Room-Temperature Qubits for Quantum Computing PI: Saritha Nellutla, Department of Chemistry and Biochemistry, Florida State University PI: Gregory S. Boebinger, Director National High Magnetic Field Laboratory Supported by NSF (No. DMR ), and State of Florida S. Nellutla, K.-Y. Choi, M. Pati, J. van Tol, I. Chiorescu and N. S. Dalal, Physical Review Letters 99, (2007) Single crystals of K 3 NbO 8 doped with Cr 5+ constitute a new candidate material for quantum computing. Cr atoms carry a spin with projection ±1/2, thus defining the two qubit states. Qubit interactions with the local environment - mostly neighboring nuclear spins - can be suppressed to a much greater extent in Cr doped K 3 NbO 8 than is usual for a solid state material. Driven coherent spin manipulations find Rabi oscillations (figure) that are observed up to room temperature. At liquid helium temperature, the phase coherence relaxation time T 2 reaches ~ 10 μs. A Rabi oscillation frequency of 20 MHz yields a single qubit figure of merit of about 500. This demonstrates unprecedented spin coherence for a spin system that is based on transition metal oxides, which are a class of materials that offers great flexibility for designing materials with promise for future applications. Figure: The crystalline structure of chromium-doped K 3 NbO 8 results in isolated electronic spins S=1/2 at the Cr atoms (green). (a) Coherence of the quantum superposition of the two spin states is demonstrated by the oscillations in the spin component. (b) The frequency of these Rabi oscillations varies linearly with the amplitude of the applied microwave pulse. 2007

Figure: In chromium-doped K 3 NbO 8 there are isolated electronic spins (brown arrows) at the Cr atoms (green). Applied pulses of microwave radiation (suggested in black at the Cr atoms) are used to control the quantum superposition of the two spin states known as |0> and |1>. The blue lines (background) are measured oscillations between spin states |0> and |1>, a demonstration of the coherence needed to envision quantum computing applications. Computer technology is reaching its limits of performance as devices approach the size of a few dozen atoms. Research is now exploring the viability of “quantum computing”, in which the power of quantum mechanics would expand computational power beyond anything imaginable with classical computers made from semiconductors. PI: Saritha Nellutla, Department of Chemistry and Biochemistry, Florida State University PI: Gregory S. Boebinger, Director National High Magnetic Field Laboratory Supported by NSF (No. DMR ), and State of Florida In a classical computer, information is stored in “bits” that can have the value of 0 or 1. In a quantum computer, quantum bits (qubits) are described using a set of two quantum states, |0> and |1>. Due to quantum mechanics, a qubit can exist in any combination of |0> and |1> states and thus can have an infinite number of values, not just two. Coherence, the ability to cleanly manipulate a qubit, must persist for sufficiently long periods of time to make meaningful computations, otherwise a quantum computer will never become a reality. Maintaining coherence in solid state materials is extremely challenging, because any interaction between the qubit and any other electrons or atoms in the material will typically scramble the state of the qubit, yielding the wrong answer. Room-Temperature Qubits for Quantum Computing 2007