In future quantum communication systems, single photons will be required to possess very narrow linewidths and accurate wavelengths for efficient interaction with quantum memories. Spectral characterization of such single photon sources is necessary and must be performed with very high spectral resolution, wavelength accuracy and detection sensitivity. We propose a method to precisely characterize the spectral properties of narrow-linewidth single-photon sources using an atomic vapor cell based on electromagnetically-induced transparency. We have experimentally demonstrated a spectral resolution of better than 150 kHz, an absolute wavelength accuracy of within 50 kHz and an exceptional detection sensitivity suitable for optical signals as weak as -117 dBm.
We characterize spontaneous parametric downconversion in a domain-engineered, type-II periodically poled lithium niobate (PPLN) crystal using seeded emission and single-photon techniques. Using continuous-wave (CW) pumping at 775 nm wavelength, the signal and idler are at 1532.5 nm and 1567.5 nm, respectively. The domain-engineered crystal simultaneously phasematches signal and idler pairs: [H(1532.5 nm), V(1567.5 nm)] and [V(1532.5 nm), H(1567.5 nm)]. We observe the tuning curves of these processes through difference-frequency generation and through CW fiberassisted, single-photon spectroscopy. These measurements indicate good matching in amplitude and bandwidth of the two processes and that the crystal can in principle be used effectively to generate polarization-entangled photon pairs.
Spontaneous parametric down-conversion (SPDC) is a common method to generate entangled photon pairs for use in quantum communications. The generated single photon linewidth is a critical issue for photon-atom interactions in quantum memory applications. We compare the linewidths of greatly non-degenerate single photon pairs from SPDC generated in the single-pass case and the singly-resonant cavity case. For a 6 mm periodically poled lithium niobate (PPLN) crystal, the linewidth of the generated signal photons is reduced from 1 THz in the single pass case to tens of MHz in the singly-resonant cavity case, while the brightness within the modal lineiwdth is increased by a factor of the cavity finesse, though the overall SPDC generation rate remains unchanged.
Quantum memory is a key device in the implementation of quantum repeaters for quantum communications and quantum networks. We demonstrated a quantum memory based on electromagnetically-induced transparency (EIT) in a warm cesium atomic cell. The quantum memory system can avoid the need for helium temperature apparatus and it is low cost for bulk scalability.
We describe the design and application of domain-engineered, periodically poled lithium niobate (PPLN) for use to
produce entangled photons and for other tools in quantum information and communications. By specially designing and
controlling the PPLN poling pattern, multiple nonlinear optical processes can be simultaneously phasematched. This
capability can be used to generate polarization-entangled photon pairs through type-II spontaneous parametric
downconversion. The single PPLN crystal is designed to produce both the |HV〉 and |VH〉 states where the
downconverted photons are distinguishable by wavelengths, which enables generation of post-selection-free,
polarization-entangled twin photons. We describe the design and fabrication of the PPLN crystal, and initial
experimental results for downconversion of a 775 nm pump to 1532 nm and 1567 nm orthogonally polarized photons.
We also discuss other applications of engineered optical frequency conversion for quantum information including the
use of dual-wavelength upconversion as a beamsplitter to route or analyze photons.
A tunable waveguide-based frequency up-conversion detector is used for single photon level near infrared (IR) spectroscopic measurements. Applications include direct spectroscopic measurement of week near IR signals and remote bi-photon spectroscopy. We have demonstrated direct spectroscopy of single photon near IR signals from a greatly attenuated laser and a single photon source. We further applied the up-conversion spectrometer for frequency correlated bi-photon spectroscopy using a single photon source of non-degenerate photon pairs at 1310 nm (near IR) and 895 nm. In correlated bi-photon spectroscopy, the spectral function at one wavelength range of a remote object can be reproduced by locally measuring another (near IR) wavelength range using the up-conversion spectrometer and monitoring the coincidence counts. A near IR single photon detection efficiency of 32 % has been achieved with the up-conversion spectrometer. The spectral resolution of the system is approximately 0.2 nm at 1310 nm based on the acceptance width of the up-conversion chip used. In bi-photon spectroscopy, the spectral resolution for the correlated photons at 895 nm is approximately 0.1 nm. The sensitivity achieved using the up-conversion detector is -126 dBm at 1310 nm.
