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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 10 December 2010, is one of a series of advanced technology reports produced in FY 2010 under the Advanced Aerospace Weapon System Applications (AAWSA) Program. It reviews quantum computing technologies and DNA-based molecular computing for onboard supercomputing on future spacecraft. It forecasts ion-trap quantum computers within 10 years, simple DNA tile computing within 20 years, and self-repairing DNA computers on a 40-year horizon.
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UNCLASSIFIED, /P8"1 8Pfll!IJJb Wlilii IUILY - Figure 6. Bilayer graphene tunneling device structure. Two sheets of graphene are separated by a one-nanometer thick insulating of graphene. Manipulation of Spin Qubits in Graphene Quantum Dots Relative to GaAs For universal quantum computing, single-qubit and two-qubit manipulations are necessary. Single-qubit rotations of spin qubits are naturally done by electron spin resonance (ESR) (63) and by electric- dipole-induced spin resonance (EDSR) (64). The Rabi frequency faabi at which the qubit rotates, for instance, in the ESR experiment (65) is proportional to the electron spin g-factor, faabi = gμsBac/2h where μs is the Bohr magneton and Bae the external oscillating magnetic field used to rotate the spin. Notably, the electron spin g-factor differs for different materials. In GaAs quantum dots, it has been measured to be lgl < 0.43 (66) whereas, in graphene quantum dots, it has been determined to be close to 191 = 2. (67) Thus, it is possible to rotate the electron spin in graphene quantum dots using ESR about five times faster than in GaAs quantum dots using the same field strength of the external oscillating magnetic field. This is an important gain because all qubit manipulations need to be done fast to avoid decoherence and implement fault-tolerant quantum computing (68). Another important advantage of graphene spin qubits is related to the small band gap in graphene nanoribbons. (For a ribbon width of about 30nm, the band gap can be estimated to be of the order of 60 meV.) This fact yields additional flexibility for two-qubit operations. Two-qubit operations are usually done via the Heisenberg exchange interaction (69). The tunneling matrix element can, however, be easily tuned by increasing or decreasing the overlap of the wave functions of the electrons in the two quantum dots. In graphene or any small band gap semiconductor, this manipulation can be done in two distinct ways: either through tunneling via conduction band states (i.e., normal tunneling) or through tunneling via valence band states (i.e., Klein tunneling). This has been predicted for graphene nanoribbons and experimentally realized in carbon nanotube quantum dots in (70)(71). The most important physical consequence of this additional flexibility is the appearance of a new type of long-distance coupling between graphene spin qubits as illustrated in Figure 8. 18 UNCLASSIFIED/ ,'a;QA. Qi;i;1&1.«1k WIiii 811b¥
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Report, from the dia collection. The PDF is mirrored here; the original link is under it. 54 pages are in the text index: search them above, or from the library's search.