Due to its coherence properties and high optical depth, a Bose-Einstein condensate provides an id... more Due to its coherence properties and high optical depth, a Bose-Einstein condensate provides an ideal setting to investigate collective atom-light interactions. Superradiant light scattering in a Bose-Einstein condensate is a fascinating example of such an interaction. It is an analogous process to Dicke superradiance, in which an electronically inverted sample decays collectively, leading to the emission of one or more light pulses in a well-defined direction. Through time-resolved measurements of the superradiant light pulses emitted by an end-pumped BEC, we study the close connection of superradiant light scattering with Dicke superradiance. A 1D model of the system yields good agreement with the experimental data and shows that the dynamics results from the structures that build up in the light and matter-wave fields along the BEC. This paves the way for exploiting the atom-photon correlations generated by the superradiance.
Magnetic fields generated by human and animal organs, such as the heart, brain and nervous system... more Magnetic fields generated by human and animal organs, such as the heart, brain and nervous system carry information useful for biological and medical purposes. These magnetic fields are most commonly detected using cryogenically-cooled superconducting magnetometers. Here we present the first detection of action potentials from an animal nerve using an optical atomic magnetometer. Using an optimal design we are able to achieve the sensitivity dominated by the quantum shot noise of light and quantum projection noise of atomic spins. Such sensitivity allows us to measure the nerve impulse with a miniature room-temperature sensor which is a critical advantage for biomedical applications. Positioning the sensor at a distance of a few millimeters from the nerve, corresponding to the distance between the skin and nerves in biological studies, we detect the magnetic field generated by an action potential of a frog sciatic nerve. From the magnetic field measurements we determine the activity...
A common knowledge suggests that trajectories of particles in quantum mechanics always have quant... more A common knowledge suggests that trajectories of particles in quantum mechanics always have quantum uncertainties. These quantum uncertainties set by the Heisenberg uncertainty principle limit precision of measurements of fields and forces, and ultimately give rise to the standard quantum limit in metrology. With the rapid developments of sensitivity of measurements these limits have been approached in various types of measurements including measurements of fields and acceleration. Here we show that a quantum trajectory of one system measured relatively to the other "reference system" with an effective negative mass can be quantum uncertainty-free. The method crucially relies on the generation of an Einstein-Podolsky-Rosen entangled state of two objects, one of which has an effective negative mass. From a practical perspective these ideas open the way towards force and acceleration measurements at new levels of sensitivity far below the standard quantum limit. Copyright line will be provided by the publisher Quantum uncertainties set by the Heisenberg uncertainty principle determine fundamental limits on measurement precision. With rapid developments of sensitivity of measurements these limits have been approached in various types of measurements including measurements of fields, time, acceleration, and position. It is commonly accepted that the balance between the amount of information obtained from the measurement and the back action of the measurement can be at best balanced such that they result in the standard quantum limit of the position measurement [1, 2, 3, 4, 5]. Ideas for circumventing this limit have been put forward based on frequency-dependent squeezing [6], variational measurement [7], the use of Kerr media [8], dual mechanical resonators [9, 10], the optical spring effect [11], stroboscopic measurements [5], or two-tone measurements [1, 12, 2, 13, 14]. However, all those apporaches are limited to measurements of a single quadrature operator of a system and hence are intrinsically limited to measurements of the disturbance whose phase is known in advance. In practice the phase of the signal to be detected is likely to be unknown in advance and therefore another approach is desirable. A new approach to back action cancellation has been demonstrated in Wasilewski et al. [15] where a carefully engineered quantum measurement on two entangled spin systems has led to partial cancelation of the quantum noise of measurement for a sensor of magnetic fields. The basis for this approach has been an experimental demonstration of an Einstein-Podolsky-Rosen (EPR) state of two atomic spin oscillators [16], one of which has an effective negative mass. Such an entangled EPR state can also be created in a hybrid system involving a nanomechanical oscillator and an atomic spin ensemble with an effective negative mass [17]. A general theoretical frame for such an approach has been very recently formulated [18, 19]. Following these works, proposals for back action evading measurements employing a Bose-Einstein-condensate [20] or a two-tone drive [21] to realize an oscillator with an effective negative mass have been put forward, and the all-optical implementation suggested in [18, 19] has been studied further [22]. In this paper we show how using the entangled state of a positive and a negative mass oscillator one can predict the quantum trajectory of the magnetic or mechanical oscillator measured relatively to a specially chosen origin with, in principle, arbitrarily low uncertainty. Our goal is to develop an intuitive physical picture for this new approach towards metrology below the standard quantum limit, and to illustrate it on the basis of the measurements reported in [15]. We base our discussion on three principles. First, we state that a trajectory should be defined with respect to some physical origin. Second, we treat this physical origin as a quantum object. Finally, we allow this object to have an effectively negative mass. Under this condition the trajectory defined with respect to this origin can be known to arbitrary precision at any time. Besides being of fundamental interest, this approach opens the way towards force and acceleration measurements at new levels of sensitivity.