A Volume Bragg Grating (VBG) can be used to efficiently extract a narrow bandwidth, highly collimated beam from an otherwise broad spectrum beam. We use a VBG to extract a narrow bandwidth of signal spectrum from a broadband Spontaneous Parametric Down-Conversion source to optimally match the narrow detection bandwidth of our idler upconversion detector. Improved coincidence count rates and visibility can be achieved when limiting signal-spectrum detection to the narrow signal bandwidth whose photons are correlated with a narrow idler-spectrum bandwidth that has been selected by the up-conversion detector. We compare coincidence count rate and visibility for when the entire signal spectrum is detected and when the spectrum has been filtered by the VBG. We further relax the collection techniques and show that following the VBG, the coincidence count rate improves with minimal loss in visibility compared to when the entire spectrum is detected. We introduce our initial efforts at using the VBG to further narrow the signal spectrum by placing it inside a multipass cavity. Additionally, we further adapt the single photon level up-conversion spectrometer, previously developed for idler spectrum measurement, to indirectly measure the single photon level signal spectrum. We verify its capability for several different wavelength and linewidth selections.
We have studied single photon level frequency up-conversion, and developed efficient single photon detectors and a
highly sensitive spectrometer at a telecommunication wavelength (around 1310 nm). We have applied the detector and spectrometer to the implementation of a quantum key distribution system; to the characterization of an entangled photon source and a single photon source from quantum dots; to increase the temporal resolution of the single photon detector; and to study on high-order temporal correlation following frequency conversion. In this paper, we will present an overview on the frequency up-conversion technique and its applications in quantum information systems.
Frequency up-conversion technology can be used to increase detection efficiency for near infrared photons, as has been
demonstrated in fiber-based quantum communication systems. In a continuous wave pumped up-conversion detector,
the temporal resolution is limited by the timing jitter of the detector in the visible range, which limits the maximum
clock rate of a quantum communication system. In this paper we describe a scheme to improve the temporal resolution
of an up-conversion single-photon detector using multi-wavelength optical-sampling techniques, allowing for increased
transmission rates in single-photon communications systems. We experimentally demonstrate our approach with an
up-conversion detector using two spectrally and temporally distinct pump pulses, and show that it allows for high-fidelity
single-photon detection at twice the rate supported by a conventional single-pump up-conversion detector.
We propose a new method to narrow the linewidth of photon pairs generated from spontaneous
parametric down conversion (SPDC). The single structure device incorporates an internal Bragg
grating onto a nonlinear optical waveguide. We study theoretically the spectral characteristics of
SPDC under two Bragg grating structures. We show that using the Bragg grating with a midway π-
phase shifter is a promising way to implement narrow-line (~GHz to sub-GHz) entangled photon
Up-conversion single photon detector technology has become efficient for photons in the near infrared range.
However, its dark count rate is a major concern for some applications in quantum optics. We have theoretically and
experimentally studied the causes of dark counts, and developed an up-conversion detector with an ultra low dark count
rate. A reduced dark count rate of only 320 counts per second is achieved at the maximum overall detection efficiency of
18% and a dark count rate of less than 100 counts per second is achieved at a detection efficiency of 10%. The ultra
low dark count rate enables this type of up-conversion detector to be utilized in a variety of applications where weak
signals in the near IR region are only at a level of few thousand photons per second.
We developed a spectrometer for signals at single photon levels in the near infrared (NIR) region based on a tunable up-conversion
detector. This detector uses a 5-cm periodically poled lithium niobate (PPLN) waveguide to convert NIR
photons to a shorter wavelength that are then detected by a silicon avalanche photodiode. The sensitivity of this
spectrometer is -126 dBm, which is three orders-of-magnitude higher than any commercial optical spectrum analyzer in
this wavelength range. Additionally, we use two PPLN waveguides to implement a polarization-independent up-conversion
spectrometer, and use it to study a fiber-based quantum communication system.
A compact scheme for high-speed frequency doubling and down-conversion on a single dual-element PPKTP
waveguide is investigated. Optimal temperature is achieved and photon pair coincidence is observed at over GHz
repetition rate with pulsed pump input scheme.
