Applications In Quantum Cryptography Research Articles

Single-photon pump

The authors describe a gated quantum dot device called a single-photon pump that produces single photons on demand through the controlled tunneling of individual electrons and holes. Simulations using an InGaAs∕GaAs dot show that two modes of operation are possible, with one mode particularly attractive for quantum cryptography applications because it cannot produce more than one photon per cycle.

Multistage high-order microring-resonator add-drop filters

We propose and demonstrate a multistage design for microphotonic add-drop filters that provides reduced drop-port loss and relaxed tolerances for achieving high in-band extinction. As a result, the first microring-resonator filters with a rectangular notch stopband in the through port (to our knowledge) are shown, with extinctions exceeding 50 dB. Reaching 30 dB beyond previous results, without postfabrication trimming, such extinction levels open the door to microphotonic notch circuits for spectroscopy, wavelength conversion, and quantum cryptography applications. Combined with a low-loss, high-index-contrast electromagnetic design in SiN and frequency-matched microring resonators, this approach led to the first demonstration of flattop microphotonic filters meeting the stringent criteria for high-spectral-efficiency integrated add-drop multiplexers. The 40 GHz wide filters show a 20 nm free spectral range, 2 dB drop loss, and suppression of adjacent channels by over 30 dB.

Fiber-coupled single-photon detectors based on NbN superconducting nanostructures for practical quantum cryptography and photon-correlation studies

We have fabricated and tested a two-channel single-photon detector system based on two fiber-coupled superconducting single-photon detectors (SSPDs). Our best device reached the system quantum efficiency of 0.3% in the 1540-nm telecommunication wavelength with a fiber-to-detector coupling factor of about 30%. The photoresponse consisted of 2.5-ns-wide voltage pulses with a rise time of 250ps and timing jitter below 40ps. The overall system response time, measured as a second-order, photon cross-correlation function, was below 400ps. Our SSPDs operate at 4.2K inside a liquid-helium Dewar, but their optical fiber inputs and electrical outputs are at room temperature. Our two-channel detector system should find applications in practical quantum cryptography and in antibunching-type quantum correlation measurements.

ZnS:Mn2+ Phosphors Capped with Chitosan

We report on the synthesis of Nanophosphors of Zinc sulphide using a novel capping agent, ‘chitosan’. Water based wet chemical reactions to obtain stable colloids have been done that does not involve any organic solvents. The resultant nanoparticles approximately 30 nm in size, consisting of 3-5 nm crystallites, show a marked improvement in PL compared to other capping agents generally used to passivate the unsaturated defects on the nanoparticle surfaces, showing orange-red color (590 nm) of luminescence even in broad daylight when illuminated with a low power UV source. The high luminescence efficiency renders these nanophosphors ideal for applications in FEDs and quantum cryptography. The presence of biocompatible capping agent, chitosan, makes it a suitable for bio-labels (markers).

Better Single-Photon Emitters

APPLIED PHYSICS The reliable emission of single photons in a particular direction from quantum dots and quantum well structures is an important requisite for applications in quantum information processing and quantum cryptography. However, in many implementations, the photons are emitted in bunches with poor control over the direction in which they are emitted. Bennett et al. use a photolithographically defined pillar-design microcavity to restrict the optical modes into which the photons can be emitted. Using such a design for pillars 1.9 μm in diameter, they demonstrate a reproducible method providing a considerable improvement in both the single-photon emission and control over the direction of emission. — ISO Opt. Exp. 13 , 50 (2005).

Elimination of mode coupling in multimode continuous-variable key distribution

A multimode channel can be utilized to substantially increase the capacity of quantum continuous-variable key distribution. Beyond losses in the channel, an uncontrollable coupling between the modes of the channel typically degrades the capacity of multimode channels. For the key distribution protocol with simultaneous measurement of both complementary quadratures we propose a feasible method to eliminate any undesirable mode coupling by only the receiver's appropriate measurement and data manipulation. It can be used to substantially increase the capacity of the channel, which has an important application in practical continuous-variable quantum cryptography.

