Quantum memories capable of storing and retrieving coherent information for extended times at room temperature would enable a host of new technologies. Electron and nuclear spin qubits using shallow neutral donors in semiconductors have been studied extensively but are limited to low temperatures (≲10 kelvin); however, the nuclear spins of ionized donors have the potential for high-temperature operation. We used optical methods and dynamical decoupling to realize this potential for an ensemble of phosphorous-31 donors in isotopically purified silicon-28 and observed a room-temperature coherence time of over 39 minutes. We further showed that a coherent spin superposition can be cycled from 4.2 kelvin to room temperature and back, and we report a cryogenic coherence time of 3 hours in the same system.

A quantum computer requires systems that are isolated from their environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spins in solids promise long coherence lifetimes, but they are difficult to initialize into known states and to detect with high sensitivity. We show how the distinctive optical properties of enriched (28)Si enable the use of hyperfine-resolved optical transitions, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these resolved hyperfine transitions permit rapid nuclear hyperpolarization and electrical spin-readout. We combine these techniques to detect nuclear magnetic resonance from dilute (31)P in the purest available sample of (28)Si, at concentrations inaccessible to conventional measurements, measuring a solid-state coherence time of over 180 seconds.

As the deepest group-V donor in Si, Bi has by far the largest hyperfine interaction and also a large I = 9/2 nuclear spin. At zero field this splits the donor ground state into states having total spin 5 and 4, which are fully resolved in the photoluminescence spectrum of Bi donor bound excitons. Under a magnetic field, the 60 expected allowed transitions cannot be individually resolved, but the effects of the nuclear spin distribution, −9/2 ≤ Iz ≤ 9/2, are clearly observed. A strong hyperpolarization of the nuclear spin towards Iz = −9/2 is observed to result from the nonresonant optical excitation. This is very similar to the recently reported optical hyperpolarization of P donors observed by EPR at higher magnetic fields. We introduce a new model to explain this effect, and predict that it may be very fast.PACS numbers: 78.55. Ap, Recent proposals [1][2][3][4][5] to use the electron and nuclear spins of shallow donor impurities as qubits for Si-based quantum computing (QC) have led to renewed interest in these systems [5][6][7][8][9][10][11]. Most studies have focused on 31 P, the most common donor in Si, with an I = 1/2 nuclear spin. Most QC schemes involve enriched 28 Si, as this eliminates the 29 Si nuclear spin, but the removal of inhomogeneous isotope broadening [6] also enables an optical measurement of the donor electron and nuclear spin using the donor bound exciton (D 0 X) transition [7], and furthermore allows for the hyperpolarization of both spin systems at very low magnetic fields by resonant optical pumping [10]. McCamey et al. [9] have reported a different effect in which P nuclear hyperpolarization can be achieved with nonresonant optical excitation in natural Si at high magnetic field and low temperature.Bismuth is the deepest group-V donor in Si, with a binding energy of 70.98 meV [12], and is monoisotopic ( 209 Bi), with a large I = 9/2 nuclear spin and a hyperfine interaction of 1475.4 MHz, more than 12 times the 117.53 MHz value for 31 P [13]. While invoked in some QC proposals [4], Bi has not been the subject of recent study. It is interesting to note that the Bi D 0 X in Si is described in the earliest studies of bound excitons (BE) in semiconductors [14,15] but has received little attention since then [16]. This likely resulted from the scarcity of samples and, until now, their low quality.Recently [17], Si:Bi samples have been grown from ultrapure natural Si ( nat Si) using a floating-zone technique, for applications involving far-infrared lasers [18]. Samples from those same crystals are studied here, and show very reproducible D 0 X no-phonon (NP) photoluminescence (PL) structure over a wide range of Bi concentration. The spectra shown here are from a slice having a resistivity of 5.5 Ω·cm, mostly due to Bi, since the residual B and P concentrations are estimated to be at least an order of magnitude less than the Bi concentration. The sample was mounted without strain in a high homogeneity (0.01 %) split pair superconducting magnet dewar in Voigt configuration, with magnetic field ...

The electron and nuclear spins of the shallow donor 31 P are promising qubit candidates invoked in many proposed Si-based quantum computing schemes. We have recently shown that the near-elimination of inhomogeneous broadening in highly isotopically enriched 28 Si enables an optical readout of both the donor electron and nuclear spins by resolving the donor hyperfine splitting in the near-gap donor bound exciton transitions. We have also shown that pumping these same transitions can very quickly produce large electron and nuclear hyperpolarizations at low magnetic fields, where the equilibrium electron and nuclear polarizations are very small. Here we show preliminary results of the measurement of 31 P neutral donor NMR parameters using this optical nuclear hyperpolarization mechanism for preparation of the 31 P nuclear spin system, followed by optical readout of the resulting nuclear spin population after manipulation with NMR pulse sequences. This allows for the observation of singleshot NMR signals with very high signal-to-noise ratio under conditions where conventional NMR is not possible, due to the low concentration of 31 P and the small equilibrium polarization.Recent proposals [1][2][3][4][5] to use the electron and nuclear spins of shallow donor impurities as qubits for Si-based quantum computing (QC) have led to renewed interest in the properties of these systems [5][6][7][8][9][10][11][12][13][14][15][16][17]. Most of these studies focused on phosphorus, the most common shallow donor in Si, which has the added advantage of having only one stable isotope, 31 P, with an I = ½ nuclear spin. Many of these QC schemes involve enriched 28 Si as the host material, due to the elimination of the nonzero 29 Si nuclear spin, and the resulting greatly improved coherence times. It has recently been shown [6] that the elimination of inhomogeneous isotope broadening inherent in highly enriched 28 Si has the added benefit of enabling an optical measurement of the donor electron and nuclear spins using resolved hyperfine components in the donor bound exciton (D 0 X) transitions [7], and furthermore allows for the rapid hyperpolarization of both electron and nuclear spins at low magnetic fields by resonant optical pumping [10].Here we show preliminary results of NMR experiments performed on dilute 31 P in 28 Si using optical excitation of the D 0 X hyperfine components to initially hyperpolarize the nuclear spin, and optical readout of the change in nuclear spin brought about by the application of RF magnetic fields at the NMR frequencies. Previous conventional NMR studies of 31 P in Si were restricted to very heavily doped samples [18][19][20]. Our method provides information in a regime inaccessible to conventional NMR due to both the small number of spins and the low equilibrium polarization. We also obtain very high signal-to-noise ratios, allowing singleshot readout without the need for signal averaging. At higher magnetic fields, it is possible to detect neutral donor NMR indirectly in dilutely-doped sample...

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