We developed low-noise up-conversion single photon detectors for 1310 nm based on a periodically-poled LiNbO3
(PPLN) waveguide. The low-noise feature is achieved by using a pulsed optical pump at a wavelength longer than the
signal wavelength. The detectors were used in a quantum key distribution (QKD) systems based on polarization
encoding, measurement for entangled photon pairs and spectrum measurement at single photon levels. In this paper, the
overall detection efficiency and noise level of the detectors are characterized and the polarization and wavelength
sensitivity of the detection efficiency is analyzed. The applications of this detector in quantum information systems are
The recent advances in superconducting nanowire single-photon detector (SNSPD or SSPD) technology has enabled
long distance quantum key distribution (QKD) over an optical fiber. We point out that the performance of SNSPDs play
a crucial role in achieving a secure transmission distance of 100 km or longer. We analyze such an impact from a
simplified model and use it to interpret results from our differential-phase-shift (DPS) QKD experiment. This allows us
to discuss the optimization of the detection time window and the clock frequency given the detector characteristics such
as dark count rate, detection efficiency, and timing jitter.
Complete high-speed quantum key distribution (QKD) systems over fiber networks for campus and metro areas have
been developed at NIST. The systems include an 850-nm QKD system for a campus network, a 1310-nm QKD system
for metro networks, and a 3-user QKD network and network manager. In this paper we describe the key techniques
used to implement these systems, including polarization recovery, noise reduction, frequency up-conversion detection
based on PPLN waveguide, custom high-speed data handling and network management. A QKD-secured video
surveillance system has been used to experimentally demonstrate these systems.
Detection-time-bin-shift (DTBS) is a scheme that projects the measurement bases or measured photon values into
detection time-bins and then time division multiplexes a single photon detector in a quantum key distribution (QKD)
system. This scheme can simplify the structure of a QKD system, reduce its cost and overcome the security problems
caused by the dead-time introduced self-correlation and the unbalanced characteristics of detectors. In this paper, we
present several DTBS schemes for QKD systems based on attenuated laser pulses and entangled photon sources. We
study the security issues of these DTBS schemes, especially the time-bin-shift intercept-resend attack and its
countermeasures. A fiber-based DTBS QKD system has been developed and its results are presented in this paper.
The desire for quantum-generated cryptographic key for broadband encryption services has motivated the development
of high-transmission-rate single-photon quantum key distribution (QKD) systems. The maximum operational
transmission rate of a QKD system is ultimately limited by the timing resolution of the single-photon detectors and
recent advances have enabled the demonstration of QKD systems operating at transmission rates well in to the GHz
regime. We have demonstrated quantum generated one-time-pad encryption of a streaming video signal with high
transmission rate QKD systems in both free-space and fiber. We present an overview of our high-speed QKD
architecture that allows continuous operation of the QKD link, including error correction and privacy amplification, and
increases the key-production rate by maximizing the transmission rate and minimizing the temporal gating on the
single-photon channel. We also address count-rate concerns that arise at transmission rates that are orders of magnitude
higher than the maximum count rate of the single-photon detectors.
This paper describes the detection of single photons, which have been transmitted through standard fiber at the telecom
wavelength of 1310 nm. Following transmission, the 1310-nm photon is up-converted to 710 nm in a periodical-poled
LiNbO3 (PPLN) waveguide and then detected by a silicon-based avalanche photodiode (Si-APD). The overall detection
efficiency of the detector is 20%. We have also characterized the sensitivity of the PPLN's efficiency to temperature and
wavelength changes. We focused on the noise property of the up-conversion detector. Without classical channel co-propagation,
the dark count rate is 2.2 kHz, which is lower than current up-conversion detectors by more than one order
of magnitude. The up-conversion detector is then applied to a QKD system, which is characterized and is shown to have
a very strong performance.
Quantum Cryptography has demonstrated the potential for ultra-secure communications. However, with quantumchannel
transmission rates in the MHz range, typical link losses and signal-to-noise ratios have resulted in keyproduction
rates that are impractical for continuous one-time-pad encryption of high-bandwidth communications. We have developed high-speed data handling electronics that support quantum-channel transmission rates up to 1.25 GHz.
