Despite the tremendous progress of quantum cryptography, efficient quantum communication over long distances (≥1000 km) remains an outstanding challenge due to fiber attenuation and operation errors accumulated over the entire communication distance. Quantum repeaters (QRs), as a promising approach, can overcome both photon loss and operation errors, and hence significantly speedup the communication rate. Depending on the methods used to correct loss and operation errors, all the proposed QR schemes can be classified into three categories (generations). Here we present the first systematic comparison of three generations of quantum repeaters by evaluating the cost of both temporal and physical resources, and identify the optimized quantum repeater architecture for a given set of experimental parameters for use in quantum key distribution. Our work provides a roadmap for the experimental realizations of highly efficient quantum networks over transcontinental distances.

We develop a layered quantum-computer architecture, which is a systematic framework for tackling the individual challenges of developing a quantum computer while constructing a cohesive device design. We discuss many of the prominent techniques for implementing circuit-model quantum computing and introduce several new methods, with an emphasis on employing surface-code quantum error correction. In doing so, we propose a new quantum-computer architecture based on optical control of quantum dots. The time scales of physical-hardware operations and logical, error-corrected quantum gates differ by several orders of magnitude. By dividing functionality into layers, we can design and analyze subsystems independently, demonstrating the value of our layered architectural approach. Using this concrete hardware platform, we provide resource analysis for executing fault-tolerant quantum algorithms for integer factoring and quantum simulation, finding that the quantum-dot architecture we study could solve such problems on the time scale of days.

Quantum repeaters (QRs) provide a way of enabling long distance quantum communication by establishing entangled qubits between remote locations. In this Letter, we investigate a new approach to QRs in which quantum information can be faithfully transmitted via a noisy channel without the use of long distance teleportation, thus eliminating the need to establish remote entangled links. Our approach makes use of small encoding blocks to fault-tolerantly correct both operational and photon loss errors. We describe a way to optimize the resource requirement for these QRs with the aim of the generation of a secure key. Numerical calculations indicate that the number of quantum memory bits at each repeater station required for the generation of one secure key has favorable poly-logarithmic scaling with the distance across which the communication is desired.PACS numbers: 03.67. Dd, 03.67.Hk, 03.67.Pp. Quantum communication across long distances (10 3 -10 4 km) can significantly extend the applications of quantum information protocols such as quantum cryptography [1] and quantum secret sharing [2, 3] which can be used for the creation of a secure quantum internet [4]. Quantum communication can be carried out by first establishing a remote entangled pair between the sender and the receiver and using teleportation to transmit information faithfully. However, there are two main challenges that have to be overcome. First, fiber attenuation during transmission leads to an exponential decrease in entangled pair generation rate. Second, several operational errors such as channel errors, gate errors, measurement errors and quantum memory errors severely degrade the quality of entanglement used for secure key generation. In addition, quantum states cannot be amplified or duplicated deterministically in contrast to classical information [5]. Establishing quantum repeater (QR) stations based on entanglement distribution is the only currently known approach to long-distance quantum communication using conventional optical fibers without exponential penalty in time and resources.A number of schemes have been proposed for long distance quantum communication using , most of which could be broadly classified into three classes. The first class of QRs [6][7][8][9] reduces the exponential scaling of fiber loss to polynomial scaling by introducing intermediate QR nodes. However, this scheme for long distance quantum communication is relatively slow [13], even after optimization [14], limited by the time associated with two-way classical communication between remote stations required for the entanglement purification process needed to correct operational errors [15]. In contrast, the second class of QRs introduce quantum encoding and classical error correction to replace the entanglement purification with classical error correction, handling all operational errors [10,16]. As a consequence, the entanglement generation rate further improves from 1/O(poly(L tot )) to 1/O(poly(log(L tot ))) where L tot is the total distance of communicat...

Fermion anti-bunching was directly observed by measuring the cross-covariance of the current fluctuations of partitioned electrons. A quantum point contact was used to inject single-mode electrons into a mesoscopic electron beam splitter device. The beam splitter output currents showed negative cross-covariance, indicating that the electrons arrived individually at the beam splitter and were randomly partitioned into two output channels. As the relative time delay between the outputs was changed, the observed ringing in the cross-covariance was consistent with the bandwidths used to monitor the fluctuations. The result demonstrates a fermion complement to the Hanbury Brown and Twiss experiment for photons.

