DEMONSTRATION OF A SPIN COHERENT PHOTON TRANSMITTER/RECEIVER SYSTEM
An opportunity has emerged for secure communication and secure networking by quantum communication protocols. To fulfill this opportunity, we will need to transmit quantum information over long distances, and/or store quantum information for correspondingly long times. Such operations require an active optical medium that can store qubits, and act on the information. Two of the most promising forms of such active media are: atoms in vacuum, and spins in low temperature semiconductors.
We propose a semiconductor technology that most closely resembles conventional optical communications technology. The main added feature is that the semiconductor photo-detector and light-emitter are designed to transfer photon polarization information, to electron spins in a semiconductor, and back to photons again.
Even if one has a perfect source of qubits, channel losses severely limit the distance over which quantum information can be transmitted and shared. Longer codes will be required to allow for higher channel losses, mode-matching impediments, and finite quantum efficiency. Quantum repeater stations must store the quantum information, perform error recovery and then retransmit to the next station. These tasks are quite complex, and may require a medium scale integration capability that is available in semiconductors.
Spins in semiconductors can store quantum phase coherence for very long time periods. For example the homogeneous T2 dephasing time for a doped electron spin in Silicon has been measured by the spin-echo technique to be » 0.5msec, at low temperatures. This is comparable to interesting photon travel times for long distance communication. The T2 lifetime for the corresponding Phosphorus donor nuclear spin is many hours, more than enough for reliable mass storage.
If this proposal is funded, the material technology that we will use, is InxGa1‑xAs/InP wafer‑fusion bonded to Silicon. This material system is currently being commercialized by Ciena Corp., for a semiconductor avalanche photodetector technology. The same material system is also suitable for the single photon light emitter that was recently demonstrated by Yamamoto, our Co PI at Stanford. In both transmitter and receiver, the preservation of photon entanglement during the transfer to/from electron spins, requires the spectral selection of a single valence band Zeeman level in a magnetic field. The received photo-electrons are clocked into a shift register, similar to a CCD, except that spin information rather than electric charge is exchanged at each transistor. The qubit rotations and memory occur in SixGe1‑x, an electronic material that has now entered mass production. If this program is successful, it could lead to a spin-coherent transmit/receive front end, integrated onto medium scale SixGe1‑x quantum memory and quantum logic.
Repeaters, quantum memory, error correction, can all play a role. As each improvement is implemented, the secure communication system bit-rate increases, permitting an ongoing evolutionary path. Quantum memory is particularly important, as a large integrated storage buffer is needed for the highest, secure, bit-rate. It appears that semiconductor technology could put us on a predictable, evolutionary path, toward these very high levels of complexity.
DEMONSTRATION OF A SPIN COHERENT PHOTON TRANSMITTER/RECEIVER SYSTEM
| University of California, Los Angeles:
Principal Investigator: Prof. Eli Yablonovitch |
Stanford University:
Co-P. I: Prof. Yoshi Yamamoto |
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Co-P. I. Prof. Vwani Roychowdhury Co-P. I. Prof. Hong Wen Jiang Co-P. I. Prof. Jason Woo
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There is likely to be a continuous, evolutionary, improvement in the secure throughput of quantum communication systems, as better repeaters, more quantum memory, and error correction are implemented. Large quantum storage buffers will fulfill the Defense Dept.’s need for broadband secure networking. Semiconductor technology would provide a predictable, evolutionary, path toward these very high levels of complexity. In the long run these networks will have much in common with conventional optical communications and will run on semiconductor opto-electronic technology.
In this proposal, we will develop an optical communications receiver and transmitter technology that can transfer quantum coherence and entanglement from photons, to photo-electron spins in a semiconductor, and back to photons again. The preservation of photon entanglement during the transfer to/from electron spins, requires the spectral selection of a single valence band Zeeman level in a magnetic field. The InxGa1‑xAs quantum wells will be engineered for a maximum g-factor, g³9, in the valence band holes, to permit operation with a tiny permanent magnet.
Spins in cold semiconductors can store quantum phase coherence for very long time periods. For example the homogeneous T2 dephasing time for a doped electron spin in Silicon has been measured by the spin-echo technique to be » 0.5msec, at low temperatures. This is comparable to interesting photon travel times for long distance communication. The corresponding T2 dephasing lifetime for the matching donor 31P nuclear spin is probably many hours, more than enough for reliable mass storage of quantum information.
The proposed materials strategy is analogous to current thinking in the information industry: III‑V semiconductors for optical communication and silicon-based-semiconductors for memory and logic gates. Specifically we will adopt the InxGa1‑xAs/Si wafer fusion approach that is currently being commercialized by Ciena Corp. for solid-state avalanche photomultipliers (for classical optical communication.) By trapping the photo-electron spins at donors in a SixGe1‑x heterostructure, qubit spin operations can be controlled by an electrostatic gate that tunes the wave function between SixGe1‑x layers of different g‑factor. SixGe1‑x is now entering mass production for high speed transistors. Thus we are invoking relatively mature, established, semiconductor material systems, that are under commercialization, or are already in mass production.
Therefore we have assembled a scientific team with the particular skills, track record, and breadth to develop quantum communication technology and turn it into a reality. A key role will be played by Yoshi Yamamoto of Stanford, a pioneer in quantum opto-electronics, who has just demonstrated a semiconductor light emitter, in which small voltage pulses produce precisely one photon per pulse. He will participate jointly with a UCLA team that has just commenced a DARPA sponsored program to develop transistor detected electron spin resonance, sometimes called the Spin Resonance Transistor (SRT). Kang Wang, a pioneer in SixGe1‑x and III‑V MBE growth, brings over 15 years of experience in converting these heterostructures into innovative infrared devices for the Defense Dept. Kang will take responsibility for epi-growth. HongWen Jiang is a leader in the Physics of low temperature mesoscopic transport, including SET’s, the Coulomb blockade and the quantum Hall effect. He will develop our simple robust SRT’s as quantum logic gates, and for memory. Vwani Roychowdhury will be responsible for quantum communication system modeling and optimization, and for device modeling. Jason Woo, an authority on low temperature CMOS design and fabrication, will supply the design expertise for the many classical and quantum circuits that will be fabricated. Yablonovitch will act as PI, merging his knowledge of spin resonance physics, opto-electronic device design, and heterogeneous material integration.
The UCLA Spin Resonance Transistor (DARPA) program, that has just commenced, will leverage this proposed MURI, and permit it to concentrate on the communications aspects, while both programs jointly develop the quantum logic gates needed for quantum computing and communication.
The organization chart will be as follows: