, and quantum metrology. In particular, there is great interest to realize scalable solid state platforms that can emit triggered photons on demand to achieve scalable nanophotonic networks.We report on a visible-spectrum single photon emitter in 4H-silicon carbide (SiC). The emitter is photostable at room-and low-temperature enabling photon counts per second (cps) in excess of 2×10 6 from unpatterned, bulk SiC. It exists in two orthogonally polarized states, which have parallel absorption and emission dipole orientations. Low temperature measurements reveal a narrow zero phonon line (linewidth < 0.1 nm) that accounts for > 30 % of the total photoluminescence spectrum.

We demonstrate the fabrication of photonic crystal nanobeam cavities with rectangular cross section into bulk diamond. In simulation, these cavities have an unloaded quality factor (Q) of over 1 × 10 6 . Measured cavity resonances show fundamental modes with spectrometer-limited quality factors ≥ 14 × 10 3 within 1 nm of the NV center's zero phonon line at 637 nm. We find high cavity yield across the full diamond chip with deterministic resonance trends across the fabricated parameter sweeps.A central aim of quantum information science is the efficient generation of large entangled states of stationary quantum memories with high-fidelity single and twoqubit gates. Among solid-state qubits, a leading system consists of the nitrogen vacancy (NV) center in diamond. Recently, entanglement between distant NV nodes has been demonstrated via single-photon measurements of the zero phonon line (ZPL) emission [1,2]. Such heralded entanglement can be used to build large cluster states which are a resource for universal quantum computation [3] or quantum repeater networks [4,5]. However, the coherent ZPL optical transition of the NV accounts for only 3% of the emission due to phonon interactions even at cryogenic temperatures. This severely limits the entanglement rate, even with broadband collection enhancement structures such as a solid immersion lens [2]. To improve the entanglement rate, the collection rate into the desired frequency and spatial mode must be increased. Much recent work has focused on maximizing the coherent ZPL collection efficiency via the Purcell effect in a photonic crystal nanocavity.One major challenge in coupling the NV to a nanocavity is that high-quality diamond cannot currently be grown in thin (wavelength-scale) waveguiding membranes. Hybrid structures have been used to enhance the ZPL emission [6][7][8], though the maximum enhancement rate is limited as the NV must be placed out of the cavity's mode maximum. Advances in diamond patterning have enabled the fabrication of photonic crystal cavities in diamond [9] with two main methods: fabrication into thinned diamond membranes [10-17] and angled etching of bulk diamond [18][19][20][21]. However, the measured quality (Q) factors have been limited to a few thousand near the NV ZPL. Low cavity yield or inconsistency across the diamond substrate also presents a major scaling challenges, especially for membrane-based approaches.Recently, Khanaliloo et al. [22,23] introduced a technique to undercut lithographically defined structures in bulk diamond, allowing for the fabrication of high quality free-standing diamond optomechanical devices. In this Letter, we apply a similar isotropic undercut pro- * S.M. and N.H.W. contributed equally to this work † englund@mit.edu cess to the fabrication of photonic crystal nanobeam cavities. Figure 1(a) shows a schematic of the rectangular nanobeam photonic crystal fabricated from bulk diamond. We find that the quasi-isotropic etching procedure is highly repeatable and consistent across the full chip. In addition, t...

The advancement of quantum optical science and technology with solid-state emitters such as nitrogen-vacancy (NV) centers in diamond critically relies on the coherence of the emitters' optical transitions. A widely employed strategy to create NV centers at precisely controlled locations is nitrogen ion implantation followed by a high-temperature annealing process. We report on experimental data directly correlating the NV center optical coherence to the origin of the nitrogen atom. These studies reveal low-strain, narrow-optical-linewidth (< 500 MHz) NV centers formed from naturally-occurring 14 N atoms. In contrast, NV centers formed from implanted 15 N atoms exhibit significantly broadened optical transitions (> 1 GHz) and higher strain. The data show that the poor optical coherence of the NV centers formed from implanted nitrogen is not due to an intrinsic effect related to the diamond or isotope. These results have immediate implications for the positioning accuracy of current NV center creation protocols and point to the need to further investigate the influence of lattice damage on the coherence of NV centers from implanted ions.