arXiv subjects

Gunnar Björk

Publications and source records attributed to Gunnar Björk.

At least 19 recordsLinked to original sources

arXiv2020

Temporal array with superconducting nanowire single-photon detectors for photon-number-resolution

We present an experimental realization of a 16 element, temporal-array, photon-number-resolving (PNR) detector, which is a multiplexed single-photon detector that splits an input signal over multiple time-bins, and the time-bins are detected using two superconducting nanowire single-photon detectors (SNSPD). A theoretical investigation of the PNR capabilities of the detector is performed and it is concluded that compared to a single-photon detector, our array detector can resolve one order of magnitude higher mean photon numbers, given the same number of input pulses to measure. This claim is experimentally verified and we show that the detector can accurately predict photon numbers between $10^{-3}$ to $10^{2}$. Our present detector is incapable of single-shot photon-number measurements with high precision since its effective quantum efficiency is $49\,\%$. Using SNSPDs with a higher quantum efficiency the PNR performance will improve, but the photon-number resolution will still be limited by the array size.

quant-ph↗

arXiv2020

Contrast resolution of few-photon detectors

We analyse the capability of distinguishing between different intensities in a monochromatic, pixellated image acquisition system at low light intensities. In practice, the latter means that each pixel detects a countable number of photons per acquired image frame. Primarily we compare systems based on pixels of the click-detection type and photon-number resolving (PNR) type of detectors, but our model can seamlessly interpolate between the two. We also discuss the probability of errors in assigning the correct intensity (or "gray level"), and finally we discuss how the estimated levels, that need to be based on threshold levels due to the stochastic nature of the detected photon number, should be assigned. Overall, we find that PNR detector-based system offer advantages over click-detector-based systems even under rather non-ideal conditions.

physics.ins-det↗

arXiv2019

Photon Counting Distribution for Arrays of Single-Photon Detectors

We derive a computationally efficient expression of the photon counting distribution for a uniformly illuminated array of single photon detectors. The expression takes the number of single detectors, their quantum efficiency, and their dark-count rate into account. Using this distribution we compute the error of the array detector by comparing the output to that of a ideal detector. We conclude from the error analysis that the quantum efficiency must be very high in order for the detector to resolve a hand-full of photons with high probability. Furthermore, we conclude that in the worst-case scenario the required array size scales quadratically with the number of photons that should be resolved. We also simulate a temporal array and investigate how large the error is for different parameters and we compute optimal size of the array that yields the smallest error.

quant-ph↗

arXiv2018

Evaluating Performance of Photon-Number-Resolving Detectors

We analyze the performance of photon-number-resolving (PNR) detectors and introduce a figure of merit for the accuracy of such detectors. This figure of merit is the (worst-case) probability that the photon-number-resolving detector correctly predicts the input photon number. Simulations of various PNR detectors based on multiplexed single-photon `click detectors' is performed. We conclude that the required quantum efficiency is very high in order to achieve even moderate (up to a handful) photon resolution, we derive the required quantum efficiency as a function of the the maximal photon number one wants to resolve, and we show that the number of click detectors required grows quadratically with the maximal number of photons resolvable.

quant-ph↗

arXiv2017

Two-photon interference from two blinking quantum emitters

We investigate the effect of blinking on the two-photon interference measurement from two independent quantum emitters. We find that blinking significantly alters the statistics in the second-order intensity correlation function g$^{(2)}(τ)$ and the outcome of two-photon interference measurements performed with independent quantum emitters. We theoretically demonstrate that the presence of blinking can be experimentally recognized by a deviation from the g$^{(2)}_{D}(0)=0.5$ value when distinguishable photons impinge on a beam splitter. Our results show that blinking imposes a mandatory cross-check measurement to correctly estimate the degree of indistinguishablility of photons emitted by independent quantum emitters.

