Quantum computing experiments are moving into a new realm of increasing size and complexity, with the short-term goal of demonstrating an advantage over classical computers. Boson sampling is a promising platform for such a goal, however, the number of involved single photons was up to five so far, limiting these small-scale implementations to a proof-of-principle stage. Here, we develop solidstate sources of highly efficient, pure and indistinguishable single photons, and 3D integration of ultra-low-loss optical circuits. We perform an experiment with 20 single photons fed into a 60-mode interferometer, and, in its output, sample over Hilbert spaces with a size of 10 14 -over ten orders of magnitude larger than all previous experiments. The results are validated against distinguishable samplers and uniform samplers with a confidence level of 99.9%.

We study experimentally the fundamental limits of sensitivity of an atomic radio-frequency magnetometer. First we apply an optimal sequence of state preparation, evolution, and the back-action evading measurement to achieve a nearly projection noise limited sensitivity. We furthermore experimentally demonstrate that Einstein-Podolsky-Rosen (EPR) entanglement of atoms generated by a measurement enhances the sensitivity to pulsed magnetic fields. We demonstrate this quantum limited sensing in a magnetometer utilizing a truly macroscopic ensemble of 1.5 · 10 12 atoms which allows us to achieve sub-femtoTesla/ √ Hz sensitivity.Ultra-sensitive atomic magnetometry is based on the measurement of the polarization rotation of light transmitted through an ensemble of atoms placed in the magnetic field [1]. For N A atoms in a state with the magnetic quantum number m F = F along a quantization axis x the collective magnetic moment (spin) of the ensemble has the length J x = F N A . A magnetic field along the y axis causes a rotation of J in the x − z plane. Polarization of light propagating along z will be rotated proportional to J z due to the Faraday effect. From a quantum mechanical point of view, this measurement is limited by quantum fluctuations (shot noise) of light, the projection noise (PN) of atoms, and the quantum backaction noise of light onto atoms. PN originates from the Heisenberg uncertainty relation δJ z · δJ y ≥ J x /2, and corresponds to the minimal transverse spin variances δJ 2 z,y = J x /2 = F N A /2 for uncorrelated atoms in a coherent spin state [2]. Quantum entanglement leads to the reduction of the atomic noise below PN and hence is capable of enhancing the sensitivity of metrology and sensing as discussed theoretically in [2][3][4][5][6][7][8][9]. In [10,11] entanglement of a few ions have been used in spectroscopy. Recently proof-of-principle measurements with larger atomic ensembles, which go beyond the PN limit have been implemented in interferometry with 10 3 atoms [12], in Ramsey spectroscopy [13,14] with up to 10 5 atoms, and in Faraday spectroscopy with 10 6 spin polarized cold atoms [15].In this Letter we demonstrate PN limited and entanglement-assisted measurement of a radio-frequency (RF) magnetic field by an atomic caesium vapour magnetometer. In the magnetometer J precesses at the Larmor frequency Ω/2π = 322kHz around a dc field B = 0.92G applied along the x axis and an RF field with the frequency Ω is applied in the y − z plane (Fig. 1a). The magnetometer (Fig. 2a) detects an RF pulse with a constant amplitude B RF and duration τ (Fourier limited full width half maximum bandwidth δ = 0.88τ −1 ≈ τ −1 ). The mean value of the projection of the atomic spin on the y − z plane in the rotating frame after the RF pulse is ΓB RF J x T 2 [1 − exp(−τ /T 2 )]/2. Here T 2 is the spin decoherence time during the RF pulse and Γ = Ω/B = 2.2 · 10 10 rad/(sec·Tesla) for caesium. Equating the mean value to the PN uncertainty we get for the minimal detectable field under the PN limited measurementThe PN...

Our Letter ''Quantum Noise Limited and Entanglement-Assisted Magnetometry'' by W. Wasilewski et al. contained a reference to M. Koschorreck, M. Napolitano, B. Dubost, and M. Mitchell, arXiv:0911.449. This reference contained a misprint and should read M. Koschorreck, M. Napolitano, B. Dubost, and M. Mitchell, arXiv:0911.4491. We regret this unfortunate error. This Letter has also appeared in press [1]. [1] M. Koschorreck, M. Napolitano, B. Dubost, and M. W. Mitchell, Phys. Rev. Lett. 104, 093602 (2010).

We report phase-programmable Gaussian boson sampling (GBS) which produces up to 113 photon detection events out of a 144-mode photonic circuit. A new high-brightness and scalable quantum light source is developed, exploring the idea of stimulated emission of squeezed photons, which has simultaneously near-unity purity and efficiency. This GBS is programmable by tuning the phase of the input squeezed states. The obtained samples are efficiently validated by inferring from computationally friendly subsystems, which rules out hypotheses including distinguishable photons and thermal states. We show that our GBS experiment passes a nonclassicality test based on inequality constraints, and we reveal nontrivial genuine high-order correlations in the GBS samples, which are evidence of robustness against possible classical simulation schemes. This photonic quantum computer, Jiuzhang 2.0, yields a Hilbert space dimension up to ∼10 43 , and a sampling rate ∼10 24 faster than using brute-force simulation on classical supercomputers.

Abstract. We use a quantum non-demolition measurement to generate a spin squeezed state and to create entanglement in a cloud of 10 5 cold cesium atoms. For the first time we operate an atomic clock improved by spin squeezing beyond the projection noise limit in a proof-of-principle experiment. For a clockinterrogation time of 10 µs, the experiments show an improvement of 1.1 dB in the signal-to-noise ratio, compared to the atomic projection noise limit.