A single-photon detector absorbs photons (particles of light), while faithfully transmitting quantum information onto individual, spinning electrons. In order for this data transfer to be efficient, electrons in the devices must reside within a thin layer referred to as a quantum well and have the property that, in a magnetic field, the energy for “spin-up” and “spin-down” electrons are equal. Through a process known as g-factor engineering, it is well-known that the magnetically-induced energy difference between “spin-up” and “spin-down” electrons can be controlled by varying the composition and thickness of the quantum well and the surrounding material (the barrier). For additional information see Kiselev, Kim and Yablonovitch “Designing a heterostructure for the quantum receiver” Applied Physics Ltrs. vol. 80, num. 16, pg. 2857-2859, 22 Apr. 2002, which is hereby incorporated herein by reference.
A prior art spin-coherent, single-photon detector is described in some detail already in R. Vrijen and E. Yablonovitch, “A spin-coherent semiconductor photo-detector for quantum communication” Physica E., vol. 10, pg. 569 (2001), which is hereby incorporated herein by reference.
The present disclosure suggests an improvement, namely, adding a third material, located within the quantum well itself, which should relax certain constraints on the design of so-called zero g-factor devices. The new material should possess a different g-factor and a different band alignment than the quantum well material and typically also different than the barrier material.
The addition of a third material to the design of zero g-factor devices is expected to provide the following advantages:
1. The quantum well should be less susceptible to fluctuations in its thickness because the presence of the third material within the well will allow it to be much thicker.
2. The spin-resonant transistor is a device for which the individual electrons are held in position by applying a positive voltage to a gate electrode. In some situations, it is desirable to modulate the g-factor by varying the bias on the gate electrode. The new quantum well disclosed herein will allow for the g-factor of the trapped electrons to be more sensitive to applied gate bias and therefore, will require less modulation to change the g-factor by a given amount. This is an advantage because too large a change in gate bias can either cause a trapped electron to leave or attract a second electron to enter the device. Both of these alternatives can be detrimental to the performance of the device.
3. The new quantum well is more flexible with respect to engineering tradeoffs between well thickness, g-factor, and band gap energies of the well and barrier materials. The added flexibility in the design makes the devices more easily compatible with commercial, networking technologies.
The term “zero g-factor devices” embraces, of course, devices which have a zero effective, weighted g-factor. But since devices whose effective, weighted g-factor is dose to zero are also satisfactory, one issue becomes how dose to zero is close enough? For a spin-coherent detector to work properly, the device is put in a magnetic field, which results in an energy difference between “spin-up” and “spin-down” electrons. This energy difference (referred to a the Zeeman energy) is directly proportional to both the strength of the aforementioned magnetic field and the g-factor. The choice of the magnetic field can vary significantly. The g-factor must be close enough to zero to make the Zeeman energy less than a linewidth (expressed in terms of energy) of the photons to be detected.
The disclosed technology is expected to be important to the development of novel, quantum information processing devices. Such devices will have applications in secure communications and quantum computing, currently active areas of research funded by the US Government (notably DARPA and ARDA).
The prior art is described in the aforementioned paper by Kiselev et al. (Applied Physics Letters, v. 80, p. 2857 (2002)) in which the authors thereof discuss how to design a heterostructure to achieve zero g-factor in a quantum receiver application. The results show that by varying the quantum well thickness and Ga concentration, for an InGaAs quantum well in InP, the g-factor can be tuned from large and negative, to somewhat positive. For a g-factor dose to zero, however, the thickness of the quantum well becomes very small and the Ga composition becomes large, resulting in films that are highly strained and metastable. In addition, single monolayer fluctuations in quantum well thickness can cause large variations (percentagewise) in the performance of these devices.
Modifying the structure of a quantum well to change its properties is not a new idea; however, this is believed to be the first time that anyone has applied these concepts to g-factor engineering. We have experimented with very thin InGaAs quantum wells in InP at HRL Laboratories LLC in Malibu, Calif., and know from experience that electrons within these layers are extremely difficult to control. The advantages of using a different material for the barrier and a third material within the well became clear to us during the course of our experimental work.