Decoherence-protected memory for a single-photon qubit

Abstract

The long-lived, efficient storage and retrieval of a qubit encoded on a photon is an important ingredient for future quantum networks [1,2]. Although systems with intrinsically long coherence times have been demonstrated [3][4][5][6][7][8], the combination with an efficient light-matter interface [9][10][11][12][13][14] remains an outstanding challenge. In fact, the coherence times of memories for photonic qubits are currently limited to a few milliseconds [15,16]. Here we report on a qubit memory based on a single atom coupled to a highfinesse optical resonator. By mapping and remapping the qubit between a basis used for light-matter interfacing and a basis which is less susceptible to decoherence, a coherence time exceeding 100 ms has been measured with a time-independant storage-andretrieval efficiency of 22%. This demonstrates the first photonic qubit memory with a coherence time that exceeds the lower bound needed for teleporting qubits in a global quantum internet.Photons are convenient carriers for encoding both classical and quantum information. To transport a pulse of light between the most distant locations on earth, it has to travel about 20,000 km, which takes at least 66 ms. For future quantum networks allowing for distributed quantum computations, the exchange of quantum states between network nodes is indispensable. In principle, it can be achieved by using single photons, but unavoidable losses in glass fibres in combination with the impossibility to amplify a quantum state renders the direct distribution of quantum states over long distances extremely inefficient. A possible solution is the teleportation of qubits between the end nodes of a quantum repeater link [1,17]. While teleportation can be done deterministically, another challenge occurs: as quantum teleportation requires classical communication between the end nodes, the receiver has to preserve its quantum state for at least the time it takes for the classical information to arrive [18]. This establishes the above-mentioned 66 ms as a minimum requirement for global quantum-state distribution.A qubit memory for photons has to combine a lightmatter interface used to efficiently map the qubit between the photon and a material system with the ability to preserve quantum coherence for the duration of storage. It has been shown that neutral atoms are suitable systems for both requirements [19,20] by mapping the photonic qubit onto the atom and end by recreating the photon (upward pointing red wavy arrow). a, In an elementary store-and-retrieve experiment (red area) the qubit dephases within hundreds of microseconds. The dephasing is dominantly caused by magnetic field fluctuations. This is illustrated as the deformation of the Bloch sphere which represents the qubit. The labels |F, mF denote the qubit basis states. b, By temporarily mapping the qubit to a memory basis which is less sensitive to those fluctuations, the rate of dephasing can be drastically reduced (blue area). This allows for a total storage time on the order of tens of millise...

Optimal storage of a single photon by a single intra-cavity atom

“…In this section we determine the efficiency of storage protocols derived in [19][20][21] for the setup of [17] in the adiabatic regime. We then analyze how the efficiency of these protocols is modified by the presence of parasitic losses at rate k loss .…”

“…Here we simulate the full Hamiltonian dynamics of the external field in the transmission line and consider a quantum memory composed of a single atom inside a reasonably good cavity. The parameters we refer to in our study are the ones of the setup of [17]: corresponding to the cooperativity C=3.27 and to the maximal storage efficiency η max =0.77. When we analyze the dependence of the efficiency on γ or κ, we vary the parameters around the values given in equation (20).…”

“…The efficiency of the transfer, equation (9), corresponds to the population of the state ñ |r , r rr . For the parameters of [17] the final efficiency is…”

“…In addition, schemes based on heralded state transfer have been realized [10,[12][13][14], and fiber-coupled resonators coupled to single atoms have been used to perform SWAP gates [15,16]. Most recently, storage efficiencies of the order of 22% have been reported for a quantum memory composed by a single atom in an optical cavity [17]. This value lies well below the value one can extract from theoretical works on spin ensembles for photon storage [18].…”

Langlebiger Qubitspeicher für Einzelphotonen

“…Der am MPQ entwickelte photonische Qubitspeicher basiert auf einem einzelnen, in einem optischen Resonator gefangenen, Rubidiumatom . Das zu speichernde Photon wird in den Resonator eingekoppelt und kann dort effizient mit dem Atom wechselwirken (Abbildungen und a, rote Pfeile).…”