The human eye contains millions of rod photoreceptor cells, and each one is a single-photon detector. Whether people can actually see a single photon|which requires the rod signal to propagate through the rest of the noisy visual system and be perceived in the brain|has been the subject of research for nearly 100 years. Early experiments hinted that people could see just a few photons, but classical light sources are poor tools for answering these questions. Single-photon sources have opened up a new area of vision research, providing the best evidence yet that humans can indeed see single photons, and could even be used to test quantum effects through the visual system. We discuss our program to study the lower limits of human vision with a heralded single-photon source based on spontaneous parametric downconversion, and present two proposed experiments to explore quantum effects through the visual system: testing the perception of superposition states, and using a human observer as a detector in a Bell test.
We can envision an eventual global multi-node quantum network, with hubs located around the planet. This, however, is still a far reach from current state of the art. Here we discuss some of our approaches to bridge the gap. Specifically, we are pursuing airborne and satellite-based free-space quantum communication. Free-space platforms naturally lend themselves to reconfiguration - likely required by a future quantum-secure network -- as nodes may be easily moved/reoriented to target new nodes. We are implementing a multi-copter drone-based quantum cryptography link, including fast, high-resolution optical stabilization; compact, independent sources; and lightweight single-photon detection. Having access to an agile, reconfigurable QKD networking system will enable quantum cryptography to reach applications prohibited by current approaches, such as temporary networks in seaborne, urban, or even battlefield situations. By using transmitters and receivers at higher altitudes, deleterious effects weather events like fog and turbulence can be mitigated. At longer scale, we are pursuing a quantum link from the International Space Station to earth, which will use hyperentanglement to enable a variety of advanced quantum communication protocols, including multi-bit-per-photon key distribution and "superdense" teleportation. With our table-top experiment we have investigated the effects of loss and turbulence, and demonstrated a system to compensate for the otherwise devastating effect of the Doppler effect from the rapidly moving ISS platform.
As optical quantum information processing protocols and experiments become increasingly more complex, integrated optics provide a small and robust alternative to traditional bulk optics. Specifically, waveguide technology allows for the creation of bright single-photon sources based on the fact that photon pairs can be created at any location along the waveguide. For our goals, we are working on the characterization of a highly nondegenerate Spontaneous Parametric Down-Conversion (SPDC) waveguide source on a periodically poled KTP (PPKTP) crystal. Our current waveguide source uses type-II phase-matching to create collinear signal and idler photons at 1550 nm and 810 nm, respectively, with the promise of generating simultaneous time-bin and polarization entanglement in future iterations. Our intended source application is for use in quantum key distribution and superdense teleportation protocols between a space platform and collection telescopes on Earth.
Superdense Teleportation (SDT) is a suitable protocol to choose for an advanced demonstration of quantum communication in space. We have taken further steps towards the realization of SDT in such an endeavor. Our system uses polarization and time-bin hyperentanglement via non-degenerate spontaneous parametric downconversion to implement SDT of 4-dimensional equimodular states. Previously, we have shown high fidelity (>90%) SDT implementation and the feasibility to perform SDT on an orbiting platform by correcting the Doppler shift. Here we discuss new analysis of the received state reconstruction performance in the presence of high channel loss and multiple pair events. Additionally, initial characterization of a waveguide-based entanglement source intended for space will be presented.
Commercial photon-counting modules, often based on actively quenched solid-state avalanche photodiode sensors, are used in wide variety of applications. Manufacturers characterize their detectors by specifying a small set of parameters, such as detection efficiency, dead time, dark counts rate, afterpulsing probability and single photon arrival time resolution (jitter), however they usually do not specify the conditions under which these parameters are constant or present a sufficient description. In this work, we present an in-depth analysis of the active quenching process and identify intrinsic limitations and engineering challenges. Based on that, we investigate the range of validity of the typical parameters used by two commercial detectors. We identify an additional set of imperfections that must be specified in order to sufficiently characterize the behavior of single-photon counting detectors in realistic applications. The additional imperfections include rate-dependence of the dead time, jitter, detection delay shift, and "twilighting." Also, the temporal distribution of afterpulsing and various artifacts of the electronics are important. We find that these additional non-ideal behaviors can lead to unexpected effects or strong deterioration of the system's performance. Specifically, we discuss implications of these new findings in a few applications in which single-photon detectors play a major role: the security of a quantum cryptographic protocol, the quality of single-photon-based random number generators and a few other applications. Finally, we describe an example of an optimized avalanche quenching circuit for a high-rate quantum key distribution system based on time-bin entangled photons.
