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1.
Science ; 2020 Dec 03.
Artigo em Inglês | MEDLINE | ID: mdl-33273064

##### RESUMO

Quantum computers promises to perform certain tasks that are believed to be intractable to classical computers. Boson sampling is such a task and is considered as a strong candidate to demonstrate the quantum computational advantage. We perform Gaussian boson sampling by sending 50 indistinguishable single-mode squeezed states into a 100-mode ultralow-loss interferometer with full connectivity and random matrix-the whole optical setup is phase-locked-and sampling the output using 100 high-efficiency single-photon detectors. The obtained samples are validated against plausible hypotheses exploiting thermal states, distinguishable photons, and uniform distribution. The photonic quantum computer generates up to 76 output photon clicks, which yields an output state-space dimension of 1030 and a sampling rate that is ~1014 faster than using the state-of-the-art simulation strategy and supercomputers.

2.
Opt Lett ; 45(9): 2640-2643, 2020 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-32356836

##### RESUMO

Photon-counting optical time-domain reflectometry (PC-OTDR) based on single photon detection is an effective scheme to attain the high spatial resolution for optical fiber fault monitoring. Currently, due to the spatial resolution of PC-OTDR being proportional to the pulse width of a laser beam, short laser pulses are essential for a high spatial resolution. However, short laser pulses have a large bandwidth, which would be widened by the dispersion of fiber, causing inevitable deterioration in the spatial resolution, especially for long-haul fiber links. In this Letter, we propose a scheme of dispersion independent PC-OTDR based on an infinite backscatter technique. Our experimental results-with more than 45 km long fiber-show that the spatial resolution of the PC-OTDR system is independent with the total dispersion of the fiber under test. Our method provides an avenue for developing long-haul PC-OTDR with high performance.

3.
Appl Opt ; 59(7): 1841-1845, 2020 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-32225699

##### RESUMO

The development of photonic quantum information technologies requires research on the properties of optical adhesives at cryogenic temperatures. In the process of developing microfiber (MF)-coupled superconducting nanowire single-photon detectors (SNSPDs), we invented a cryogenic-temperature refractive index (RI) measurement method based on a kind of MF device. The device was put into the cryostat to observe the variance of MF transmittance with temperature. Then an RI-temperature relationship was established through the correspondence between the confinement conditions of MFs of various diameters in an optical adhesive-${{\rm MgF}_2}$MgF2 environment and transmittance-temperature curves. Using this method, we analyzed the thermal-optical properties of a commercial fluorinated acrylic optical adhesive and obtained the RI values of the adhesive at various temperatures. The results were successfully applied in the development of broadband and high-efficiency MF-coupled SNSPDs.

4.
Phys Rev Lett ; 124(7): 070501, 2020 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-32142314

##### RESUMO

Twin-field (TF) quantum key distribution (QKD) promises high key rates over long distances to beat the rate-distance limit. Here, applying the sending-or-not-sending TF QKD protocol, we experimentally demonstrate a secure key distribution that breaks the absolute key-rate limit of repeaterless QKD over a 509-km-long ultralow loss optical fiber. Two independent lasers are used as sources with remote-frequency-locking technique over the 500-km fiber distance. Practical optical fibers are used as the optical path with appropriate noise filtering; and finite-key effects are considered in the key-rate analysis. The secure key rate obtained at 509 km is more than seven times higher than the relative bound of repeaterless QKD for the same detection loss. The achieved secure key rate is also higher than that of a traditional QKD protocol running with a perfect repeaterless QKD device, even for an infinite number of sent pulses. Our result shows that the protocol and technologies applied in this experiment enable TF QKD to achieve a high secure key rate over a long distribution distance, and is therefore practically useful for field implementation of intercity QKD.

5.
Nature ; 578(7794): 240-245, 2020 02.
Artigo em Inglês | MEDLINE | ID: mdl-32051600

##### RESUMO

A quantum internet that connects remote quantum processors1,2 should enable a number of revolutionary applications such as distributed quantum computing. Its realization will rely on entanglement of remote quantum memories over long distances. Despite enormous progress3-12, at present the maximal physical separation achieved between two nodes is 1.3 kilometres10, and challenges for longer distances remain. Here we demonstrate entanglement of two atomic ensembles in one laboratory via photon transmission through city-scale optical fibres. The atomic ensembles function as quantum memories that store quantum states. We use cavity enhancement to efficiently create atom-photon entanglement13-15 and we use quantum frequency conversion16 to shift the atomic wavelength to telecommunications wavelengths. We realize entanglement over 22 kilometres of field-deployed fibres via two-photon interference17,18 and entanglement over 50 kilometres of coiled fibres via single-photon interference19. Our experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment of the atomic quantum network, paving the way towards establishing atomic entanglement over many nodes and over much longer distances.

