arXiv subjects

Daniel Oblak

Publications and source records attributed to Daniel Oblak.

50 records · Page 3Linked to original sources

arXiv2015

Quantum storage of entangled telecom-wavelength photons in an erbium-doped optical fibre

The realization of a future quantum Internet requires processing and storing quantum information at local nodes, and interconnecting distant nodes using free-space and fibre-optic links. Quantum memories for light are key elements of such quantum networks. However, to date, neither an atomic quantum memory for non-classical states of light operating at a wavelength compatible with standard telecom fibre infrastructure, nor a fibre-based implementation of a quantum memory has been reported. Here we demonstrate the storage and faithful recall of the state of a 1532 nm wavelength photon, entangled with a 795 nm photon, in an ensemble of cryogenically cooled erbium ions doped into a 20 meter-long silicate fibre using a photon-echo quantum memory protocol. Despite its currently limited efficiency and storage time, our broadband light-matter interface brings fibre-based quantum networks one step closer to reality. Furthermore, it facilitates novel tests of light-matter interaction and collective atomic effects in unconventional materials.

quant-ph↗

arXiv2014

Efficient Bell state analyzer for time-bin qubits with fast-recovery WSi superconducting single photon detectors

We experimentally demonstrate a high-efficiency Bell state measurement for time-bin qubits that employs two superconducting nanowire single-photon detectors with short dead-times, allowing projections onto two Bell states, |Psi>- and |Psi+>. Compared to previous implementations for time-bin qubits, this yields an increase in the efficiency of Bell state analysis by a factor of thirty.

quant-ph↗

arXiv2014

Spectral multiplexing for scalable quantum photonics using an atomic frequency comb quantum memory and feed-forward control

Future multi-photon applications of quantum optics and quantum information science require quantum memories that simultaneously store many photon states, each encoded into a different optical mode, and enable one to select the mapping between any input and a specific retrieved mode during storage. Here we show, with the example of a quantum repeater, how to employ spectrally-multiplexed states and memories with fixed storage times that allow such mapping between spectral modes. Furthermore, using a Ti:Tm:LiNbO3 waveguide cooled to 3 Kelvin, a phase modulator, and a spectral filter, we demonstrate storage followed by the required feed-forward-controlled frequency manipulation with time-bin qubits encoded into up to 26 multiplexed spectral modes and 97% fidelity.

quant-ph↗

arXiv2014

An integrated processor for photonic quantum states using a broadband light-matter interface

Faithful storage and coherent manipulation of quantum optical pulses are key for long distance quantum communications and quantum computing. Combining these functions in a light-matter interface that can be integrated on-chip with other photonic quantum technologies, e.g. sources of entangled photons, is an important step towards these applications. To date there have only been a few demonstrations of coherent pulse manipulation utilizing optical storage devices compatible with quantum states, and that only in atomic gas media (making integration difficult) and with limited capabilities. Here we describe how a broadband waveguide quantum memory based on the Atomic Frequency Comb (AFC) protocol can be used as a programmable processor for essentially arbitrary spectral and temporal manipulations of individual quantum optical pulses. Using weak coherent optical pulses at the few photon level, we experimentally demonstrate sequencing, time-to-frequency multiplexing and demultiplexing, splitting, interfering, temporal and spectral filtering, compressing and stretching as well as selective delaying. Our integrated light-matter interface offers high-rate, robust and easily configurable manipulation of quantum optical pulses and brings fully practical optical quantum devices one step closer to reality. Furthermore, as the AFC protocol is suitable for storage of intense light pulses, our processor may also find applications in classical communications.

quant-ph↗

arXiv2013

Two-photon interference of weak coherent laser pulses recalled from separate solid-state quantum memories