Establishing a quantum communication network would provide advantages in areas such as security and information processing. Such a network would require the implementation of quantum teleportation between remote parties. However, for photonic "qudits" of dimension greater than two, this teleportation always fails due to the inability to carry out the required quantum Bell-state measurement. A quantum communication protocol called Superdense Teleportation (SDT) can allow the reconstruction of a state without the usual 2-photon Bell-state measurements, enabling the protocol to succeed deterministically even for high dimensional qudits. This technique restricts the class of states transferred to equimodular states, a type of superposition state where each term can differ from the others in phase but not in amplitude; this restricted space of transmitted states allows the transfer to occur deterministically. We report on our implementation of SDT using photon pairs that are entangled in both polarization and temporal mode. After encoding the phases of the desired equimodular state on the signal photon, we perform a complete tomography on the idler photon to verify that we properly prepared the chosen state. Beyond our tabletop demonstration, we are working towards an implementation between a space platform in low earth orbit and a ground telescope, to demonstrate the feasibility of space-based quantum communication. We will discuss the various challenges presented by moving the experiment out of the laboratory, and our proposed solutions to make Superdense Teleportation realizable in the space setting.
Free-space quantum key distribution (QKD) over water (e.g., ship to ship) may be limited by ship motion and atmospheric effects, such as mode distortion and beam wander due to turbulence. We report on a technique which reduces noise by excluding spatial modes which are less likely to contain QKD signal photons and experimentally demonstrate an improvement in QKD key generation rates in various noise and turbulence regimes.
We present a method, known as hyperdense coding, which uses photons hyperentangled in polarization and temporal
mode to transmit up to 2.81 bits/photon of classical information over a two-qubit quantum channel. Furthermore, the
hyperentangled photons used in this approach are much less susceptible to the influences of turbulence than spatial
qubits, allowing for turbulence-resistant communication. We compare this technique to previously implemented
hyperentanglement-enhanced superdense coding implementations which have a maximum theoretical channel capacity
of 2 bits/photon.
We report the implementation of a novel entanglement-enabled quantum state communication protocol, known as
SuperDense Teleportation, using photons hyperentangled in polarization and orbital angular momentum. We used these
techniques to transmit unimodular ququart states between distant parties with an averaged fidelity of 86.2±3%; almost
twice the classical limit of 44%. We also propose a method to use SuperDense Teleportation to communicate quantum
states from a space platform, such as the International Space Station, to a terrestrial optical telescope. We evaluate
several configurations and investigate the challenges arising from the movement of the space station with respect to the
Efficiently creating optical quantum states, both simple (e.g., pure single-photon states) and complex (e.g., polarization-entangled but spectrally unentangled photon pairs), remains an experimental challenge. We report on a novel method that allows for efficiently preparing certain classes of states: by weakly driving repeated downconversion in a cavity, we can pseudo-deterministically add photons to a state, preparing Fock states of definite photon number. We discuss expected performance and experimental limitations, including the difficulty of creating pure photons at a high rate. Additionally, we report on our progress in engineering high-rate spatio-spectrally unentangled downconversion, a key technology for optical quantum information processing, and propose a novel 4-photon experimental scheme to test the intrinsic indistinguishability of the photons from this source.
Quantum entanglement is known to enable otherwise impossible feats in various communication protocols, such
as quantum key distribution and super-dense coding. Here we describe efforts to further enhance the usual
benefits, by incorporating quantum states that are simultaneously entangled in multiple degrees of freedom -
"hyperentangled". Via the process of spontaneous parametric down conversion, we have demonstrated photon
pairs simultaneously entangled in polarization and spatial mode, and have used these to realize remote entangled
state preparation, full polarization Bell-state analysis, and the highest reported capacity quantum dense coding.
