Quantum memory

Photons in quantum optical cavities also constitute excellent qubit candidates [52]. Resonant coupling of atoms with a single mode of the radiation field was experimentally achieved 25 years ago [53], and eventually the coherent coupling of quantum optical cavities with atoms or (simple) molecules was suggested as a means to achieve stable quantum memories in a hybrid quantum processor [54]. There might be a role to play for molecular spin qubits in this kind of hybrid quantum devices that combine solid-state with flying qubits. 

Quantum memory with a single photon in a cavity. Phys. Rev. Lett., 79, 769-772. 

Rabi, P., DeMille, D., Doyle, J.M., Lukin, M.D., Schoelkopf, R.J. and Zoller, P. (2006) Hybrid quantum processors molecular ensembles as quantum memory for solid state circuits. Phys. Rev. Lett, 97, 033003. 

An additional asset of both donors in silicon and NV centres is that nuclear spins, of 31P in the former and of N and neighbouring 13C in the latter, can be employed as long-term storage quantum memories [63], or even to build multiple qubit registers [22, 67]. 

The advantages of enhanced coupling between collective many-atom states and the radiation field have to be checked against the worry that these states are highly entangled if non-classical fight is stored. Entangled states are known to be very sensitive to decoherence and one could naively expect their lifetime to decrease with the number of atoms. It is therefore important to analyze the effect of unwanted environmental influences on the fidelity of the collective quantum memory. 

In order to simplify the discussion we will here restrict ourselves to a quantum memory for a two-mode radiation field, realized for example in a weak-coupling resonator allowing for two orthogonal polarization modes of the same frequency described by annihilation and creation operators , [Lukin 2000... 

However, in the read-out process of the quantum memory, i. e., when rotating 9 from 7r/2 to 0, only the dark polariton excitations T arc transferred to the cavity modes and thus only them are relevant. This can be seen upon inverting Eqs. (14) and (16) ... 

Thus the decoherence model of Fig. 8 exactly reproduces the anticipated behavior. All decoherence processes resulting from individual and uncorrelated reservoir interactions of the atoms are either exponentially suppressed by the energy gap (33) or are proportional to l/N. The latter is due to the large effective distance of the collective states in state space. In this way a quasi decoherence free subspace of dimension two is generated which allows to protect a stored photonic qubit from decoherence much more efficiendy than possible in quantum memories based on single particles. 

Accessibility of single spins for a manipulation even at room temperature, coherent control and read-out, demonstrated by the investigations of the N-V colour centers, together with the proposals of application of these centers for room temperature single-photon emitters [17], quantum cryptography [18], quantum memory and quantum repeaters [19], puts the diamond-based systems on much more higher level in quantum information race . 

Diamond-based single-photon sources, QKD, quantum memory and repeaters... 

Rabl, P. and Zoller, P, Molecular dipolar crystals as high-fidebty quantum memory for hybrid quantum computing, Phys. Rev. A, 76, 042308, 2007. 

If prepared in a general superposition state, quantum registers consisting of N qubits can store 2 bits of information simultaneously, as compared to classical registers where only N bits of information are stored. However, not all the information contained in quantum memories can be accessed by physical measurements. Nevertheless, so-called quantum parallelism makes quantum computers very fast they can process quantum superpositions of many numbers in one computational step, where each computational step is a unitary transformation of quantum registers. To achieve this, a universal quantum computer should be able to perform an arbitrary unitary transformation on any superposition of states. 

Various schemes for hybrid quantum processors based on molecular ensembles as quantum memories and optical interfaces have been proposed. In Ref. [17], a hybrid quantum circuit using ensembles of cold polar molecules with solid-state quantum processors is discussed. As described above, the quantum memory is realized by collective spin states (ensemble qubit), which are coupled to a high-Q stripline cavity via microwave Raman processes. This proposal combines both molecular ensemble and stripline resonator ideas. A variant of this scheme using collective excitations of rotational and spin states of an ensemble of polar molecules prepared in a dipolar... 

Choi KS, Deng H, Laurat J and et al. Mapping photonic entanglement into and out of a quantum memory. Nature 2008 Mar 6 452 67-71. 

Phase-dependent coherence and interference can be induced in a multi-level atomic system coupled by multiple laser fields. Two simple examples are presented here, a three-level A-type system coupled by four laser fields and a four-level double A-type system coupled also by four laser fields. The four laser fields induce the coherent nonlinear optical processes and open multiple transitions channels. The quantum interference among the multiple channels depends on the relative phase difference of the laser fields. Simple experiments show that constructive or destructive interference associated with multiple two-photon Raman channels in the two coherently coupled systems can be controlled by the relative phase of the laser fields. Rich spectral features exhibiting multiple transparency windows and absorption peaks are observed. The multicolor EIT-type system may be useful for a variety of application in coherent nonlinear optics and quantum optics such as manipulation of group velocities of multicolor, multiple light pulses, for optical switching at ultra-low light intensities, for precision spectroscopic measurements, and for phase control of the quantum state manipulation and quantum memory. 

Chaneliere T, Matsukevich DN, Jenkins SD et al. Storage and retrieval of single photons transmitted between remote quantum memories. Nature (London) 2005 Dec 8 438 833-836. 

Appel J, Figueroa E, Korystov D et al. Quantum memory for squeezed light. Physical Review Letters 2008 Mar 5 100(9) 093602(4). 

Exchange-Correlation Functionals with Quantum Memory. 

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