This article reviews recent research towards a universal light-matter interface. Such an interfac... more This article reviews recent research towards a universal light-matter interface. Such an interface is an important prerequisite for long distance quantum communication, entanglement assisted sensing and measurement, as well as for scalable photonic quantum computation. We review the developments in light-matter interfaces based on room temperature atomic vapors interacting with propagating pulses via the Faraday effect. This interaction has long been used as a tool for quantum nondemolition detections of atomic spins via light. It was discovered recently that this type of light-matter interaction can actually be tuned to realize more general dynamics, enabling better performance of the light-matter interface as well as rendering tasks possible, which were before thought to be impractical. This includes the realization of improved entanglement assisted and backaction evading magnetometry approaching the Quantum Cramer-Rao limit, quantum memory for squeezed states of light and the dissipative generation of entanglement. A separate, but related, experiment on entanglement assisted cold atom clock showing the Heisenberg scaling of precision is described. We also review a possible interface between collective atomic spins with nano-or micromechanical oscillators, providing a link between atomic and solid state physics approaches towards quantum information processing.
We explore the fundamental noise of the atomic spin measurement performed via polarization analys... more We explore the fundamental noise of the atomic spin measurement performed via polarization analysis of the probe light. The noise is shown to consist of the quantum noise of the probe and the quantum noise of atomic spins. In the experiment with cold atoms in a magneto-optical trap we demonstrate the reduction of the former by 2.5 dB below the standard quantum limit. For the latter we reach the quantum limit set by fluctuations of uncorrelated individual atomic spins. We outline the way to overcome this limit using a recent theoretical proposal on spin squeezing. [S0031-9007(98)05843-8]
We propose a method for the detection of ground state quantum phases of spinor gases through a se... more We propose a method for the detection of ground state quantum phases of spinor gases through a series of two quantum nondemolition measurements performed by sending off-resonant, polarized light pulses through the gas. Signatures of various mean-field as well as strongly-correlated phases of F = 1 and F = 2 spinor gases obtained by detecting quantum fluctuations and mean values of polarization of transmitted light are identified.
A method for generating a mesoscopic superposition state of the collective spin variable of a gas... more A method for generating a mesoscopic superposition state of the collective spin variable of a gas of atoms is proposed. The state consists of a superposition of the atomic spins pointing in two slightly different directions. It is obtained by using off resonant light to carry out Quantum Non Demolition Measurements of the spins. The relevant experimental conditions, which require very dense atomic samples, can be realized with presently available techniques. Long-lived atomic superposition states may become useful as an off-line resource for quantum computing with otherwise linear operations.
