Christoph Simon

Quantum Cryptography with Entangled Photons

By realizing a quantum cryptography system based on polarization entangled photon pairs we establish highly secure keys, because a single photon source is approximated and the inherent randomness of quantum measurements is exploited. We implement a novel key distribution scheme using Wigners inequality to test the security of the quantum channel, and, alternatively, realize a variant of the BB84 protocol. Our system has two completely independent users separated by 360 m, and generates raw keys at rates of 400-800 bits/s with bit error rates around 3%.

Towards quantum superpositions of a mirror.

We propose an experiment for creating quantum superposition states involving of the order of 10(14) atoms via the interaction of a single photon with a tiny mirror. This mirror, mounted on a high-quality mechanical oscillator, is part of a high-finesse optical cavity which forms one arm of a Michelson interferometer. By observing the interference of the photon only, one can study the creation and decoherence of superpositions involving the mirror. A detailed analysis of the requirements shows that the experiment is within reach using a combination of state-of-the-art technologies.

Entanglement purification for quantum communication.

The distribution of entangled states between distant locations will be essential for the future large-scale realization of quantum communication schemes such as quantum cryptography and quantum teleportation. Because of unavoidable noise in the quantum communication channel, the entanglement between two particles is more and more degraded the further they propagate. Entanglement purification is thus essential to distil highly entangled states from less entangled ones. Existing general purification protocols are based on the quantum controlled-NOT (CNOT) or similar quantum logic operations, which are very difficult to implement experimentally. Present realizations of CNOT gates are much too imperfect to be useful for long-distance quantum communication. Here we present a scheme for the entanglement purification of general mixed entangled states, which achieves 50 per cent of the success probability of schemes based on the CNOT operation, but requires only simple linear optical elements. Because the perfection of such elements is very high, the local operations necessary for purification can be performed with the required precision. Our procedure is within the reach of current technology, and should significantly simplify the implementation of long-distance quantum communication.

Quantum repeaters with photon pair sources and multimode memories.

We propose a quantum repeater protocol which builds on the well-known Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L. M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature (London) 414, 413 (2001)10.1038/35106500], but which uses photon pair sources in combination with memories that allow to store a large number of temporal modes. We suggest to realize such multimode memories based on the principle of photon echo, using solids doped with rare-earth-metal ions. The use of multimode memories promises a speedup in entanglement generation by several orders of magnitude and a significant reduction in stability requirements compared to the DLCZ protocol.

A solid-state light–matter interface at the single-photon level

Coherent and reversible mapping of quantum information between light and matter is an important experimental challenge in quantum information science. In particular, it is an essential requirement for the implementation of quantum networks and quantum repeaters. So far, quantum interfaces between light and atoms have been demonstrated with atomic gases, and with single trapped atoms in cavities. Here we demonstrate the coherent and reversible mapping of a light field with less than one photon per pulse onto an ensemble of ∼107 atoms naturally trapped in a solid. This is achieved by coherently absorbing the light field in a suitably prepared solid-state atomic medium. The state of the light is mapped onto collective atomic excitations at an optical transition and stored for a pre-determined time of up to 1 μs before being released in a well-defined spatio-temporal mode as a result of a collective interference. The coherence of the process is verified by performing an interference experiment with two stored weak pulses with a variable phase relation. Visibilities of more than 95 per cent are obtained, demonstrating the high coherence of the mapping process at the single-photon level. In addition, we show experimentally that our interface makes it possible to store and retrieve light fields in multiple temporal modes. Our results open the way to multimode solid-state quantum memories as a promising alternative to atomic gases.

Polarization entanglement purification using spatial entanglement.

We present a scheme for entanglement purification with linear optics that works for currently available parametric down-conversion sources, in contrast to a previous scheme [J. W. Pan, Nature (London) 410, 1067 (2001)]] that relied on ideal single-pair sources. The present scheme makes use of spatial entanglement in order to purify polarization entanglement. Surprisingly, spatial entanglement as an additional resource also leads to a substantial improvement in entanglement output compared to the previous scheme.

Multimode quantum memory based on atomic frequency combs

An efficient multimode quantum memory is a crucial resource for long-distance quantum communication based on quantum repeaters. We propose a quantum memory based on spectral shaping of an inhomogeneously broadened optical transition into an atomic frequency comb (AFC). The spectral width of the AFC allows efficient storage of multiple temporal modes without the need to increase the absorption depth of the storage material, in contrast to previously known quantum memories. Efficient readout is possible thanks to rephasing of the atomic dipoles due to the AFC structure. Long-time storage and on-demand readout is achieved by use of spin states in a lambda-type configuration. We show that an AFC quantum memory realized in solids doped with rare-earth-metal ions could store hundreds of modes or more with close to unit efficiency, for material parameters achievable today.

Entangling independent photons by time measurement

Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in quantum communication. Remarkably, entanglement can be ‘swapped’: if we prepare two independent entangled pairs A1–A2 and B1–B2, a joint measurement on A1 and B1 (called a ‘Bell-state measurement’, BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted nor share any common past1,2. Entanglement swapping with photon pairs has already been experimentally demonstrated3,4,5,6 using pulsed sources—where the challenge was to achieve sufficiently sharp synchronization of the photons in the BSM—but never with continuous-wave sources, as originally proposed2. Here, we present an experiment where the coherence time of the photons exceeds the temporal resolution of the detectors. Hence, photon timing can be obtained by the detection times, and pulsed sources can be replaced by continuous-wave sources, which do not require any synchronization6,7. This allows for the first time the use of completely autonomous sources, an important step towards real-world quantum networks with truly independent and distant nodes.

Robust long-distance entanglement and a loophole-free bell test with ions and photons.

Two trapped ions that are kilometers apart can be entangled by the joint detection of two photons, each coming from one of the ions, in a basis of entangled states. Such a detection is possible with linear optical elements. The use of two-photon interference allows entanglement distribution free of interferometric sensitivity to the path length of the photons. The present method of creating entangled ions also opens up the possibility of a loophole-free test of Bells inequalities.

Demonstration of Atomic Frequency Comb Memory for Light with Spin-Wave Storage

We present a light-storage experiment in a praseodymium-doped crystal where the light is mapped onto an inhomogeneously broadened optical transition shaped into an atomic frequency comb. After absorption of the light, the optical excitation is converted into a spin-wave excitation by a control pulse. A second control pulse reads the memory (on-demand) by reconverting the spin-wave excitation to an optical one, where the comb structure causes a photon-echo-type rephasing of the dipole moments and directional retrieval of the light. This combination of photon-echo and spin-wave storage allows us to store submicrosecond (450 ns) pulses for up to 20 mus. The scheme has a high potential for storing multiple temporal modes in the single-photon regime, which is an important resource for future long-distance quantum communication based on quantum repeaters.