Quantum optics of single atoms

AbstractThe widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory using the one‐atom maser or micromaser to produce these fields on demand. In order to achieve this goal the maser is operating under the condition of the so‐called trapping states. In this situation the field of the micromaser stabilises to a photon number state. In the second part of the paper a single photon source on the basis of a trapped ion will be described. The strong localization and position control available when an ion trap is combined with an optical cavity is a big step forward in producing a deterministic atom‐field coupling necessary for quantum information processing.

Continuous generation of single photons with controlled waveform in an ion-trap cavity system

The controlled production of single photons is of fundamental and practical interest; they represent the lowest excited quantum states of the radiation field, and have applications in quantum cryptography and quantum information processing. Common approaches use the fluorescence of single ions, single molecules, colour centres and semiconductor quantum dots. However, the lack of control over such irreversible emission processes precludes the use of these sources in applications (such as quantum networks) that require coherent exchange of quantum states between atoms and photons. The necessary control may be achieved in principle in cavity quantum electrodynamics. Although this approach has been used for the production of single photons from atoms, such experiments are compromised by limited trapping times, fluctuating atom-field coupling and multi-atom effects. Here we demonstrate a single-photon source based on a strongly localized single ion in an optical cavity. The ion is optimally coupled to a well-defined field mode, resulting in the generation of single-photon pulses with precisely defined shape and timing. We have confirmed the suppression of two-photon events up to the limit imposed by fluctuations in the rate of detector dark counts. The stream of emitted photons is uninterrupted over the storage time of the ion, as demonstrated by a measurement of photon correlations over 90 min.

Single photon counting at telecom wavelength and quantum key distribution

A brief overview is given of single photon detector performance requirements for quantum cryptography applications. The analysis is made with respect to restrictions necessary to secure the quantum key distribution channel. InGaAs/InP avalanche photodiode performance is analysed for single photon counting at 1550 nm. Quantum efficiency, dark current and afterpulsing probability (for times up to 100μs after an initial avalanche) are studied in a wider temperature range than previously reported (0deg; C to –80deg;C). We show that photon counting is a bottle-neck in current quantum key distribution systems and provides the source for future performance improvement.

Measurement of number states and phase diffusion using the micromaser

Abstract The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory using the one-atom maser or micromaser to produce these fields on demand. In order to achieve this goal the maser is operating under the condition of the so-called trapping states. In this situation the field of the micromaser stabilises to a photon number state. Recently, we have also succeeded in determining the Wigner function of a single-photon state. Presently measurements of the phase diffusion of the one-atom maser field are underway, whereby the case will be studied in detail when the field approaches a number state. The measurements will also give information on the decoherence of the maser field.

Measurement of number states and phase diffusion using the micromaser

The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory using the one-atom maser or micromaser to produce these fields on demand. In order to achieve this goal the maser is operating under the condition of the so-called trapping states. In this situation the field of the micromaser stabilises to a photon number state. Recently, we have also succeeded in determining the Wigner function of a single-photon state. Presently measurements of the phase diffusion of the one-atom maser field are underway, whereby the case will be studied in detail when the field approaches a number state. The measurements will also give information on the decoherence of the maser field.

Single photon counting at telecom wavelength and quantum key distribution

Abstract A brief overview is given of single photon detector performance requirements for quantum cryptography applications. The analysis is made with respect to restrictions necessary to secure the quantum key distribution channel. InGaAs/InP avalanche photodiode performance is analysed for single photon counting at 1550 nm. Quantum efficiency, dark current and afterpulsing probability (for times up to 100μs after an initial avalanche) are studied in a wider temperature range than previously reported (0deg; C to –80deg;C). We show that photon counting is a bottle-neck in current quantum key distribution systems and provides the source for future performance improvement.

