February 2009

Identical photons from a semiconductor diode

Applied Physics Letters 92, 193503 (2008)

A.J. Bennett, R. B. Patel, A.J. Shields, K. Cooper, P. Atkinson, C.A. Nicoll, and D. A. Ritchie

Real-world applications of optical quantum information processing could become more practical if small and robust light sources were available that could generate identical, single photons. An experimental test of whether two photons are identical can be made by colliding two photons at a 50% reflecting, 50% transmitting mirror: if they are identical a quantum interference effect occurs whereby both photons exit along the same direction, as illustrated in Figure 1. This effect is the cornerstone of many proposed quantum computing schemes which operate with light.
 

Our goal was to demonstrate that it is possible to observe this effect with a simple light emitting diode (LED). Most LEDs currently produced emit uncontrolled numbers of photons at random times and with variable energies. Thus any given photon is unlikely to be identical to the others. For the last few years we have been developing a particular type of LED that contains a single semiconductor quantum dot, which is capable of emitting single photons, one at a time. Our recent advance is to find ways of reducing the error on the time and energy of photon emission so each photon can be identical. Shown in Figure 2 is a measurement where successive photons from one of our LEDs are collided on a mirror, from opposite directions. The probability of the two photons leaving in opposite directions falls when the photons arrive at identical times (reference 1). In this experiment the LED was operated in a pulsed mode at a rate of 500MHz. We have recently reported another experiment with the LED run on a DC current, in which case interference can surprisingly still be observed if fast enough detectors are used to make the measurements (reference 2).

 

References

[1] A. J. Bennett, R. B. Patel, A. J. Shields, P. Atkinson, C. Nicoll, K. Cooper and D. A. Ritchie, Appl. Phys. Lett. 92 (2008) 193503.
[2] R. B. Patel, A. J. Bennett, A. J. Shields, P. Atkinson, C. Nicoll, K. Cooper and D. A. Ritchie, Phys. Rev. Lett. 100 (2008) 207405.
 

April 2009

Ultralong spin coherence time in isotopically engineered diamond

Nature Materials 8, 383 - 387 (2009)

G. Balasubramanian, P. Neumann, D. Twitchen, M. Markham, R. Kolesov, N. Mizuochi1, J. Isoya, J. Achard, J. Beck, J. Tisler, V. Jacques, P. R. Hemmer, F. Jelezko and J. Wrachtrup

Overview

Apart from being valuable gem stones, diamonds possess superior material properties, which are of immense use in modern technology. Diamond tops in certain properties like thermal conductivity, hardness, charge carrier mobility, chemical inertness and optical transparency. In addition to these properties, doped diamond is gaining importance in the thriving age of spintronics.

Diamond crystals on doping with Nitrogen atoms forms defect centers called Nitrogen – Vacancy (NV) color centers. The structure of NV center consists of substitutional nitrogen at the lattice site neighboring a carbon vacancy (Figure 1a). Several unique properties make the NV centers particularly suitable for applications related to quantum information processing. Firstly the NV center exhibit strong optical absorption and high fluorescence yield that allows us to detect and address single defect centers using confocal fluorescence microscopy. Secondly it is an extraordinarily photostable single photon source. Third being the paramagnetic ground state of a NV defect can be used as qubit. Finally the fluorescence intensity of a NV defect is spin dependent, which allow us to readout the spin state via counting the number of scattered photons.
 

The negative charge state of the defect center is formed by three electrons associated with dangling bonds of vacancy, and the two electrons of the nitrogen and additional electron form an external donor. Two out of six electrons are unpaired forming a triplet spin system. Spin-spin interactions spit the energy levels with magnetic quantum numbers m_s=0 and m_s=1 by about 2.88 GHz. The degeneracy of m_s +/- 1 state that arise because of the C_3v symmetry can be lifted further by applying external magnetic field (Figure 1b). Under optical illumination, spin-selective relaxations lead to an efficient optical pumping of the system into m_s=0 state that allows fast initialization of the spin qubit. The spin state of a NV defect can be manipulated by applying resonant microwave fields. Hence all the necessary ingredients to prepare, manipulate and readout single spin qubit are readily available in diamond. Single defects can be isolated and individually addressed using confocal microscopy and nonlinear microscopic techniques that allow far field addressing of defects with a few nanometers spatial resolution.

NV centers in diamonds are robust spin system because of its coherence time is weakly affected by temperature. The two principal causes of decoherence in NV center in diamond are due to the magnetic field fluctuations caused by spins of substitutional nitrogen impurity and the presence of ¹³C isotope in the diamond lattice. Advances in synthetic diamond growth has successfully minimized these two factors, thus promoting diamond based spin systems to have ultra long coherence times ever achieved for a solid state system at room temperature. In this paper, the coherence time of about 1.8 ms is achieved for a NV spin in diamond made up of 99.7% of ¹²C isotopes and Nitrogen content less than 1 ppb. (Figure 1c) Such a Quantum grade diamond offers a spin free lattice and preserves the spin coherence time of the qubit very long, enabling numerous benchmark experiments in quantum information processing.

Taking into account the time required for single qubit gate of a few nanoseconds and a MHz speed for two-qubit CNOT, fidelity limit necessary for quantum error correction come within reach. Furthermore, ultra long coherence times potentially allow building quantum register based on magnetic dipolar coupling between isolated NV spins. The strength of dipolar interaction is in the range of few kHz for qubits spaced few tens of nanometers apart. Hence quantum register with individual addressable qubits using nonlinear microscopy technique can potentially be build at room temperature in solid state.

One application that was demonstrated is for sensing very weak magnetic fields. Using a single NV defect center magnetic fields as small as few nanoTesla was measured, with a sensitivity of about 4nT/√Hz (Figure 1d). Such sensitivity would allow probing external spins by measuring the coupling/decoherence. Nanoscale positioning/scanning offers advantages in developing novel microscope capable of imaging magnetic fields of nanostructure. Being an atomistic sensor, scanning them over the samples/molecules of interest would readily give the image of spin densities and dynamics. The method has far reaching potential in solving structure of biomolecules under ambient conditions. These NV magnetometers also find applications in sensing weak magnetic fields associated with ion currents through membrane channels in living cells.