This system has demonstrated error-corrected and privacy-amplified key rates above 1 Mbps over a free-space link.
While the transmission rate is ultimately limited by timing jitter in the single-photon avalanche photodiodes (SPADs),
we find the timing resolution of silicon SPADs sufficient to operate efficiently with temporal gates as short as 100 ps.
We have developed systems to implement such high-resolution gating in our system, and anticipate the attendant
reduction in noise to produce significantly higher secret-key bitrates.
Free-space Quantum key distribution (QKD) has shown the potential for the practical production of cryptographic key for ultra-secure communications. The performance of any QKD system is ultimately limited by the signal to noise ratio on the single-photon channel, and over most useful communications links the resulting key rates are impractical for performing continuous one-time-pad encryption of today's broadband communications. We have adapted clock and data recovery techniques from modern telecommunications practice, combined with a synchronous classical free-space optical communications link operating in parallel, to increase the repetition rate of a free-space QKD system by roughly 2 orders of magnitude over previous demonstrations. We have also designed the system to operate in the H-alpha Fraunhofer window at 656.28 nm, where the solar background is reduced by roughly 7 dB. This system takes advantage of high efficiency silicon single-photon avalanche photodiodes with <50ps timing resolution that are expected to enable operation at a repetition rate of 2.5 GHz. We have identified scalable solutions for delivering sustained one-time-pad encryption at 10 Mbps, thus making it possible to integrate quantum cryptography into first-generation Ethernet protocols.
A Quantum Key Distribution (QKD) network can allow multi-user communication via secure key. Moreover, by
actively switching communication nodes, one can achieve high key transmission rate for the selected nodes. However,
the polarization properties of different fiber path are different and these properties also randomly drift over time.
Therefore, polarization recovery after the switching and auto-compensation during key transmission are critical for the
QKD network. In this work, we use programmable polarization controllers to implement polarization recovery and
auto-compensation in the QKD network. We will also discuss its time limitation and future improvement.
We previously demonstrated a high speed, point to point, quantum key distribution (QKD) system with polarization
coding over a fiber link, in which the resulting cryptographic keys were used for one-time pad encryption of real time
video signals. In this work, we extend the technology to a three-node active QKD network - one Alice and two Bobs. A
QKD network allows multiple users to generate and share secure quantum keys. In comparison with a passive QKD
network, nodes in an active network can actively select a destination as a communication partner and therefore, its
sifted-key rate can remain at a speed almost as high as that in the point-to-point QKD. We demonstrate our three-node
QKD network in the context of a QKD secured real-time video surveillance system. In principle, the technologies for the
three-node network are extendable to multi-node networks easily. In this paper, we report our experiments, including
the techniques for timing alignment and polarization recovery during switching, and discuss the network architecture and
its expandability to multi-node networks.
NIST has developed a high-speed quantum key distribution (QKD) test bed incorporating both free-space and fiber systems. These systems demonstrate a major increase in the attainable rate of QKD systems: over two orders of magnitude faster than other systems. NIST's approach to high-speed QKD is based on a synchronous model with hardware support. Practical one-time pad encryption requires high key generation rates since one bit of key is needed for each bit of data to be encrypted. A one-time pad encrypted surveillance video application was developed and serves as a demonstration of the speed, robustness and sustainability of the NIST QKD systems. We discuss our infrastructure, both hardware and software, its operation and performance along with our migration to quantum networks.
Proc. SPIE. 6244, Quantum Information and Computation IV
KEYWORDS: Avalanche photodetectors, Coarse wavelength division multiplexing, Clocks, Polarization, Single mode fibers, Computer programming, Vertical cavity surface emitting lasers, Local area networks, Quantum key distribution, Picture Archiving and Communication System
A complete fiber-based polarization encoding quantum key distribution (QKD) system based on the BB84 protocol has been developed at National Institute of Standard and Technology (NIST). The system can be operated at a sifted key rate of more than 4 Mbit/s over optical fiber of length 1 km and mean photon number 0.1. The quantum channel uses 850 nm photons from attenuated high speed VCSELs and the classical channel uses 1550 nm light from normal commercial coarse wavelength division multiplexing devices. Sifted-key rates and quantum error rates at different transmission rates are measured as a function of distance (fiber length). A polarization auto-compensation module has been developed and utilized to recover the polarization state and to compensate for temporal drift. An automatic timing alignment device has also been developed to quickly handle the initial configuration of quantum channels so that detection events fall into the correct timing window. These automated functions make the system more practical for integration into existing optical local area networks.