Most protocols for quantum information processing consist of a series of quantum gates, which are... more Most protocols for quantum information processing consist of a series of quantum gates, which are applied sequentially. In contrast, interactions between matter and fields, for example, as well as measurements such as homodyne detection of light, are typically continuous in time. We show how the ability to perform quantum operations continuously and deterministically can be leveraged for inducing nonlocal dynamics between two separate parties. We introduce a scheme for the engineering of an interaction between two remote systems and present a protocol that induces a dynamics in one of the parties, that is controlled by the other one. Both schemes apply to continuous variable systems, run continuously in time and are based on real-time feedback.
Entanglement is a striking feature of quantum mechanics and an essential ingredient in most appli... more Entanglement is a striking feature of quantum mechanics and an essential ingredient in most applications in quantum information. Typically, coupling of a system to an environment inhibits entanglement, particularly in macroscopic systems. Here we report on an experiment, where dissipation continuously generates entanglement between two macroscopic objects. This is achieved by engineering the dissipation using laser-and magnetic fields, and leads to robust event-ready entanglement maintained for 0.04s at room temperature. Our system consists of two ensembles containing about 10 12 atoms and separated by 0.5m coupled to the environment composed of the vacuum modes of the electromagnetic field. By combining the dissipative mechanism with a continuous measurement, steady state entanglement is continuously generated and observed for up to an hour.
We present a protocol for the teleportation of the quantum state of a pulse of light onto the col... more We present a protocol for the teleportation of the quantum state of a pulse of light onto the collective spin state of an atomic ensemble. The entangled state of light and atoms employed as a resource in this protocol is created by probing the collective atomic spin, Larmor precessing in an external magnetic field, off resonantly with a coherent pulse of light. We take here for the first time full account of the effects of Larmor precession and show that it gives rise to a qualitatively new type of multimode entangled state of light and atoms. The protocol is shown to be robust against the dominating sources of noise and can be implemented with an atomic ensemble at room temperature interacting with free space light. We also provide a scheme to perform the readout of the Larmor precessing spin state enabling the verification of successful teleportation as well as the creation of spin squeezing.
A scheme for retrieving quantum information stored in collective atomic spin systems onto optical... more A scheme for retrieving quantum information stored in collective atomic spin systems onto optical pulses is presented. Two off-resonant light pulses cross the atomic medium in two orthogonal directions and are interferometrically recombined in such a way that one of the outputs carries most of the information stored in the medium. In contrast to previous schemes our approach requires neither multiple passes through the medium nor feedback on the light after passing the sample which makes the scheme very efficient. The price for that is some added noise which is however small enough for the method to beat the classical limits.
We present an experimentally feasible protocol for the complete storage and retrieval of arbitrar... more We present an experimentally feasible protocol for the complete storage and retrieval of arbitrary light states in an atomic quantum memory using the well-established Faraday interaction between light and matter. Our protocol relies on multiple passages of a single light pulse through the atomic ensemble without the impractical requirement of kilometer long delay lines between the passages. Furthermore, we introduce a time dependent interaction strength which enables storage and retrieval of states with arbitrary pulse shapes. The fidelity approaches unity exponentially without squeezed or entangled initial states, as illustrated by explicit calculations for a photonic qubit.
We show how high fidelity quantum teleportation of light to atoms can be achieved in the same set... more We show how high fidelity quantum teleportation of light to atoms can be achieved in the same setup as was used in the recent experiment [J. Sherson et.al., quant-ph/0605095, accepted by Nature], where such an inter-species quantum state transfer was demonstrated for the first time. Our improved protocol takes advantage of the rich multimode entangled structure of the state of atoms and scattered light and requires simple post-processing of homodyne detection signals and squeezed light in order to achieve fidelities up to 90% (85%) for teleportation of coherent (qubit) states under realistic experimental conditions. The remaining limitation is due to atomic decoherence and light losses.
We investigate theoretically and experimentally a nondestructive interferometric measurement of t... more 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 133 Cs 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 √ Nat in a cloud of Nat cold atoms, which is an important benchmark towards the experimental realisation of the theoretically analyzed protocol.
