Michał Lipka

Optical frequency locked loop for long-term stabilization of broad-line DFB laser frequency difference

We present an experimental realization of the optical frequency locked loop applied to long-term frequency difference stabilization of broad-line DFB lasers along with a new independent method to characterize relative phase fluctuations of two lasers. The presented design is based on a fast photodiode matched with an integrated phase-frequency detector chip. The locking setup is digitally tunable in real time, insensitive to environmental perturbations and compatible with commercially available laser current control modules. We present a simple model and a quick method to optimize the loop for a given hardware relying exclusively on simple measurements in time domain. Step response of the system as well as phase characteristics closely agree with the theoretical model. Finally, frequency stabilization for offsets within 4–15 GHz working range achieving <0.1 Hz long-term stability of the beat note frequency for 500 s averaging time period is demonstrated. For these measurements we employ an I/Q mixer that allows us to precisely and independently measure the full phase trace of the beat note signal.

Wavevector multiplexed atomic quantum memory via spatially-resolved single-photon detection

Parallelized quantum information processing requires tailored quantum memories to simultaneously handle multiple photons. The spatial degree of freedom is a promising candidate to facilitate such photonic multiplexing. Using a single-photon resolving camera, we demonstrate a wavevector multiplexed quantum memory based on a cold atomic ensemble. Observation of nonclassical correlations between Raman scattered photons is confirmed by an average value of the second-order correlation function

Feasibility of quantum fingerprinting using optical signals with random global phase

g_{{mathrm{S,AS}}}^{{mathrm{(2)}}} = 72 pm 5

Quantum fingerprinting without a shared phase reference

gS,AS(2)=72±5 in 665 separated modes simultaneously. The proposed protocol utilizing the multimode memory along with the camera will facilitate generation of multi-photon states, which are a necessity in quantum-enhanced sensing technologies and as an input to photonic quantum circuits.Multiplexing of quantum memories could boost the efficiency of photon state preparation. Here, the authors use a cold atomic ensemble and a single-photon resolving camera to exploit emission multiplexing of Raman photons from 665 different angular modes, confirming nonclassical photon-number correlations.

Certification of high-dimensional entanglement and Einstein-Podolsky-Rosen steering with cold atomic quantum memory

We present a quantum fingerprinting protocol relying on two-photon interference which does not require a shared phase reference between the parties preparing optical signals carrying data fingerprints. We show that the scaling of the protocol, in terms of transmittable classical information, is analogous to the recently proposed and demonstrated scheme based on coherent pulses and first-order interference, offering comparable advantage over classical fingerprinting protocols without access to shared prior randomness. We analyze the protocol taking into account non-Poissonian photon statistics of optical signals and a variety of imperfections, such as transmission losses, dark counts, and residual distinguishability. The impact of these effects on the protocol performance is quantified with the help of Chernoff information.

Wavevector-multiplexed and memory-enabled source of multimode nonclassical light

Quantum fingerprinting allows two remote parties to determine whether their datasets are identical or different by sending exponentially less information compared to the classical protocol with equivalent performance. Optical implementations of the quantum protocol based on weak coherent states have been proposed and realized. However, in these realizations phase stability between the sending parties is required. Here we analyze the practical feasibility of the recently introduced quantum fingerprinting protocol based on higher-order interference, which does not require a phase lock between transmitters. We show that an experimental demonstration of a quantum fingerprinting protocol beating the known bound on the performance of classical schemes should be possible using currently available technology.

Spatially-resolved spin manipulation using ac-Stark effect

Microchannel plates (MCP) are the basis for many spatially-resolved single-particle detectors such as ICCD or I-sCMOS cameras employing image intensifiers (II), MCPs with delay-line anodes for the detection of cold gas particles or Cherenkov radiation detectors. However, the spatial characterization provided by an MCP is severely limited by cross-talk between its microchannels, rendering MCP and II ill-suited for autocorrelation measurements. Here we present a cross-talk subtraction method experimentally exemplified for an I-sCMOS based measurement of pseudo-thermal light second-order intensity autocorrelation function at the single- photon level. The method merely requires a dark counts measurement for calibration. A reference cross- correlation measurement certifies the cross-talk subtraction. While remaining universal for MCP applications, the presented cross-talk subtraction in particular simplifies quantum optical setups. With the possibility of autocorrelation measurement the signal needs no longer to be divided into two camera regions for a cross- correlation measurement, reducing the experimental setup complexity and increasing at least twofold the simultaneously employable camera sensor region.

Certification of high-dimensional entanglement and Einstein-Podolsky-Rosen steering with quantum memory

One of the standard communication complexity problems is deciding whether bit strings held by two separate parties Alice (A) and Bob (B) are equal or different. In the simultaneous message passing model, A and B send certain information to a Referee, who makes the decision. The overall goal is to minimize the amount of communication involved while not exceeding desired error probability. This is achieved by sending only fingerprints of the original n-bit strings. In the best classical protocols, these scale as O(√n) if A and B have no access to shared randomness [1]. The recently proposed [2] and demonstrated quantum fingerprinting [3], exploiting phase-modulated weak coherent pulses, provides O(log 2 n) scaling in the number of used qubits, but needs phase stability between A and B. Here we introduce and analyse a quantum fingerprinting protocol based on two-photon interference which does require a shared phase reference and works in scenarios when the global phase of transmitted signals becomes completely random. Interestingly, the performance of this protocol, quantified in terms of the Chernoff information [4], is enhanced through the use of non-classical single-photon states, even in the regime of high transmission loss.