A protocol is presented for the practical generation and coherent manipulation of high-dimensional frequency-bin entangled photon states using integrated micro-cavities and standard telecommunications components, respectively.
We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional states on-chip in a practical way have remained elusive due to the increasing complexity of the quantum circuitry needed to prepare and process such states. Here, we outline how high-dimensional, frequency-bin entangled, two-photon states can be generated at a stable, high generation rate by using a nested-cavity, actively mode-locked excitation of a nonlinear micro-cavity. This technique is used to produce pulsed quantum frequency combs. Moreover, we present how the quantum states can be coherently manipulated using standard telecommunications components such as programmable filters and electro-optic modulators. In particular, we show in detail how to accomplish state characterization measurements such as density matrix reconstruction, coincidence detection, and single photon spectrum determination. The presented methods form an accessible, reconfigurable, and scalable foundation for complex high-dimensional state preparation and manipulation protocols in the frequency domain.
The control of quantum phenomena opens the possibility for new applications in fields as diverse as secure quantum communications1, powerful quantum information processing2, and quantum sensing3. While a variety of physical platforms are actively being researched for the realizations of quantum technologies4, optical quantum states are important candidates as they can exhibit long coherence times and stability from external noise, excellent transmission properties, as well as compatibility with existing telecommunications and silicon chip (CMOS) technologies.
Towards fully realizing the potential of photons for quantum technologies, state complexity and information content can be increased through the use of multiple entangled parties and/or high-dimensionality. However, the on-chip generation of such optical states lacks practicality as setups are complicated, not perfectly scalable, and/or use highly-specialized components. Specifically, high-dimensional path-entanglement requires coherently-excited identical sources and elaborate circuits of beam-splitters5 (where is the state dimensionality), while time-entanglement needs complex multi-arm interferometers6. Remarkably, the frequency-domain is well-suited for the scalable generation and control of complex states, as shown by its recent exploitation in quantum frequency combs (QFC)7,8 using a combination of integrated optics and telecommunication infrastructures9, and provides a promising framework for future quantum information technologies.
On-chip QFCs are generated using nonlinear optical effects in integrated micro-cavities. Using such a nonlinear micro-resonator, two entangled photons (noted as signal and idler) are produced by spontaneous four-wave mixing, via the annihilation of two excitation photons – with the resultant pair generated in a superposition of the cavity's evenly-spaced resonant frequency modes (Figure 1). If there is coherence between the individual frequency modes, a frequency-bin entangled state is formed10, which is often referred to as a mode-locked two photon state11. This state wave-function can be described by,
Here, and are the single-frequency-mode idler and signal components, respectively, and is the probability amplitude for the -th signal-idler mode pair.
Previous demonstrations of on-chip QFCs highlight their versatility as viable quantum information platforms, and include combs of correlated photons12, cross-polarized photons13, entangled photons14,15,16, multi-photon states15, and frequency-bin entangled states9,17. Here, we provide a detailed overview of the QFC platform and a protocol for high-dimensional frequency-bin entangled optical state generation and control.
Future quantum applications, especially those to be interfaced with high-speed electronics (for timely information processing), demand the high-rate generation of high-purity photon states in a compact and stable setup. We use an actively mode-locked, nested cavity scheme to produce QFCs within the telecommunications S, C, and L frequency bands. A micro-ring is incorporated into a larger pulsed laser cavity, with optical gain (provided by an erbium-doped fiber amplifier, EDFA) filtered to match the micro-ring excitation bandwidth18. Mode-locking is actively realized via electro-optic modulation of the cavity losses19. An isolator ensures that pulse propagation follows a single direction. The resulting pulse train has very low root mean square (RMS) noise and exhibits tunable repetition rates and pulse powers. A high isolation notch filter separates the emitted QFC photons from the excitation field. These single photons are then guided through fibers for control and detection.
Our scheme is a step towards a high generation-rate, small-footprint QFC source, as all components used can potentially be integrated onto a photonic chip. Additionally, pulsed excitation is particularly well-suited for quantum applications. First, looking at a pair of micro-cavity resonances symmetric to the excitation, it generates two-photon states where each photon is characterized by a single-frequency mode– central for linear optical quantum computing20. As well, multi-photon states can be generated by moving to higher power excitation regimes and selecting multiple signal-idler pairs15. Second, as photons are emitted in known time windows corresponding to the pulsed excitation, post-processing and gating can be implemented to improve state detection. Perhaps most significantly, our scheme supports high generation rates of photon states using harmonic mode-locking without reducing the coincidence-to-accidental ratio (CAR) – which could pave the way for high-speed, multi-channel quantum information technologies.