6.
Phys Rev Lett ; 124(1): 010502, 2020 Jan 10.
Artigo em Inglês | MEDLINE | ID: mdl-31976724

##### RESUMO

Ensuring the nonentanglement-breaking (non-EB) property of quantum channels is crucial for the effective distribution and storage of quantum states. However, a practical method for direct and accurate certification of the non-EB feature is highly desirable. Here, we propose and verify a realistic source based measurement device independent certification of non-EB channels. Our method is resilient to repercussions on the certification from experimental conditions, such as multiphotons and imperfect state preparation, and can be implemented with an information incomplete set. We achieve good agreement between experimental outcomes and theoretical predictions, which is validated by the expected results of the ideal semiquantum signaling game, and accurately certify the non-EB channels. Furthermore, our approach is highly robust to effects from noise. Therefore, the proposed approach can be expected to play a significant role in the design and evaluation of realistic quantum channels.

7.
Phys Rev Lett ; 125(26): 260503, 2020 Dec 31.
Artigo em Inglês | MEDLINE | ID: mdl-33449747

##### RESUMO

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates, and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over a free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step toward satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

8.
Appl Opt ; 58(30): 8148-8152, 2019 Oct 20.
Artigo em Inglês | MEDLINE | ID: mdl-31674484

##### RESUMO

In this work, we report a large-active-area multispectral superconducting nanowire single-photon detector for free-space applications. The detector is realized by fabricating NbTiN nanowire with an active area of 35 µm diameter on two serially connected dielectric mirrors that can simultaneously and efficiently detect single photons at the three typical wavelengths employed in free-space applications, namely, 532, 850, and 1064 nm. Maximal system detection efficiencies (SDEs) of 80.0% at 532 nm and 850 nm and 75.8% at 1064 nm are achieved for polarized light obtained by coupling the detector with an SMF-28 fiber. Upon coupling with a 50 µm multimode fiber, SDEs of 68.6%, 59.6%, and 47.0%, are achieved for 532, 850, and 1064 nm wavelength unpolarized light, respectively. Moreover, the detector shows timing jitters of 37.1 and 41.0 ps when coupled with SMF-28 fiber and 50 µm multimode fiber. This type of detector with a large active area and multiwavelength detection capability is promising for both single and multiwavelength free-space applications.

9.
Phys Rev Lett ; 123(10): 100503, 2019 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-31573287

##### RESUMO

Quantum computing has seen tremendous progress in past years. Due to implementation complexity and cost, the future path of quantum computation is strongly believed to delegate computational tasks to powerful quantum servers on the cloud. Universal blind quantum computing (UBQC) provides the protocol for the secure delegation of arbitrary quantum computations, and it has received significant attention. However, a great challenge in UBQC is how to transmit a quantum state over a long distance securely and reliably. Here, we solve this challenge by proposing a resource-efficient remote blind qubit preparation (RBQP) protocol, with weak coherent pulses for the client to produce, using a compact and low-cost laser. We experimentally verify a key step of RBQP-quantum nondemolition measurement-in the field test over 100 km of fiber. Our experiment uses a quantum teleportation setup in the telecom wavelength and generates 1000 secure qubits with an average fidelity of (86.9±1.5)%, which exceeds the quantum no-cloning fidelity of equatorial qubit states. The results prove the feasibility of UBQC over long distances, and thus serves as a key milestone towards secure cloud quantum computing.

10.
Phys Rev Lett ; 123(10): 100505, 2019 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-31573314

##### RESUMO

Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution. The idea of twin-field quantum key distribution can improve the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. However, the technical demands are rather tough because they require single photon level interference of two remote independent lasers. Here, we adopt the technology developed in the frequency and time transfer to lock two independent laser wavelengths and utilize additional phase reference light to estimate and compensate the fiber fluctuation. Further, with a single photon detector with a high detection rate, we demonstrate twin field quantum key distribution through the sending-or-not-sending protocol with a realistic phase drift over 300 km optical fiber spools. We calculate the secure key rates with the finite size effect. The secure key rate at 300 km (1.96×10^{-6}) is higher than that of the repeaterless secret key capacity (8.64×10^{-7}).

11.
Opt Express ; 27(18): 25241-25250, 2019 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-31510399

##### RESUMO

Broadband photon detectors are a key enabling technology for various applications such as spectrometers, light detection and ranging. In this work, we report on an ultra-broadband single-photon detector based on a microfiber (MF)-coupled superconducting nanowires structure operating in the spectral range from visible to near-infrared light. The MF-coupled superconducting nanowire single-photon detector (SNSPD) is formed by placing an MF on top of superconducting niobium nitride (NbN) nanowires, allowing ultra-broadband photon detection due to their nearly lossless transmission/absorption and nearly unity internal efficiency for ultra-broad waveband. The simulation results indicate that with optimal device structure, the optical absorption with efficiency > 90% can be realized over a wavelength range of 350 nm to 2150 nm. The fabricated MF-coupled SNSPD shows unparalleled broadband system detection efficiencies (SDEs) of more than 50% from 630 nm to 1500 nm. The SDEs reach 66% at 785 nm and 45% at 1550 nm. These results pave the way for ultra-broadband weak light detection with quantum-limit sensitivity.