Due to its coherence properties and high optical depth, a Bose-Einstein condensate provides an id... more Due to its coherence properties and high optical depth, a Bose-Einstein condensate provides an ideal setting to investigate collective atom-light interactions. Superradiant light scattering in a Bose-Einstein condensate is a fascinating example of such an interaction. It is an analogous process to Dicke superradiance, in which an electronically inverted sample decays collectively, leading to the emission of one or more light pulses in a well-defined direction. Through time-resolved measurements of the superradiant light pulses emitted by an end-pumped BEC, we study the close connection of superradiant light scattering with Dicke superradiance. A 1D model of the system yields good agreement with the experimental data and shows that the dynamics results from the structures that build up in the light and matter-wave fields along the BEC. This paves the way for exploiting the atom-photon correlations generated by the superradiance.
Magnetic fields generated by human and animal organs, such as the heart, brain and nervous system... more Magnetic fields generated by human and animal organs, such as the heart, brain and nervous system carry information useful for biological and medical purposes. These magnetic fields are most commonly detected using cryogenically-cooled superconducting magnetometers. Here we present the first detection of action potentials from an animal nerve using an optical atomic magnetometer. Using an optimal design we are able to achieve the sensitivity dominated by the quantum shot noise of light and quantum projection noise of atomic spins. Such sensitivity allows us to measure the nerve impulse with a miniature room-temperature sensor which is a critical advantage for biomedical applications. Positioning the sensor at a distance of a few millimeters from the nerve, corresponding to the distance between the skin and nerves in biological studies, we detect the magnetic field generated by an action potential of a frog sciatic nerve. From the magnetic field measurements we determine the activity...
A common knowledge suggests that trajectories of particles in quantum mechanics always have quant... more A common knowledge suggests that trajectories of particles in quantum mechanics always have quantum uncertainties. These quantum uncertainties set by the Heisenberg uncertainty principle limit precision of measurements of fields and forces, and ultimately give rise to the standard quantum limit in metrology. With the rapid developments of sensitivity of measurements these limits have been approached in various types of measurements including measurements of fields and acceleration. Here we show that a quantum trajectory of one system measured relatively to the other "reference system" with an effective negative mass can be quantum uncertainty-free. The method crucially relies on the generation of an Einstein-Podolsky-Rosen entangled state of two objects, one of which has an effective negative mass. From a practical perspective these ideas open the way towards force and acceleration measurements at new levels of sensitivity far below the standard quantum limit. Copyright line will be provided by the publisher Quantum uncertainties set by the Heisenberg uncertainty principle determine fundamental limits on measurement precision. With rapid developments of sensitivity of measurements these limits have been approached in various types of measurements including measurements of fields, time, acceleration, and position. It is commonly accepted that the balance between the amount of information obtained from the measurement and the back action of the measurement can be at best balanced such that they result in the standard quantum limit of the position measurement [1, 2, 3, 4, 5]. Ideas for circumventing this limit have been put forward based on frequency-dependent squeezing [6], variational measurement [7], the use of Kerr media [8], dual mechanical resonators [9, 10], the optical spring effect [11], stroboscopic measurements [5], or two-tone measurements [1, 12, 2, 13, 14]. However, all those apporaches are limited to measurements of a single quadrature operator of a system and hence are intrinsically limited to measurements of the disturbance whose phase is known in advance. In practice the phase of the signal to be detected is likely to be unknown in advance and therefore another approach is desirable. A new approach to back action cancellation has been demonstrated in Wasilewski et al. [15] where a carefully engineered quantum measurement on two entangled spin systems has led to partial cancelation of the quantum noise of measurement for a sensor of magnetic fields. The basis for this approach has been an experimental demonstration of an Einstein-Podolsky-Rosen (EPR) state of two atomic spin oscillators [16], one of which has an effective negative mass. Such an entangled EPR state can also be created in a hybrid system involving a nanomechanical oscillator and an atomic spin ensemble with an effective negative mass [17]. A general theoretical frame for such an approach has been very recently formulated [18, 19]. Following these works, proposals for back action evading measurements employing a Bose-Einstein-condensate [20] or a two-tone drive [21] to realize an oscillator with an effective negative mass have been put forward, and the all-optical implementation suggested in [18, 19] has been studied further [22]. In this paper we show how using the entangled state of a positive and a negative mass oscillator one can predict the quantum trajectory of the magnetic or mechanical oscillator measured relatively to a specially chosen origin with, in principle, arbitrarily low uncertainty. Our goal is to develop an intuitive physical picture for this new approach towards metrology below the standard quantum limit, and to illustrate it on the basis of the measurements reported in [15]. We base our discussion on three principles. First, we state that a trajectory should be defined with respect to some physical origin. Second, we treat this physical origin as a quantum object. Finally, we allow this object to have an effectively negative mass. Under this condition the trajectory defined with respect to this origin can be known to arbitrary precision at any time. Besides being of fundamental interest, this approach opens the way towards force and acceleration measurements at new levels of sensitivity.