To demonstrate the impact and feasibility of the frequency-domain, control of QFC states must be accomplished in targeted ways, ensuring highly efficient transformations and state coherence. To satisfy such requirements, we use cascaded programmable filters and phase modulators – established components in the telecommunications industry. Programmable filters can be used to impose an arbitrary spectral amplitude and phase mask on the single photons, with a resolution sufficient to address each frequency mode individually; and electro-optic phase modulators driven by radio-frequency (RF) signal generators facilitate the mixing of frequency components21.
The most important aspect of this control scheme is that it operates on all quantum modes of the photons simultaneously in a single spatial mode, using single control elements. Increasing the quantum state dimensionality will not lead to an increase in the setup complexity, in contrast to path- or time-bin entanglement schemes. As well, all components are externally reconfigurable (meaning the operations can be altered without amending the setup) and use existing telecommunications infrastructure. Thus, existing and upcoming developments in the field of ultrafast optical processing can be directly transferred to the scalable control of quantum states in the future.
In summary, the exploitation of the frequency-domain by QFCs supports the high-rate generation of complex quantum states and their control, and, is thus well-suited for the harnessing of complex states towards practical and scalable quantum technologies.
1. Generation of the High-dimensional Frequency-bin Entangled States via Pulsed Excitation
2. Control of the High-dimensional Frequency-bin Entangled States
3. Processing of the High-dimensional Frequency-bin Entangled States
The outlined scheme for the generation and control of high-dimensional frequency-bin states (based on the excitation of nonlinear micro-cavities, Figure 1) is shown in Figure 2. This setup uses standard telecommunications components and is highly flexible in the photon production rate and the processing operations applied. Figure 3 shows the characterization of the generation scheme through the coincidence rate and CAR as function of the repetition rate, demonstrating that the production of photon pairs can be increased without decreasing the CAR. In the control section, programmable filters and phase modulators (Figure 4A) allow coherent control of the photon wavefunctions. Such a control scheme is used to perform quantum state tomography of a , two-photon system to reconstruct the state density matrix, as shown in Figure 4B. The results demonstrate excellent agreement between the measured and maximally entangled states, with an achieved fidelity of 80.9%.
Figure 1: Pulsed quantum frequency comb generation. A pulsed field excites a single nonlinear micro-cavity resonance (green). Spontaneous four-wave mixing mediates the annihilation of two photons from the excitation spectral-mode and the generation of two daughter photons, called signal and idler (red and blue), spectrally symmetric to the excitation. The photon pair is also in a quantum superposition of the frequency modes defined by the resonances, such that in the eigenbasis defined by the state Hamiltonian, the wavefunction is represented by a normalized sum of the symmetric frequency-mode eigenvectors. Please click here to view a larger version of this figure.
Figure 2: Platform for practical high-dimensional quantum state generation and control. The micro-ring resonator26,27 is embedded in a larger, external cavity. This external cavity comprises an active electro-optic amplitude modulator driven by a signal generator, an optical gain component, and a narrow band-pass filter, with the latter limiting the circulating excitation pulse to a pass-band corresponding to a single micro-cavity resonance. Quantum frequency combs generated through this scheme (Figure 1) are filtered from the excitation field and pass on to the control stage via a notch filter. Here, a concatenation of programmable filters and electro-optic phase modulators (driven by an amplified signal from an RF signal generator) can be used for manipulating the state. In the processing stage, the idler and signal photons are routed to separate single-photon detectors using a DWDM, and the time delay is measured using timing electronics. Please click here to view a larger version of this figure.
Figure 3: The measured coincidence rate (top) and coincidence-to-accidental ratio (CAR) (bottom) for photon pairs corresponding to the signal-2 and idler-2 frequency modes as a function of increasing repetition rate for harmonically mode-locked pulsed excitation. As the pulse shape and peak powers were maintained for different repetition rates, the coincidence rate was found to grow linearly while the CAR was largely preserved. The slight reduction in CAR and its imperfect linear decrease is imputable to small deviations from the targeted excitation power. The error bars correspond to the standard deviation calculated for five measurements. Please click here to view a larger version of this figure.
Figure 4: The generation of sidebands via electro-optic phase modulation (top) and example density matrix reconstruction for D = 3 (bottom). (a) Frequency sideband generation by an electro-optic modulator as a function of the frequency , with side-bands spaced by the frequency of the modulating signal, . FSR: example free spectral range of a micro-ring resonator. (b) Experimental density matrix reconstruction of a D = 3 frequency-bin entangled two-photon state (real and imaginary parts on the left and right, respectively). Please click here to view a larger version of this figure.