12.
Phys Rev Lett ; 122(16): 160501, 2019 Apr 26.
Artigo em Inglês | MEDLINE | ID: mdl-31075015

##### RESUMO

Measurement-device-independent quantum key distribution (MDI-QKD) can eliminate all detector side channels and it is practical with current technology. Previous implementations of MDI-QKD all used two symmetric channels with similar losses. However, the secret key rate is severely limited when different channels have different losses. Here we report the results of the first high-rate MDI-QKD experiment over asymmetric channels. By using the recent 7-intensity optimization approach, we demonstrate>10×higher key rate than the previous best-known protocols for MDI-QKD in the situation of large channel asymmetry, and extend the secure transmission distance by more than 20-50 km in standard telecom fiber. The results have moved MDI-QKD towards widespread applications in practical network settings, where the channel losses are asymmetric and user nodes could be dynamically added or deleted.

13.
Phys Rev Lett ; 122(12): 120504, 2019 Mar 29.
Artigo em Inglês | MEDLINE | ID: mdl-30978079

##### RESUMO

Finding exponential separation between quantum and classical information tasks is like striking gold in quantum information research. Such an advantage is believed to hold for quantum computing but is proven for quantum communication complexity. Recently, a novel quantum resource called the quantum switch-which creates a coherent superposition of the causal order of events, known as quantum causality-has been harnessed theoretically in a new protocol providing provable exponential separation. We experimentally demonstrate such an advantage by realizing a superposition of communication directions for a two-party distributed computation. Our photonic demonstration employs d-dimensional quantum systems, qudits, up to d=2^{13} dimensions and demonstrates a communication complexity advantage, requiring less than (0.696±0.006) times the communication of any causally ordered protocol. These results elucidate the crucial role of the coherence of communication direction in achieving the exponential separation for the one-way processing task, and open a new path for experimentally exploring the fundamentals and applications of advanced features of indefinite causal structures.

14.
Opt Express ; 27(4): 4727-4733, 2019 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-30876083

##### RESUMO

In this work, we report multispectral superconducting nanowire single photon detectors (SNSPDs) that can simultaneously detect single photons at multiple wavelengths with high efficiency. The superconducting nanowires are fabricated on an all-dielectric mirror consisting of two quarter-wave stack reflectors with separated central wavelengths. The unique optical structure results in serially coupled optical cavities, leading to multiple resonant absorption bands that are utilized for high-efficiency single photon detection. The fabricated detector shows system detection efficiencies of >80% at the three target wavelengths of 1550 nm, 1310 nm, and 1064 nm. The multispectral detector may eliminate the need for multiple SNSPDs for different wavelengths in a system, potentially resulting in a reduction in size, weight, and power, as well as in the cost of the overall detection system. The detector may also find interesting use for applications such as multispectral ranging or imaging.

15.
Appl Opt ; 58(8): 1868-1872, 2019 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-30874050

##### RESUMO

Fast and high efficiency single-photon detectors have important applications in the fields of life science and quantum information. We report, herein, a serially connected two superconducting nanowire avalanche photon detector (SC2-SNAP) fabricated on a dielectric mirror aiming to 630 nm wavelength. This detector shows system detection efficiency (SDE) of 84.8% at a dark count rate of 10 Hz and offers fast detection speed while maintaining a high SDE, where the counting rate reaches 53.9 MHz at an SDE of 60%. This fast and high efficiency single-photon detector may find applications in fluorescence correlation spectroscopy and quantum key distribution.

16.
Opt Lett ; 44(3): 614-617, 2019 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-30702692

##### RESUMO

Quantum clock synchronization schemes utilizing frequency-entangled pulses have flourished for their potentially superior precision to the classical protocols. In this Letter, a new experimental record based on the second-order quantum interference algorithm is reported, to the best of our knowledge. The synchronization accuracy between two parties separated by a 6 km fiber coiling link, which is evaluated by the time offset shift relative to that with the fibers removed, has been measured to be 13±1 ps. The stability in terms of time deviation (TDEV) of 0.81 ps at an averaging time of 100 s has been achieved. The long-term synchronization stability is seen determined by the measurement device, and a minimum stability of 60 fs has been reached at 25,600 s. Furthermore, for the first time to the best of our knowledge, we quantify the performance of this quantum synchronization scheme, and very good agreements with the experimental results have been achieved. According to the quantum simulation, further improvements for both the synchronizing stability and accuracy can be expected.