We have implemented a quantum key distribution (QKD) system with polarization encoding at 850 nm over 1 km of optical fiber. The high-speed management of the bit-stream, generation of random numbers and processing of the sifting algorithm are all handled by a pair of custom data handling circuit boards. As a complete system using a clock rate of 1.25 Gbit/s, it produces sifted keys at a rate of 1.1 Mb/s with an error rate lower than 1.3% while operating at a transmission rate of 312.5 Mbit/s and a mean photon number μ = 0.1. With a number of proposed improvements this system has a potential for a higher key rate without an elevated error rate.
In this paper, a new kind of automatic method to test the Parameters of CD/DVD Optical Discs is introduced and named near-line inspector. Being connected to product lines, sampling and testing the discs automatically in batches, our inspector can provide a brief yet revealing profile of optical disc parameters such as electrical variables (HF, Jitter, etc.), physical variables and digital error variables in an acceptable short time so as to reflect the up-to-date condition of the product lines. As a real-time and relatively high-efficiency measurement method, it will be integrated into a perfect test system together with on-line and off-line inspectors. Such a system would be especially suitable for the replication of high-quality optical discs in large volume. The layout of the whole set of test system is described. The feasibility and reliability of near-line method are analyzed based on study by means of a high-precision DVD off-line test system developed by us.
The process of replication, such as molding, clamping, injection, metalizing, even extrusion and printing, put an important impact on the quality of the finished DVD discs. They are crucial to duplicator, but not so easy to detect by conventional analysis methods provided by current commercial off-line tester. We introduce a data-analysis method to enlarge the effects of replication process on the disc parameters, by means of eliminating the disturbance from pre- replication process. This method is named RES (Remainder Elimination of the effect Stamper) parameters. An example is given to show the status of disc's edges affected by molding, and the selection of different molding patterns is analyzed.
In this paper, the objective of servo system in an optical disc player is discussed. The traditional servo method, zero- order indifferent PID adjustment, is analyzed. Based on the analysis, a new servo system, 1-order indifferent PI&PID adjustment, is provided. In the new adjustment method, the main error of low frequency from ECC and Imbalance will be reduced to minimum in theory. In the end of the paper, the stability of the new servo system is analyzed in detail.
The mechanism of three dimensions digital data storage, including the construction of optical disk, seeking method, principle and method of reading/writing is described in this paper. With the technology of multi-layer, multi-color, longitudinal coding and masking, the new scheme can overcome the bottleneck of 3-D digital storage for tracking, focusing, data rate and do most of the advantages of 3-dimension optical storage for higher density and larger capacity.
The principle of modified repetitive control is analyzed. According to the model of tracking actuator, a simplified design method of modified repetitive control was introduced. The simulation result shows the modified system has good performance. Compared with classical control system, it increases servo accuracy and steady state robustness, decreasing error signal greatly.
Jitter is defined by the variation of each pit length and timing variations between data and clock frequency on discs. As a comprehensive item, it relates with error rate directly. In this paper, the definition, determined and influence factors of jitter n are analyzed. According to the analysis, measurement methods of jitter are provided. The precision of a method. The method's precision and demand are calculated and explained in detail.
In this paper, a writing and reading method of 3D optical digital storage is presented. The novel method is based on dual photon-absorption effective and multi-layer distribution of data. The potential problems of the method are discussed and analyzed.
Magneto optical (MO) disks have shown great advantages in performance, reliability, price per bit, portability, rewrite cycles, high-density recording, and achieve life. For a 3.5' ISO standard MO media, the capacity of 1X-density disks is 128 MB, and 5X density disks have 540 MB/640 MB capacity, with track and bit density increased. Media testing is very important in research and manufacture of the disks. A media testing system for the characteristics of the recording layer of the 5X capacity MO disks is introduced in this paper. Many features and techniques are discussed such as to achieve high accuracy testing, flexibility to upgrade the system.