Session P3b: Timekeeping for Quantum Networking and Other Science Applications

Network Synchronized Source of Indistinguishable Photons
Nijil Lal, National Institute of Standards and Technology (NIST); I.A. Burenkov, NIST & Joint Quantum Institute, University of Maryland; Y-S. Li-Baboud, M.V. Jabir, P.S. Kuo, T. Gerrits, O. Slattery, S.V. Polyakov, NIST & Physics Department, University of Maryland
Location: Beacon B
Date/Time: Wednesday, Jan. 24, 5:08 p.m.

Quantum network uses unique quantum phenomena such as superposition, entanglement, no-cloning etc. for communicating quantum bits of information between interconnected nodes. Most of the network protocols, such as quantum teleportation, entanglement swapping etc., necessitate quantum interference. It is also essential for the scalability of the network that modular sources from different nodes could be plugged in to the network infrastructure and perform quantum algorithms mediated by non-classical interference. To achieve quantum interference between the nodes in a network, the clock synchronization between photon sources as well as the indistinguishability of the single photons are fundamental requisites. In this work, we build a network synchronized source of indistinguishable photons. We study the Allan deviation of the timing jitter associated with the source when synchronized to an external clock, both locally and over a network. It is also important to design a source that would produce indistinguishable photons even when a realistic jitter is present. Here we present our network source of indistinguishable photons. We demonstrate (a) sub-picosecond jitter over an offset defined by the time tagger electronics and (b) a near-perfect quantum coalescence which proves indistinguishability of single photons.
Heralded single photon sources using a spontaneous parametric down-conversion (SPDC) process are known favorites owing to their high generation rate and ease of manipulation. We use a white rabbit switch (WRS) as our external network clock distributor that follows White Rabbit Precision Time Protocol (WR-PTP) which is a part of IEEE 1588-2019 standard. This protocol can extend the synchronization across sources situated at nodes throughout a network of hundreds of kilometers with picosecond level timing jitter. Recent studies employed protocols based on Global Positioning System as well as white rabbit (WR) protocols for synchronization over quantum networks. It is shown that, while GPS fails to reach ps level synchronization, ultra-low jitter provided by WR substantially increases the fidelity of measured entanglement distributed over the network. To take a full advantage of network-based entanglement distribution and other protocols, the sources need to be designed to provide quantum interference with the existing jitter, unavoidable in a realistic network. That is, the photon pulse duration becomes crucial in quantum interference experiments and is limited by two factors the timing jitter associated with the network clock as well as the requirements arising from the transform-limited pulse shape. In the best-case scenario, the white rabbit switch can provide a timing jitter on a picosecond scale, and the pulse duration needs to be significantly larger (~ 10 ps) to establish pulse overlaps for quantum interference. On the other hand, the practical crystal dimensions along with the transform-limited pulse shape, arising from single-mode requirement of the generated photons, set an upper limit for the pulse duration at ~10-50 ps.
Here we build and characterize a source that abides by the above constraints. We use a commercial pulsed laser, with 10 ps pulse duration and 80 MHz repetition rate, to pump a 30 mm quasi-phase-matched PPKTP crystal. The laser is capable of being locked to external clocks through commercial locking electronics. The generated SPDC photon pairs provide heralded single photons, with a detected heralded efficiency (uncorrected for the propagation and detection loss in the photonic channel) of ~104 s-1mW-1 and coincidence-to-accidentals ratio of ~150, where the pump power is capable to go as high as ~ 1 W. We study synchronization between our source and different external clocks. The timing jitter of the pump laser when locked to a white rabbit switch is measured to be 2.40 ps TDEV. The instrument response function of the time tagger provides a lower limit for the measurable jitter to be ~2 ps. After deconvolution of the instrument function, the synchronization measurement yields sub-picosecond timing jitter. The Allan deviation analysis shows a stable downward trend in jitter with time averaging, except for a rise in the millisecond time scale which is explained by the intrinsic timing drift associated with the white rabbit system. This feature is further confirmed by replacing the white rabbit switch with a Rubidium-atomic clock as the external clock, which shows a stable clock synchronization over larger time scales. Thus, apart from clock-specific characteristics, Allan deviation in the timing jitter of the synchronized source is shown to be at least an order of magnitude less than the pulse duration of the interfering single photon. For feasibility of connecting distant nodes, we studied the long-distance synchronization between two white rabbit switches separated by an 88 km fiber spool and the timing jitter is measured to be sub-picosecond for this case as well. We have studied the indistinguishability of the photons generated from the source by performing Hong-Ou-Mandel interference between photons originating from pulses that are far apart in time. This quantum interference shows near-perfect indistinguishability, after correcting for the two photon contributions in coincidences. One can set an upper limit for the measurement jitter in the system by assuming that any degradation of this indistinguishability is solely due to jitter causing imperfect overlap of the interfering single photons. This independent verification shows that the maximum possible jitter in our system is lower than the pulse width of the single photons, which is the goal for this source. The actual jitter is probably significantly smaller since the indistinguishability could be affected by several other factors such as presence of multiple modes, polarization overlap, etc.
In summary, we built a source of indistinguishable photons, in the telecom c-band, successfully synchronized to an external clock that follows a high accuracy precision protocol. This network source could serve as a scalable modular unit in a multi-node, long distance quantum network infrastructure.