Quantum computing leverages the quantum resources of superposition and entanglement to efficiently solve computational problems considered intractable for classical computers. Examples include calculating molecular and nuclear structure, simulating strongly-interacting electron systems, and modeling aspects of material function. While substantial theoretical advances have been made in mapping these problems to quantum algorithms, there remains a large gap between the resource requirements for solving such problems and the capabilities of currently available quantum hardware. Bridging this gap will require a co-design approach, where the expression of algorithms is developed in conjunction with the hardware itself to optimize execution. Here, we describe a scalable co-design framework for solving chemistry problems on a trapped ion quantum computer, and apply it to compute the ground-state energy of the water molecule. The robust operation of the trapped ion quantum computer yields energy estimates with errors approaching the chemical accuracy, which is the target threshold necessary for predicting the rates of chemical reaction dynamics.

Information encoded in high-dimensional quantum states can achieve ultrahigh rates over metropolitan distances.

Visible light photon counters feature noise-free avalanche multiplication and narrow pulse height distribution for single photon detection events. Such a well-defined pulse height distribution for a single photon detection event, combined with the fact that the avalanche multiplication is confined to a small area of the whole detector, opens up the possibility for the simultaneous detection of two photons. In this letter, we investigated this capability using twin photons generated by parametric down conversion, and present a high quantum efficiency (ϳ47%) detection of two photons with good time resolution (ϳ2 ns), which can be distinguished from a single-photon incidence with a small bit-error rate (ϳ0.63%).

A high-quantum-efficiency single-photon counting system has been developed. In this system, single photons were detected by a visible light photon counter operated at 6.9 K. The visible light photon counter is a solid state device that makes use of avalanches across a shallow impurity conduction band in silicon. Threefold tight shielding and viewports that worked as infrared blocking filters were used to eliminate the dark count caused by room-temperature radiation. Corrected quantum efficiencies as high as 88.2%Ϯ5% ͑at 694 nm͒ were observed, which we believe is the highest reported value for a single-photon detector. The dark count increased as the exponential of the quantum efficiency with changing temperature or bias voltage, and was 2.0ϫ10 4 cps at the highest quantum efficiency. © 1999 American Institute of Physics. ͓S0003-6951͑99͒01308-X͔ Photon counting methods have been widely applied to the accurate measurement of very weak lights, and have recently been used in quantum key distribution systems which encode a data bit to a single photon. The sensitivity of the measurement and the data transmission efficiency depends on the quantum efficiency of photon counters. Therefore, improvements in quantum efficiency have been the main issue in the development of photon counters. Photomultiplier tubes ͑PMTs͒ have been commonly used as the photon counter, however, their quantum efficiency is 25% at its most sensitive wavelength and 15% in the near infrared region. Silicon avalanche photodiodes ͑APDs͒ operating in Geiger mode have become popular, and a quantum efficiency of 76% was recorded with the APD, 1 which is the highest value so far. One of the important applications for such a highquantum-efficiency photon counter is the loophole free test of Bell's inequality. [2][3][4][5] In order to close the loophole, the quantum efficiency of the system has to be higher than 83% when an ordinary Einstein-Podorsky-Rosen ͑EPR͒ pair source is used. 6,7 The loophole free test has not been performed yet, and the lack of a highly efficient single-photon counting system over the threshold is one of the reasons.The visible light photon counter ͑VLPC͒ is an alternative detector, which is a solid state device using the avalanche multiplication effect of electrons in an impurity band in silicon. [8][9][10] The quantum efficiency of a VLPC was estimated to be 93%, but the measured value as a system was less than 70%. 1 The difficulty was that, in order to cope with both ''high quantum efficiency'' and ''low dark counts,'' a special shielding system which has high transmittance for the desired wavelength but filters out room-temperature radiation sufficiently is required. Infrared photons of the radiation up to 28 m in wavelength can excite electrons in the shallow impurity band and might cause large dark count ͑up to 10 15 cps). We developed the cryostat system shown in Fig. 1. Threefold shields at 77, 4, and 6.5 K were used to reduce the thermal photons reaching the detector. We put felt between the shields to block the background...