cond-mat.mes-hall↗

arXiv2017

Experimental characterization of quantum polarization of three-photon states

We experimentally investigate various quantum polarization features of three-photon quantum states, including product and entangled states with varying purity. The three-photon quantum states are categorized into six classes based on the rotation symmetry of mean, variance, and skewness of the polarization distribution. The representative three-photon quantum states in each category is prepared from double-pair emission from pulsed spontaneous parametric down-conversion and quantum interferometry. We demonstrate that the three-photon quantum states show interesting quantum polarization properties, such as, maximum sum-uncertainty and hidden polarizations.

quant-ph↗

arXiv2016

Fidelity as a figure of merit in quantum error correction

We discuss the fidelity as a figure of merit in quantum error correction schemes. We show that when identifiable but uncorrectable errors occur as a result of the action of the channel, a common strategy that improves the fidelity actually decreases the transmitted mutual information. The conclusion is that while the fidelity is simple to calculate and therefore often used, it is perhaps not always a recommendable figure of merit for quantum error correction. The reason is that while it roughly speaking encourages optimisation of the "mean probability of success", it gives no incentive for a protocol to indicate exactly where the errors lurk. For small error probabilities, the latter information is more important for the integrity of the information than optimising the mean probability of success.

quant-ph↗

arXiv2016

Quantum-polarization state tomography

We propose and demonstrate a method for quantum-state tomography of qudits encoded in the quantum polarization of $N$-photon states. This is achieved by distributing $N$ photons nondeterministically into three paths and their subsequent projection, which for $N=1$ is equivalent to measuring the Stokes (or Pauli) operators. The statistics of the recorded $N$-fold coincidences determines the unknown $N$-photon polarization state uniquely. The proposed, fixed setup manifestly rules out any systematic measurement errors due to moving components and allows for simple switching between tomography of different states, which makes it ideal for adaptive tomography schemes.

quant-ph↗

arXiv2016

SU(2) uncertainty limits

Although progress has been made recently in defining nontrivial uncertainty limits for the SU(2) group, a description of the intermediate states bound by these limits remains lacking. In this paper we enumerate possible uncertainty relations for the SU(2) group that involve all three observables and that are, moreover, invariant under SU(2) transformations. We demonstrate that these relations however, even taken as a group, do not provide sharp, saturable bounds. To find sharp bounds, we systematically calculate the variance of the SU(2) operators for all pure states belonging to the $N=2$ and $N=3$ polarisation excitation manifold (corresponding to spin 1 and spin 3/2). Lastly, and perhaps counter to expectation, we note that even pure states can reach the maximum uncertainty limit.

quant-ph↗

arXiv2015

Mode density of orbital angular momentum modes in a cylindrical cavity and in free space

In this short note we derive an approximate expression for the mode density of modes carrying orbital angular momentum (OAM) in a cylindrical cavity which is large compared to the inverse wavevector in any direction. We argue that in the large cavity limit the modes asymptotically converges to the corresponding OAM modes in free space. We compare the result to Weyl's law. It is found, as expected, that the mode density does not depend on whether or not the modes carry OAM.

physics.class-ph↗

arXiv2013

Non-monotonic dependence of projection probabilities as a function of distinguishability

Typically, quantum superpositions, and thus measurement projections of quantum states involving interference, decrease (or increase) monotonically as a function of increased distinguishability. Distinguishability, in turn, can be a consequence of decoherence, for example caused by the (simultaneous) loss of excitation or due to inadequate mode matching (either deliberate or indeliberate). It is known that for some cases of multi-photon interference, non-monotonic decay of projection probabilities occurs, which has so far been attributed to interference between four or more two photons. We show that such a non-monotonic behaviour of projection probabilities is not unnatural, and can also occur for single-photon and even semiclassical states. Thus, while the effect traces its roots from indistinguishability and thus interference, the states for which this can be observed do not need to have particular quantum features.

quant-ph↗

arXiv2013

Synthesis of arbitrary, two-mode, high visibility N-photon interference patterns

Using coherent states and linear optics, we demonstrate the synthesis of arbitrary interference patterns and establish that neither the shape nor the visibility of N-photon interference patterns can be used as a quantum signature in general. Specific examples include saw-curve and rectangle curve interference patterns, and phase super-resolution with period shortening of up to 60 times compared to ordinary interference. The former two with a visibility close to 100 % and the latter with a visibility in excess of 57 %.

