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A quantum computer attains computational advantage when outperforming the best classical computers running the best-known algorithms on well-defined tasks. No photonic machine offering programmability over all its quantum gates has demonstrated quantum computational advantage: previous machines1,2 were largely restricted to static gate sequences. Earlier photonic demonstrations were also vulnerable to spoofing3, in which classical heuristics produce samples, without direct simulation, lying closer to the ideal distribution than do samples from the quantum hardware. Here we report quantum computational advantage using Borealis, a photonic processor offering dynamic programmability on all gates implemented. We carry out Gaussian boson sampling4 (GBS) on 216 squeezed modes entangled with three-dimensional connectivity5, using a time-multiplexed and photon-number-resolving architecture. On average, it would take more than 9,000 years for the best available algorithms and supercomputers to produce, using exact methods, a single sample from the programmed distribution, whereas Borealis requires only 36 μs. This runtime advantage is over 50 million times as extreme as that reported from earlier photonic machines. Ours constitutes a very large GBS experiment, registering events with up to 219 photons and a mean photon number of 125. This work is a critical milestone on the path to a practical quantum computer, validating key technological features of photonics as a platform for this goal.

The ability to perform computations on encrypted data is a powerful tool for protecting privacy. Recently, protocols to achieve this on classical computing systems have been found. Here, we present an efficient solution to the quantum analogue of this problem that enables arbitrary quantum computations to be carried out on encrypted quantum data. We prove that an untrusted server can implement a universal set of quantum gates on encrypted quantum bits (qubits) without learning any information about the inputs, while the client, knowing the decryption key, can easily decrypt the results of the computation. We experimentally demonstrate, using single photons and linear optics, the encryption and decryption scheme on a set of gates sufficient for arbitrary quantum computations. As our protocol requires few extra resources compared with other schemes it can be easily incorporated into the design of future quantum servers. These results will play a key role in enabling the development of secure distributed quantum systems.

Interference is a defining feature of both quantum and classical theories of light, enabling the most precise measurements of a wide range of physical quantities including length 1 and time 2 . Quantum metrology exploits fundamental differences between these theories for new measurement techniques and enhanced precision 3,4 . Advantages stem from several phenomena associated with quantum interferometers, including non-local interference 5,6 , phase-insensitive interference 7 , phase super-resolution and super-sensitivity 8-10 , and automatic dispersion cancellation 6,11,12 . However, quantum interferometers require entangled states that are in practice difficult to create, manipulate and detect, especially compared with robust, intense classical states. In the present work, we report an interferometer based on chirped femtosecond laser pulses and classical nonlinear optics showing all of the metrological advantages of the quantum Hong-Ou-Mandel interferometer 7 , but with 10 million times more signal. Our work emphasizes the importance of delineating truly quantum effects from those with classical analogues 10,13,14 , and shows how insights gained from quantum mechanics can inspire novel classical technologies.Arguably the best known example of quantum interference was demonstrated by Hong, Ou and Mandel 7 (HOM); their interferometer is depicted in Fig. 1a. HOM interference is central to optical quantum technologies, including quantum teleportation 15 and linear-optical quantum computing 16 . Several characteristics distinguish HOM from classical interference, such as Michelson's or Young's. The HOM signal stems from pairs of interfering photons and is manifest as a dip in the rate of coincident photon detections spanning the coherence length of the light, as opposed to classical wavelength fringes. It is therefore inherently robust against path-length fluctuations. If the photons are entangled, the visibility and width of the HOM interferogram are insensitive to loss 17 and dispersion 11 . Furthermore, HOM interferometers achieve higher resolution than classical interferometers using the same bandwidth 18,19 . These features are ideal for precision optical path measurements of dispersive and lossy materials, implemented by placing the sample in one interferometer arm and measuring the delay required to restore interference. A quantum version of optical coherence tomography 20 was proposed and demonstrated 18,19 to harness these advantages.Recently, two proposals 21,22 and one experimental demonstration 23 have described classical systems showing automatic dispersion cancellation. Drawbacks to these techniques include reliance on unavailable technology 21 or significant postprocessing 22,23 . The experimentally demonstrated technique requires wavelength path stability; the interference visibility falls precipitously with loss and is limited to 50% of that possible with the HOM effect. Alternatively, background-free autocorrelation of transform-limited pulses, recently used for optical coherence tomography 24 , s...

Photons are critical to quantum technologies since they can be used for virtually all quantum information tasks: in quantum metrology, as the information carrier in photonic quantum computation, as a mediator in hybrid systems, and to establish long distance networks. The physical characteristics of photons in these applications differ drastically; spectral bandwidths span 12 orders of magnitude from 50 THz for quantum-optical coherence tomography to 50 Hz for certain quantum memories. Combining these technologies requires coherent interfaces that reversibly map centre frequencies and bandwidths of photons to avoid excessive loss. Here we demonstrate bandwidth compression of single photons by a factor 40 and tunability over a range 70 times that bandwidth via sum-frequency generation with chirped laser pulses. This constitutes a time-to-frequency interface for light capable of converting time-bin to colour entanglement and enables ultrafast timing measurements. It is a step toward arbitrary waveform generation for single and entangled photons.Comment: 6 pages (4 figures) + 6 pages (3 figures