We present an experimental realization of a "sudden mirror replacement" thought experiment, in which a mirror that is inhibiting spontaneous emission is quickly replaced by a photodetector. The question is, can photons be counted immediately, or only after a retardation time that allows the emitter to couple to the changed modes of the cavity, and for light to propagate to the detector? Our results, obtained with a parametric downconverter, are consistent with the cavity QED prediction that photons can be counted immediately, and are in conflict with the retardation time prediction.
Visible light photon counters (VLPCs) and solid-state photomultipliers (SSPMs) are high-efficiency single-photon detectors which have multi-photon counting capability. While both the VLPCs and the SSPMs have inferred internal quantum efficiencies above 93%, the actual measured values for both the detectors were in fact limited to less than 88%, attributed to in-coupling losses. We are currently improving this overall detection efficiency via a) custom anti-reflection coating the detectors and the in-coupling fibers, b) implementing a novel cryogenic design to reduce transmission losses and, c) using low-noise electronics to obtain a better signal-to-noise ratio.
A source of single photons allows secure quantum key distribution, in addition, to being a critical resource for linear optics quantum computing. We describe our progress on deterministically creating single photons from spontaneous parametric downconversion, an extension of the Pittman, Jacobs and Franson scheme [Phys. Rev A, v66, 042303 (2002)]. Their idea was to conditionally prepare single photons by measuring one member of a spontaneously emitted photon pair and storing the remaining conditionally prepared photon until a predetermined time, when it would be "deterministically" released from storage. Our approach attempts to improve upon this by recycling the pump pulse in order to decrease the possibility of multiple-pair generation, while maintaining a high probability of producing a single pair. Many of the challenges we discuss are central to other quantum information technologies, including the need for low-loss optical storage, switching and detection, and fast feed-forward control.
By using a partial polarizer to apply a generalized polarization measurement to one photon of a polarization entangled pair, we remotely prepare single photons in arbitrary polarization qubits. Specifically, we are able to produce a range of states of any desired degree of mixedness or purity, over (and within) the entire Poincare sphere, with a typical fidelity exceeding 99.5%. Moreover, by using non-degenerate entangled pairs as a resource, we can prepare states in multiple wavelengths. Finally, we discuss the states remotely preparable given a particular two-qubit resource state.
The process of up-conversion is shown to enable superior single-photon detectors in the infrared, compared to InGaAs and germanium Avalanche Photodiodes (APDs) (normally used for IR single photon detection). After up-converting an infrared photon to a visible one in a non-linear crystal-Periodically Poled Lithium Niobate (PPLN)-we use a silicon APD to efficiently detect the frequency up-shifted IR photon. We have demonstrated this process at the "high-intensity" level and at the single-photon level, where the up-converted state is effectively a superposition of the single photon Fock state and the vacuum state. We achieve an 80% conversion efficiency over the width of the pulse, and show that this process is coherent, a necessary ingredient for many applications.
Understanding quantum noise is essential for accurately creating desired quantum states and for examining a given state's evolution in any protocol. Using spontaneous parametric downconversion, we can create a wide variety of single- and two-qubit polarization states, including nearly perfect Bell states, mixed states (i.e., "noisy" states) and maximally entangled mixed states (MEMS). To characterize these states we use several different measures, including fidelity, "tangle" and linear entropy. In the course of our experiments, we have discovered and numerically investigated an extreme imbalance in the sensitivity of these different two-qubit state measures. We have also experimentally realized a "Procrustean" filtering technique to remove noise from MEMS. For moderate amounts of filtering, the experimental procedure works as desired to increase the tangle and decrease the linear entropy. However, for large amounts of filtering, the process becomes dominated by perturbations in the starting density matrix. The final outcome is a pure (i.e., zero entropy) product state (i.e., zero entanglement).
Quantum cryptography is a method of communicating securely, the secrecy of which is guaranteed by the laws of physics and information theory. Current implementations suffer from relatively short ranges and low data rates. We are developing a system that modifies the usual protocol by incorporating elements of special relativity. The result is that in principle, every detected photon can be used in the final key, thus doubling or tripling the possible data rate. Our delayed-choice quantum cryptography (DCQC) system works by storing the photon sent to Bob in a low-loss optical delay line until a classical signal from Alice informs him which measurement basis to use.