quant-ph↗

arXiv2012

Experimental determination of the degree of quantum polarisation of continuous variable states

We demonstrate excitation-manifold resolved polarisation characterisation of continuous-variable (CV) quantum states. In contrast to traditional characterisation of polarisation that is based on the Stokes parameters, we experimentally determine the Stokes vector of each excitation manifold separately. Only for states with a given photon number does the methods coincide. For states with an indeterminate photon number, for example Gaussian states, the employed method gives a richer and more accurate description. We apply the method both in theory and in experiment to some common states to demonstrate its advantages.

quant-ph↗

arXiv2011

Deterministic teleportation using single-photon entanglement as a resource

We outline a proof that teleportation with a single particle is in principle just as reliable as with two particles. We thereby hope to dispel the skepticism surrounding single-photon entanglement as a valid resource in quantum information. A deterministic Bell state analyzer is proposed which uses only classical resources, namely coherent states, a Kerr non-linearity, and a two-level atom.

quant-ph↗

arXiv2011

Realistic limits on the nonlocality of an N-partite single-photon superposition

A recent paper [L. Heaney, A. Cabello, M. F. Santos, and V. Vedral, New Journal of Physics, 13, 053054 (2011)] revealed that a single quantum symmetrically delocalized over N modes, namely a W state, effectively allows for all-versus-nothing proofs of nonlocality in the limit of large N. Ideally, this finding opens up the possibility of using the robustness of the W states while realizing the nonlocal behavior previously thought to be exclusive to the more complex class of Greenberger-Horne-Zeilinger (GHZ) states. We show that in practice, however, the slightest decoherence or inefficiency of the Bell measurements on W states will degrade any violation margin gained by scaling to higher N. The non-statistical demonstration of nonlocality is thus proved to be impossible in any realistic experiment.

quant-ph↗

arXiv2011

On the efficiency of quantum lithography

Quantum lithography promises, in principle, unlimited feature resolution, independent of wavelength. However, in the literature at least two different theoretical descriptions of quantum lithography exist. They differ in to which extent they predict that the photons retain spatial correlation from generation to the absorption, and while both predict the same feature size, they differ vastly in predicting how efficiently a quantum lithographic pattern can be exposed. Until recently, essentially all experiments reported have been performed in such a way that it is difficult to distinguish between the two theoretical explanations. However, last year an experiment was performed which gives different outcomes for the two theories. We comment on the experiment and show that the model that fits the data unfortunately indicates that the trade-off between resolution and efficiency in quantum lithography is very unfavourable.

quant-ph↗

arXiv2010

Arbitrarily High Super-Resolving Phase Measurements at Telecommunication Wavelengths

We present two experiments that achieve phase super-resolution at telecommunication wavelengths. One of the experiments is realized in the space domain and the other in the time domain. Both experiments show high visibilities and are performed with standard lasers and single-photon detectors. The first experiment uses six-photon coincidences, whereas the latter needs no coincidence measurements, is easy to perform, and achieves, in principle, arbitrarily high phase super-resolution. Here, we demonstrate a 30-fold increase of the resolution. We stress that neither entanglement nor joint detection is needed in these experiments, demonstrating that neither is necessary to achieve phase super-resolution.

quant-ph↗

arXiv2008

Entanglement invariant for the double Jaynes-Cummings model

We study entanglement dynamics between four qubits interacting through two isolated Jaynes-Cummings hamiltonians, via the entanglement measure based on the wedge product. We compare the results with similar results obtained using bipartite concurrence resulting in what is referred to as "entanglement sudden death". We find a natural entanglement invariant under evolution demonstrating that entanglement sudden death is caused by ignoring (tracing over) some of the system's degrees of freedom that become entangled through the interaction.

quant-ph↗