This article reviews recent research towards a universal light-matter interface. Such an interfac... more This article reviews recent research towards a universal light-matter interface. Such an interface is an important prerequisite for long distance quantum communication, entanglement assisted sensing and measurement, as well as for scalable photonic quantum computation. We review the developments in light-matter interfaces based on room temperature atomic vapors interacting with propagating pulses via the Faraday effect. This interaction has long been used as a tool for quantum nondemolition detections of atomic spins via light. It was discovered recently that this type of light-matter interaction can actually be tuned to realize more general dynamics, enabling better performance of the light-matter interface as well as rendering tasks possible, which were before thought to be impractical. This includes the realization of improved entanglement assisted and backaction evading magnetometry approaching the Quantum Cramer-Rao limit, quantum memory for squeezed states of light and the dissipative generation of entanglement. A separate, but related, experiment on entanglement assisted cold atom clock showing the Heisenberg scaling of precision is described. We also review a possible interface between collective atomic spins with nano-or micromechanical oscillators, providing a link between atomic and solid state physics approaches towards quantum information processing.
We explore the fundamental noise of the atomic spin measurement performed via polarization analys... more We explore the fundamental noise of the atomic spin measurement performed via polarization analysis of the probe light. The noise is shown to consist of the quantum noise of the probe and the quantum noise of atomic spins. In the experiment with cold atoms in a magneto-optical trap we demonstrate the reduction of the former by 2.5 dB below the standard quantum limit. For the latter we reach the quantum limit set by fluctuations of uncorrelated individual atomic spins. We outline the way to overcome this limit using a recent theoretical proposal on spin squeezing. [S0031-9007(98)05843-8]
We propose a method for the detection of ground state quantum phases of spinor gases through a se... more We propose a method for the detection of ground state quantum phases of spinor gases through a series of two quantum nondemolition measurements performed by sending off-resonant, polarized light pulses through the gas. Signatures of various mean-field as well as strongly-correlated phases of F = 1 and F = 2 spinor gases obtained by detecting quantum fluctuations and mean values of polarization of transmitted light are identified.
A method for generating a mesoscopic superposition state of the collective spin variable of a gas... more A method for generating a mesoscopic superposition state of the collective spin variable of a gas of atoms is proposed. The state consists of a superposition of the atomic spins pointing in two slightly different directions. It is obtained by using off resonant light to carry out Quantum Non Demolition Measurements of the spins. The relevant experimental conditions, which require very dense atomic samples, can be realized with presently available techniques. Long-lived atomic superposition states may become useful as an off-line resource for quantum computing with otherwise linear operations.