17.
Opt Express ; 26(22): 29471-29481, 2018 Oct 29.
Artigo em Inglês | MEDLINE | ID: mdl-30470110

##### RESUMO

We report two-photon interferences on a silica-on-silicon chip of Mach-Zehnder interferometer using telecommunication-band correlated photon pairs. The photon pairs were generated by spontaneous four-waving mixing process in a dispersion-shifted fiber. The integrated chip, which was fabricated by standard silica-on-silicon planar lightwave circuit technology, contained a Mach-Zehnder interferometer with a thermo-optic phase shifter. The insertion loss of the interferometer was less than 1 dB. We demonstrated two-photon interferences with both degenerate- and non-degenerate-frequency photon pairs on the Mach-Zehnder interferometer chip. A high fringe visibility was achieved in the interference with nondegenerate-frequency photons. Properties of quantum interference were demonstrated in the interference with degenerate-frequency photon pairs, which is an important way to manipulate the quantum state. These results show great potential of silica-on-silicon photonic chips in applications for the fiber-chip scheme in quantum networks.

18.
Nature ; 562(7728): 548-551, 2018 10.
Artigo em Inglês | MEDLINE | ID: mdl-30287887

##### RESUMO

Randomness is important for many information processing applications, including numerical modelling and cryptography1,2. Device-independent quantum random-number generation (DIQRNG)3,4 based on the loophole-free violation of a Bell inequality produces genuine, unpredictable randomness without requiring any assumptions about the inner workings of the devices, and is therefore an ultimate goal in the field of quantum information science5-7. Previously reported experimental demonstrations of DIQRNG8,9 were not provably secure against the most general adversaries or did not close the 'locality' loophole of the Bell test. Here we present DIQRNG that is secure against quantum and classical adversaries10-12. We use state-of-the-art quantum optical technology to create, modulate and detect entangled photon pairs, achieving an efficiency of more than 78 per cent from creation to detection at a distance of about 200 metres that greatly exceeds the threshold for closing the 'detection' loophole of the Bell test. By independently and randomly choosing the base settings for measuring the entangled photon pairs and by ensuring space-like separation between the measurement events, we also satisfy the no-signalling condition and close the 'locality' loophole of the Bell test, thus enabling the realization of the loophole-free violation of a Bell inequality. This, along with a high-voltage, high-repetition-rate Pockels cell modulation set-up, allows us to accumulate sufficient data in the experimental time to extract genuine quantum randomness that is secure against the most general adversaries. By applying a large (137.90 gigabits × 62.469 megabits) Toeplitz-matrix hashing technique, we obtain 6.2469 × 107 quantum-certified random bits in 96 hours with a total failure probability (of producing a random number that is not guaranteed to be perfectly secure) of less than 10-5. Our demonstration is a crucial step towards transforming DIQRNG from a concept to a key aspect of practical applications that require high levels of security and thus genuine randomness7. Our work may also help to improve our understanding of the origin of randomness from a fundamental perspective.

19.
Phys Rev Lett ; 121(10): 100502, 2018 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-30240268

##### RESUMO

Periodically driven systems have displayed a variety of fascinating phenomena without analogies in static systems, which enrich the classification of quantum phases of matter and stimulate a wide range of research interests. Here, we employ discrete-time quantum walks to investigate a nontrivial topological effect unique to a two-dimensional periodically driven system: chiral edge states can exist at the interface of Floquet insulators whose Chern numbers vanish. Thanks to a resource-saving and flexible fiber-loop architecture, we realize inhomogeneous two-dimensional quantum walks up to 25 steps, over an effective 51×51 lattice with tunable local parameters. Spin-polarized chiral edge states are observed at the boundary of two distinct quantum walk domains. Our results contribute to establishing a well-controlled platform for exploring nontrivial topological phases.

20.
Phys Rev Lett ; 121(8): 080404, 2018 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-30192594

##### RESUMO

Inspired by the recent remarkable progress in the experimental test of local realism, we report here such a test that achieves an efficiency greater than (78%)^{2} for entangled photon pairs separated by 183 m. Further utilizing the randomness in cosmic photons from pairs of stars on the opposite sides of the sky for the measurement setting choices, we not only close the locality and detection loopholes simultaneously, but also test the null hypothesis against local hidden variable mechanisms for events that took place 11 years ago (13 orders of magnitude longer than previous experiments). After considering the bias in measurement setting choices, we obtain an upper bound on the p value of 7.87×10^{-4}, which clearly indicates the rejection with high confidence of potential local hidden variable models. One may further push the time constraint on local hidden variable mechanisms deep into the cosmic history by taking advantage of the randomness in photon emissions from quasars with large aperture telescopes.

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