Quantum memories for light, which allow the reversible transfer of quantum states between light and matter, are central to the development of quantum repeaters, quantum networks, and linear optics quantum computing. Significant progress has been reported in recent years, including the faithful transfer of quantum information from photons in pure and entangled qubit states. However, none of these demonstrations confirm that photons stored in and recalled from quantum memories remain suitable for two-photon interference measurements, such as C-NOT gates and Bell-state measurements, which constitute another key ingredient for all aforementioned applications of quantum information processing. Using pairs of weak laser pulses, each containing less than one photon on average, we demonstrate two-photon interference as well as a Bell-state measurement after either none, one, or both pulses have been reversibly mapped to separate thulium-doped titanium-indiffused lithium niobate (Ti:Tm:LiNbO3) waveguides. As the interference is always near the theoretical maximum, we conclude that our solid-state quantum memories, in addition to faithfully mapping quantum information, also preserves the entire photonic wavefunction. Hence, we demonstrate that our memories are generally suitable for use in advanced applications of quantum information processing that require two-photon interference.

quant-ph↗

arXiv2011

Conditional detection of pure quantum states of light after storage in a waveguide

Conditional detection is an important tool to extract weak signals from a noisy background and is closely linked to heralding, which is an essential component of protocols for long distance quantum communication and distributed quantum information processing in quantum networks. Here we demonstrate the conditional detection of time-bin qubits after storage in and retrieval from a photon-echo based waveguide quantum memory. Each qubit is encoded into one member of a photon-pair produced via spontaneous parametric down conversion, and the conditioning is achieved by the detection of the other member of the pair. Performing projection measurements with the stored and retrieved photons onto different bases we obtain an average storage fidelity of 0.885 \pm 0.020, which exceeds the relevant classical bounds and shows the suitability of our integrated light-matter interface for future applications of quantum information processing.

quant-ph↗

arXiv2010

Broadband waveguide quantum memory for entangled photons

The reversible transfer of quantum states of light in and out of matter constitutes an important building block for future applications of quantum communication: it allows synchronizing quantum information, and enables one to build quantum repeaters and quantum networks. Much effort has been devoted worldwide over the past years to develop memories suitable for the storage of quantum states. Of central importance to this task is the preservation of entanglement, a quantum mechanical phenomenon whose counter intuitive properties have occupied philosophers, physicists and computer scientists since the early days of quantum physics. Here we report, for the first time, the reversible transfer of photon-photon entanglement into entanglement between a photon and collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol, and increase the spectral acceptance from the current maximum of 100 MHz to 5 GHz. The entanglement-preserving nature of our storage device is assessed by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer, showing, within statistical error, a perfect mapping process. Our integrated, broadband quantum memory complements the family of robust, integrated lithium niobate devices. It renders frequency matching of light with matter interfaces in advanced applications of quantum communication trivial and institutes several key properties in the quest to unleash the full potential of quantum communication.

quant-ph↗

arXiv2010

Entanglement-assisted atomic clock beyond the projection noise limit

We use a quantum non-demolition measurement to generate a spin squeezed state and to create entanglement in a cloud of 10^5 cold cesium atoms, and for the first time operate an atomic clock improved by spin squeezing beyond the projection noise limit in a proof-of-principle experiment. For a clock-interrogation time of 10 μ\s the experiments show an improvement of 1.1 dB in the signal-to-noise ratio, compared to the atomic projection noise limit.

quant-ph↗

arXiv2009

Squeezing of Atomic Quantum Projection Noise

We provide a framework for understanding recent experiments on squeezing of a collective atomic pseudo-spin, induced by a homodyne measurement on off-resonant probe light interrogating the atoms. The detection of light decimates the atomic state distribution and we discuss the conditions under which the resulting reduced quantum fluctuations are metrologically relevant. In particular, we consider a dual probe scheme which benefits from a cancelation of classical common mode noise sources such that quantum fluctuations from light and atoms are the main contributions to the detected signal.