The optical frequency-domain, via QFCs, is advantageous in quantum applications for a host of reasons. Operations are global, acting on all states simultaneously, which results in a design that does not scale in size or complexity as the state dimensionality increases. This is enhanced as the components can be reconfigured on-the-fly without changing the setup and are capable of being integrated on-chip by exploiting existing and/or developing semiconductor and telecommunications infrastructures. The generation techniques could also be adopted for other optical micro-cavities — such as second-order nonlinear micro-cavities28, micro-disks29, photonic crystal waveguides30,31, etc.
Advances in the excitation scheme will pave the way for high production rates, necessary for quantum information processing applications. While the production rate of our generation scheme can be increased by mode-locking at higher harmonic frequencies, supermode noise can lead to instabilities at these higher repetition rates. Suppression of this noise could be accomplished with techniques such as cavity length modulation32,33, nonlinear compensation34, and high-finesse supermode filtering techniques35,36.
Improvements in the system will result in even higher photon production rates. The total losses for the control portion was 14.5 dB (1 dB for the notch filter, 4.5 dB for the first programmable filter, 3.5 dB for the phase modulator, and 4.5 dB for the second programmable filter). Production rates could be increased many-fold through realizable reductions in losses – with a readily available improvement of 5 dB by integrating many of the control components used in the setup into a single compact, lower-loss optical chip.
Improved control of the frequency-mode mixing through better targeted side-band creation will provide more efficient gates and higher production rates. As the probability scattering depends on the modulation driving signal (pattern, frequency, and amplitude) and electro-optic modulator specifications, these must be in the realm to effectively overlap modes (generate side-bands) at the desired mixing frequencies — requiring RF (GHz) signal speeds, state-of-the-art voltage amplifiers and low phase modulators.
Current programmable filters are limited in the spectral bandwidth and resolution; the equipment used in the original demonstrations had a bandwidth from 1527.4 nm to 1567.5 nm and a resolution of 12.5 GHz. With a micro-ring FSR of 200 GHz, this programmable filter provides access to 10 signal and 10 idler frequencies. The dimensionality of these quantum states could readily reach values upwards of (corresponding to as many as 14 qubits) with advances in programmable filter bandwidth/resolution and optical cavity FSR — all without increasing the footprint of the setup.
With the QFC platform outlined here, we demonstrate the generation and control of complex quantum states in a compact, reconfigurable, and practical way. The highlights of our schemes are the capability for high generation rates of pure single photons and global operation on all states with single components, allowing scalability in the form of mass-produced, low-cost, integrated photonic chips and accessible telecommunications components. Using this QFC platform, significant steps are made towards quantum information processing technologies. Quantum communication at high rates is realizable with multiplexed channels, allowing secure information transfer at very efficient rates, while high-dimensional quantum computing is a developing field that could help overcome the limitations of qubit-based computation37.
We thank R. Helsten for technical insights; P. Kung from QPS Photronics for the help and processing equipment; as well as QuantumOpus and N. Bertone of OptoElectronics Components for their support and for providing us with state-of-the-art photon detection equipment. This work was made possible by the following funding sources: Natural Sciences and Engineering Research Council of Canada (NSERC) (Steacie, Strategic, Discovery, and Acceleration Grants Schemes, Vanier Canada Graduate Scholarships, USRA Scholarship); Mitacs (IT06530) and PBEEE (207748); MESI PSR-SIIRI Initiative; Canada Research Chair Program; Australian Research Council Discovery Projects (DP150104327); European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant (656607); CityU SRG-Fd program (7004189); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB24030300); People Programme (Marie Curie Actions) of the European Union's FP7 Programme under REA grant agreement INCIPIT (PIOF-GA-2013-625466); Government of the Russian Federation through the ITMO Fellowship and Professorship Program (Grant 074-U 01); 1000 Talents Sichuan Program (China)
Superconducting Nanowire Single-Photon Detector System | Quantum Opus | Opus One | |
Electro-optic phase modulator | EO-Space | Low loss model | |
Programmable filter | Finisar | WaveShaper 4000s | |
Timing electronics | PicoQuant | HydraHarp 400 | |
Micro-ring resonator | 200 GHz FSR micro-ring resonator made from high refractive index glass. See Ref. 24 for platform details. | ||
Erbium-doped fiber amplifier | Keopsys | PEFA-SP-C-PM-27-B202-FA-FA | |
Electro-optic amplitude modulator | Oclaro | SD40 | |
RF tone source | Rohde & Schwarz | SMP 04 | |
RF tone amplifier | RF-Lambda | RFLUPA27G34GA | |
Function generator | Tetronix | AFG 3251 | |
Isolator | General Photonics | NISO-S-15-SS-FC/APF | |
Oscilloscope | Tetronix | TDS5052B | |
Photodiode | Finisar | XPDV 50 GHz | |
DWDM | OptiWorks | DWFUQUMD08BN | |
Power supply | Madell | CA18303D |