Control of Photon Storage Time in Photon Echoes using a Deshelving Process

Abstract

Since the first protocol of quantum algorithm was put forth in 1994 (Shor, 1994), quantum information processing has been intensively studied (Nielsen & Chuang, 2000). The quantum approach has benefits over the classical optical information processing in areas such as prime number factorization (Shor, 1994), data searching (Grover, 1996), and highresolution lithography (Boto et al., 2000; Yablonovitch, 1999). Compared to conventional cryptography based on public key cryptosystem (RSA cryptosystem) using conventional computers, prime number factorization using quantum computers has demonstrated a potential for a formidable attack on existing cryptographic systems. Like conventional memory which serves in the information processing unit, such as a processing unit together with logic gates, quantum memory is also essential to quantum information and communications networks. For quantum communications via a classical optical channel, the longest communication distance a quantum light can be transmitted is determined by the sensitivity of optical detectors and a lossy classical channel such as an optical fibre. Based on current technologies, the longest distance a single photon can propagate through an optical fibre is about 100 km (Zbinden et al., 1998). This distance should limit applications of quantum information especially for long-distance quantum communications. To solve the limited photon transmission, a quantum repeater has been introduced for virtually unlimited transmission distance (Duan et al., 2001; Jiang et al., 2007; Simon et al., 2007; Waks et al., 2002). Quantum memory is an essential element for the entangled photon swapping in the quantum repeaters. Because quantum repeaters swap entangled photons shared by neighboring remote quantum nodes in a quantum network, and the quantum information must be kept coherently through the quantum network, the minimum storage time of quantum memory is determined by the longest transmission distance of the lossy optical channel. For tranceatlantic quantum communications, roughly a one-second or more storage time is required. So far, such a long photon storage has not been demonstrated, where conventional quantum memory protocols limit the storage time to spin phase decay time at most (≤10-3 second). Unlike classical memories, quantum memory must satisfy a coherent process. Since the first observation of coherent retrieval of a stored optical pulse in a Bose Einstein condensate using slow light (Liu et al., 2001), interest in quantum memories has increased in the last decade (Alexander et al., 2006; Afzelius et al., 2010; Chaneliere et al., 2005. Choi et al., 2008;