quant-ph↗

arXiv2008

Echo Spectroscopy of Atomic Dynamics in a Gaussian Trap via Phase Imprints

We report on the collapse and revival of Ramsey fringe visibility when a spatially dependent phase is imprinted in the coherences of a trapped ensemble of two-level atoms. The phase is imprinted via the light shift from a Gaussian laser beam which couples the dynamics of internal and external degrees of freedom for the atoms in an echo spectroscopy sequence. The observed revivals are directly linked to the oscillatory motion of atoms in the trap. An understanding of the effect is important for quantum state engineering of trapped atoms.

quant-ph↗

arXiv2008

Inhomogeneous Light Shift Effects on Atomic Quantum State Evolution in Non-Destructive Measurements

Various parameters of a trapped collection of cold and ultracold atoms can be determined non--destructively by measuring the phase shift of an off--resonant probe beam, caused by the state dependent index of refraction of the atoms. The dispersive light--atom interaction, however, gives rise to a differential light shift (AC Stark shift) between the atomic states which, for a nonuniform probe intensity distribution, causes an inhomogeneous dephasing between the atoms. In this paper, we investigate the effects of this inhomogeneous light shift in non--destructive measurement schemes. We interpret our experimental data on dispersively probed Rabi oscillations and Ramsey fringes in terms of a simple light shift model which is shown to describe the observed behavior well. Furthermore, we show that by using spin echo techniques, the inhomogeneous phase shift distribution between the two clock levels can be reversed.

quant-ph↗

arXiv2007

Compensation of eddy-current-induced magnetic field transients in a MOT

The design and implementation of a current driver for the quadrupole coil of a magneto-optical trap are presented. The control and the power stages of the driver, both based on a push-pull configuration with high-speed transistors, are separated, allowing for a protection of the control electronics. Moreover, the coil current can be set by an analog voltage, a feature that makes possible both fast and adiabatic switching of the quadrupole magnetic field. In order to compensate magnetic field transients induced by eddy currents, the driver allows a quick reversal of the quadrupole coil current providing in such a way the required magnetic flux compensation. From a fit to the measured magnetic field transients, we have extracted the decay constants which confirm the efficiency of the compensation. Furthermore, we measured the influence of eddy currents on a sample of cold caesium atoms, testing at the same time the ultimate performance of the driver for our applications. The results show that the implemented electronic circuit is able to reduce substantially the transient effects with switching times of the magnetic field below 100 microseconds.

physics.ins-det↗

arXiv2006

Non-destructive interferometric characterization of an optical dipole trap

A method for non-destructive characterization of a dipole trapped atomic sample is presented. It relies on a measurement of the phase-shift imposed by cold atoms on an optical pulse that propagates through a free space Mach-Zehnder interferometer. Using this technique we are able to determine, with very good accuracy, relevant trap parameters such as the atomic sample temperature, trap oscillation frequencies and loss rates. Another important feature is that our method is faster than conventional absorption or fluorescence techniques, allowing the combination of high-dynamical range measurements and a reduced number of spontaneous emission events per atom.

quant-ph↗

arXiv2005

Quantum noise limited interferometric measurement of atomic noise: towards spin squeezing on the Cs clock transition

We investigate theoretically and experimentally a nondestructive interferometric measurement of the state population of an ensemble of laser cooled and trapped atoms. This study is a step towards generation of (pseudo-) spin squeezing of cold atoms targeted at the improvement of the Caesium clock performance beyond the limit set by the quantum projection noise of atoms. We calculate the phase shift and the quantum noise of a near resonant optical probe pulse propagating through a cloud of cold 133Cs atoms. We analyze the figure of merit for a quantum non-demolition (QND) measurement of the collective pseudo-spin and show that it can be expressed simply as a product of the ensemble optical density and the pulse integrated rate of the spontaneous emission caused by the off-resonant probe light. Based on this, we propose a protocol for the sequence of operations required to generate and utilize spin squeezing for the improved atomic clock performance via a QND measurement on the probe light. In the experimental part we demonstrate that the interferometric measurement of the atomic population can reach the sensitivity of the order of N_at^1/2 in a cloud of N_at cold atoms, which is an important benchmark towards the experimental realisation of the theoretically analyzed protocol.

quant-ph↗