Most protocols for quantum information processing consist of a series of quantum gates, which are... more Most protocols for quantum information processing consist of a series of quantum gates, which are applied sequentially. In contrast, interactions between matter and fields, for example, as well as measurements such as homodyne detection of light, are typically continuous in time. We show how the ability to perform quantum operations continuously and deterministically can be leveraged for inducing nonlocal dynamics between two separate parties. We introduce a scheme for the engineering of an interaction between two remote systems and present a protocol that induces a dynamics in one of the parties, that is controlled by the other one. Both schemes apply to continuous variable systems, run continuously in time and are based on real-time feedback.
Entanglement is a striking feature of quantum mechanics and an essential ingredient in most appli... more Entanglement is a striking feature of quantum mechanics and an essential ingredient in most applications in quantum information. Typically, coupling of a system to an environment inhibits entanglement, particularly in macroscopic systems. Here we report on an experiment, where dissipation continuously generates entanglement between two macroscopic objects. This is achieved by engineering the dissipation using laser-and magnetic fields, and leads to robust event-ready entanglement maintained for 0.04s at room temperature. Our system consists of two ensembles containing about 10 12 atoms and separated by 0.5m coupled to the environment composed of the vacuum modes of the electromagnetic field. By combining the dissipative mechanism with a continuous measurement, steady state entanglement is continuously generated and observed for up to an hour.
We present a protocol for the teleportation of the quantum state of a pulse of light onto the col... more We present a protocol for the teleportation of the quantum state of a pulse of light onto the collective spin state of an atomic ensemble. The entangled state of light and atoms employed as a resource in this protocol is created by probing the collective atomic spin, Larmor precessing in an external magnetic field, off resonantly with a coherent pulse of light. We take here for the first time full account of the effects of Larmor precession and show that it gives rise to a qualitatively new type of multimode entangled state of light and atoms. The protocol is shown to be robust against the dominating sources of noise and can be implemented with an atomic ensemble at room temperature interacting with free space light. We also provide a scheme to perform the readout of the Larmor precessing spin state enabling the verification of successful teleportation as well as the creation of spin squeezing.
A scheme for retrieving quantum information stored in collective atomic spin systems onto optical... more A scheme for retrieving quantum information stored in collective atomic spin systems onto optical pulses is presented. Two off-resonant light pulses cross the atomic medium in two orthogonal directions and are interferometrically recombined in such a way that one of the outputs carries most of the information stored in the medium. In contrast to previous schemes our approach requires neither multiple passes through the medium nor feedback on the light after passing the sample which makes the scheme very efficient. The price for that is some added noise which is however small enough for the method to beat the classical limits.
We present an experimentally feasible protocol for the complete storage and retrieval of arbitrar... more We present an experimentally feasible protocol for the complete storage and retrieval of arbitrary light states in an atomic quantum memory using the well-established Faraday interaction between light and matter. Our protocol relies on multiple passages of a single light pulse through the atomic ensemble without the impractical requirement of kilometer long delay lines between the passages. Furthermore, we introduce a time dependent interaction strength which enables storage and retrieval of states with arbitrary pulse shapes. The fidelity approaches unity exponentially without squeezed or entangled initial states, as illustrated by explicit calculations for a photonic qubit.
We show how high fidelity quantum teleportation of light to atoms can be achieved in the same set... more We show how high fidelity quantum teleportation of light to atoms can be achieved in the same setup as was used in the recent experiment [J. Sherson et.al., quant-ph/0605095, accepted by Nature], where such an inter-species quantum state transfer was demonstrated for the first time. Our improved protocol takes advantage of the rich multimode entangled structure of the state of atoms and scattered light and requires simple post-processing of homodyne detection signals and squeezed light in order to achieve fidelities up to 90% (85%) for teleportation of coherent (qubit) states under realistic experimental conditions. The remaining limitation is due to atomic decoherence and light losses.
We investigate theoretically and experimentally a nondestructive interferometric measurement of t... more 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 133 Cs 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 √ Nat in a cloud of Nat cold atoms, which is an important benchmark towards the experimental realisation of the theoretically analyzed protocol.
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Papers by E. Polzik