Eden Figueroa

An elementary quantum network of single atoms in optical cavities

Quantum networks are distributed quantum many-body systems with tailored topology and controlled information exchange. They are the backbone of distributed quantum computing architectures and quantum communication. Here we present a prototype of such a quantum network based on single atoms embedded in optical cavities. We show that atom–cavity systems form universal nodes capable of sending, receiving, storing and releasing photonic quantum information. Quantum connectivity between nodes is achieved in the conceptually most fundamental way—by the coherent exchange of a single photon. We demonstrate the faithful transfer of an atomic quantum state and the creation of entanglement between two identical nodes in separate laboratories. The non-local state that is created is manipulated by local quantum bit (qubit) rotation. This efficient cavity-based approach to quantum networking is particularly promising because it offers a clear perspective for scalability, thus paving the way towards large-scale quantum networks and their applications.

Quantum Memory for Squeezed Light

We produce a 600-ns pulse of 1.86-dB squeezed vacuum at 795 nm in an optical parametric amplifier and store it in a rubidium vapor cell for 1 mus using electromagnetically induced transparency. The recovered pulse, analyzed using time-domain homodyne tomography, exhibits up to 0.21+/-0.04 dB of squeezing. We identify the factors leading to the degradation of squeezing and investigate the phase evolution of the atomic coherence during the storage interval.

A single-atom quantum memory

The faithful storage of a quantum bit (qubit) of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing. The required optical quantum memory must be able to receive and recreate the photonic qubit; additionally, it must store an unknown quantum state of light better than any classical device. So far, these two requirements have been met only by ensembles of material particles that store the information in collective excitations. Recent developments, however, have paved the way for an approach in which the information exchange occurs between single quanta of light and matter. This single-particle approach allows the material qubit to be addressed, which has fundamental advantages for realistic implementations. First, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection; this can be used to combat inevitable losses and finite efficiencies. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory, by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is tested with weak coherent pulses and analysed using full quantum process tomography. The average fidelity is measured to be 93%, and low decoherence rates result in qubit coherence times exceeding 180u2009microseconds. This makes our system a versatile quantum node with excellent prospects for applications in optical quantum gates and quantum repeaters.

Electromagnetically induced transparency with single atoms in a cavity

Optical nonlinearities offer unique possibilities for the control of light with light. A prominent example is electromagnetically induced transparency (EIT), where the transmission of a probe beam through an optically dense medium is manipulated by means of a control beam. Scaling such experiments into the quantum domain with one (or just a few) particles of light and matter will allow for the implementation of quantum computing protocols with atoms and photons, or the realization of strongly interacting photon gases exhibiting quantum phase transitions of light. Reaching these aims is challenging and requires an enhanced matter–light interaction, as provided by cavity quantum electrodynamics. Here we demonstrate EIT with a single atom quasi-permanently trapped inside a high-finesse optical cavity. The atom acts as a quantum-optical transistor with the ability to coherently control the transmission of light through the cavity. We investigate the scaling of EIT when the atom number is increased one-by-one. The measured spectra are in excellent agreement with a theoretical model. Merging EIT with cavity quantum electrodynamics and single quanta of matter is likely to become the cornerstone for novel applications, such as dynamic control of the photon statistics of propagating light fields or the engineering of Fock state superpositions of flying light pulses.

Phase shaping of single-photon wave packets

While the phase of a coherent light field can be precisely known, the phase of the individual photons that create this field, considered individually, cannot [1]. Phase changes within singlephoton wave packets, however, have observable effects. In fact, actively controlling the phase of individual photons has been identified as a powerful resource for quantum communication protocols [2, 3]. Here we demonstrate the arbitrary phase control of a single photon. The phase modulation is applied without affecting the photon’s amplitude profile and is verified via a two-photon quantum interference measurement [4, 5], which can result in the fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photon’s amplitude [6, 7, 8, 9, 10], frequency [11], and polarisation [12], the fully deterministic phase shaping presented here allows for the complete control of single-photon wave packets. Consider two identical photons mode-matched at the two input ports (A and B) of a 50/50 non-polarising beam splitter (NPBS), represented by the initial state |Ψi〉 = |1A1B〉 (see Fig. 1). Due to the indistinguishability of the photons, the detection of one photon in output port C or D at time t0 projects the initial product state |Ψi〉 into the “which path” superposition state |Ψ±(t0)〉 = (|1A, 0B〉 ± |0A, 1B〉)/ √ 2 of the remaining photon. As first demonstrated by Hong, Ou and Mandel [4], the bosonic nature of photons always results in the detection of the second photon in the same output port as the first. However, we can alter this coalescence behaviour by introducing an arbitrary differential phase ∆φ between the two components of |Ψ±〉. This results in a phase-dependent wave function of the remaining single photon

Complete Characterization of Quantum-Optical Processes

The technologies of quantum information and quantum control are rapidly improving, but full exploitation of their capabilities requires complete characterization and assessment of processes that occur within quantum devices. We present a method for characterizing, with arbitrarily high accuracy, any quantum optical process. Our protocol recovers complete knowledge of the process by studying, via homodyne tomography, its effect on a set of coherent states, that is, classical fields produced by common laser sources. We demonstrate the capability of our protocol by evaluating and experimentally verifying the effect of a test process on squeezed vacuum.

Memory for Light as a Quantum Process

We report complete characterization of an optical memory based on electromagnetically induced transparency. We recover the superoperator associated with the memory, under two different working conditions, by means of a quantum process tomography technique that involves storage of coherent states and their characterization upon retrieval. In this way, we can predict the quantum state retrieved from the memory for any input, for example, the squeezed vacuum or the Fock state. We employ the acquired superoperator to verify the nonclassicality benchmark for the storage of a Gaussian distributed set of coherent states.

Adiabatic frequency conversion of optical information in atomic vapor

We experimentally demonstrate a communication protocol that enables frequency conversion and routing of quantum information in an adiabatic and thus robust way. The protocol is based on electromagnetically induced transparency (EIT) in systems with multiple excited levels: transfer and/or distribution of optical states between different signal modes is implemented by adiabatically changing the control fields. The proof-of-principle experiment is performed using the hyperfine levels of the rubidium D1 line.

Propagation of squeezed vacuum under electromagnetically induced transparency

We analyze the transmission of continuous-wave and pulsed squeezed vacuum through rubidium vapor under the conditions of electromagnetically induced transparency. Our analysis is based on a full theoretical treatment for a squeezed state of light propagating through temporal and spectral filters and detected using time and frequency-domain homodyne tomography. A model based on a three-level atom allows us to evaluate the linear losses and extra noise that degrade the nonclassical properties of the squeezed vacuum during the atomic interaction and eventually predict the quantum states of the transmitted light with a high precision.

Electromagnetically Induced Transparency with Single Atoms in a Cavity

We report on the observation of Electromagnetically Induced Transparency (EIT) with a single atom quasi‐permanently trapped inside a high‐finesse optical cavity. In the experiment, the atom acts as a quantum‐optical transistor with the ability to coherently control the transmission of light through the cavity.