On quantum operations as quantum states

We formalize Jamiolkowski’s correspondence between quantum states and quantum operations isometrically, and harness its consequences. This correspondence was already implicit in Choi’s proof of the operator sum representation of Completely Positive-preserving linear maps; we go further and show that all of the important theorems concerning quantum operations can be derived directly from those concerning quantum states. As we do so the discussion first provides an elegant and original review of the main features of quantum operations. Next (in the second half of the paper) we find more results stemming from our formulation of the correspondence. Thus, we provide a factorizability condition for quantum operations, and give two novel Schmidt-type decompositions of bipartite pure states. By translating the composition law of quantum operations, we define a group structure upon the set of totally entangled states. The question whether the correspondence is merely mathematical or can be given a physical interpretation is addressed throughout the text: we provide formulae which suggest quantum states inherently define a quantum operation between two of their subsystems, and which turn out to have applications in quantum cryptography.

Metal organic vapor phase epitaxy and Raman spectroscopy of InN for nanostructure applications

Self ordered InN dots are grown by Metal Organic Vapor Phase Epitaxy onto GaN, with very low surface densities. Such nano-objects may be employed in the realization of single photon emitters, for quantum cryptography applications. The growth parameters, like growth temperature, V/III molar ratio and deposition time are investigated. First “large” dots are grown, in order to easily assess the material quality, and are studied both by X-ray diffraction and Raman spectroscopy. After optimising the growth conditions, we decrease the deposition time, in order to obtain nanometer size structures. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Indium nitride quantum dots grown by metalorganic vapor phase epitaxy

With respect to growing indium nitride quantum dots with very low surface densities for quantum cryptography applications, we have studied the metalorganic vapor phase epitaxy of InN onto GaN buffer layers. From lattice mismatch results the formation of self-assembled dots. The effects of the growth temperature, V/III molar ratio, and deposition time are studied, and we demonstrate that quantum-sized dots of InN can be grown with a material crystalline quality similar to the quality of the GaN buffer layer, in densities of 107 to 108 cm−2. Such low densities of dots allow for the realization of experiments or devices in which a single dot is isolated, and may be used in the near future to produce single-photon sources.

Generation of photon number states on demand

AbstractThe widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory to produce these fields on demand. Two different methods are discussed. The first is based on the one‐atom maser or the micromaser operating under the conditions of the so‐called trapping states. In this situation the micromaser stabilises to a photon number state. The second device uses a single ion in an optical cavity. The latter setup was recently realised in our laboratory.

Generation of photon number states on demand

The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory to produce these fields on demand. Two different methods are discussed. The first is based on the one-atom maser or micromaser operating under the conditions of the so-called trapping states. In this situation the micromaser stabilises to a photon number state. Recently, we also succeeded in determining the Wigner function of a single-photon state. The second device, recently realized in our laboratory, uses a single trapped ion in an optical cavity.

Generation of non-classical light by single quantum dots

The emission statistics of single semiconductor quantum dots is investigated in view of generating quantum states of light. Correlation experiments show that the emission of a quantum dot can be regulated so as to produce only one photon at a time as well as correlated photon pairs. This observation opens the way to the use of semiconductor quantum dots as triggered sources of single photons, which could find applications in quantum cryptography. We then discuss the potentiality of quantum dots for the production of mutually coherent single photons as well as entangled photons that could be used in quantum information processing schemes.

Generation of photon number states on demand

Abstract The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory to produce these fields on demand. Two different methods are discussed. The first is based on the one-atom maser or micromaser operating under the conditions of the so-called trapping states. In this situation the micromaser stabilises to a photon number state. Recently, we also succeeded in determining the Wigner function of a single-photon state. The second device, recently realized in our laboratory, uses a single trapped ion in an optical cavity.

Time-division phase modulated single-photon interference in a Sagnac interferometer

We introduce a stable, long-distance singlephoton Sagnac interferometer, which has a balanced configuration to efficiently compensate phase drift caused by change of the fiber-optic path. By using time-division phase modulation, single-photon interference was realized at 1550 nm in a 5-km-long as well as 27-km-long Sagnac fiber loops, with a fringe visibility higher than 90% and long-term stability. The stable performance of the single-photon interference indicated that the time-division phase-modulated Sagnac interferometer might readily lead to practical applications in single-photon routing and quantum cryptography.