A number of optical technologies remain to be developed and
optimized for various applications in quantum information processing,
especially quantum communication. We will give an overview of our
approach to some of these, including periodic heralded single-photon sources based on spontaneous parametric down-conversion, ultrabright sources of tunable entangled photons, near unit efficiency single- and multi-photon detectors based on an atomic vapor interaction, quantum state transducers based on high efficiency frequency up-conversion, and low-loss optical quantum memories.
We propose a method of single photon detection of infrared (IR) photons at potentially higher efficiencies and lower noise than allowed by traditional IR band Avalanche Photodiodes (APD). By up-converting the photon from IR, e.g., 1550 nm, to a visible wavelength in a nonlinear crystal, we can utilize the much higher efficiency of visible wavelength APDs. We have used a nonlinear crystal -- Periodically Poled Lithium Niobate (PPLN) -- and a pulsed 1064-nm Nd:YAG laser to perform the up-conversion to a 631-nm photon. When properly quasi-phase-matched, PPLN provides a large enough second order nonlinear susceptibility that near unit conversion efficiency of the IR photon into the visible should be possible. We have been able to observe peak conversion efficiencies as high as 80%, and have demonstrated scaling down to the single photon level while maintaining a background of 3 x 10-4 dark counts/count. Since the PPLN only acts on one polarization of the single photon, we also propose a 2-crystal extension of this scheme whereby orthogonal polarizations may be up-converted coherently, thereby enabling complete quantum state transduction.
Quantum cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light. The security of these transmissions is based on the inviolability of the laws of quantum mechanics and information-theoretically secure post-processing methods. An adversary can neither successfully tap the quantum transmissions, nor evade detection, owing to Heisenberg's uncertainty principle. In this paper we describe the theory of quantum cryptography, and the most recent results from our experimental free-space system with which we have demonstrated for the first time the feasibility of quantum key generation over a point-to-point outdoor atmospheric path in daylight. We achieved a transmission distance of 0.5 km, which was limited only by the length of the test range. Our results provide strong evidence that cryptographic key material could be generated on demand between a ground station and a satellite (or between two satellites), allowing a satellite to be securely re-keyed on orbit. We present a feasibility analysis of surface-to-satellite quantum key generation.
We have demonstrated point-to-point single-photon quantum key distribution over a free-space optical path of approximately 475 m under daylight conditions. This represents an increase of > 1,000 times farther than any reported point-to-point demonstration, and > 6 times farther than the previous folded path daylight demonstration. We expect to extend the daylight range to 2 km or more within the next few months. A brief description of the system is given here.
The secure distribution of the secret random bit sequences known as 'key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is a new technique for secure key distribution with single-photon transmissions: Heisenberg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non- orthogonal photon polarization states to generate shared key material over line-of-sight optical links. Key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. We have developed and tested a free-space quantum key distribution (QKD) system over an outdoor optical path of approximately 1 km at Los Alamos National Laboratory under nighttime conditions. Results show that free-space QKD can provide secure real-time key distribution between parties who have a need to communicate secretly. Finally, we examine the feasibility of surface to satellite QKD.
An experimental free-space quantum key distribution (QKD) system has been tested over an outdoor optical path of approximately 1 km under nighttime conditions at Los Alamos National Laboratory. This system employs the Bennett 92 protocol; here we give a brief overview of this protocol, and describe our experimental implementation of it. An analysis of the system efficiency is presented as well as a description of our error detection protocol, which employs a 2D parity check scheme. Finally, the susceptibility of this system to eavesdropping by various techniques is determined, and the effectiveness of privacy amplification procedures is discussed. Our conclusions are that free-space QKD is both effective and secure; possible applications include the rekeying of satellites in low earth orbit.
The secure distribution of the secret random bit sequences known as `key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions: Heisenburg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over multi-kilometer optical fiber paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single- photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24-km of underground optical fiber using single-photon interference states, demonstrating that secure, real-time key generation over `open' multi-km node-to-node optical fiber communications links is possible.