Optical interconnect in spin-based computation and communication systems

Techniques are described for data transfer in spin-based systems where digital bit values are represented by magnetization states of magnetoresistive devices rather than voltages or currents. For data transmission, a spin-based signal is converted to an optical signal and transmitted via an optical transport. For data reception, the optical signal is received via the optical transport and converted back to a spin-based signal. Such data transfer may not require an intervening conversion of the spin-based signal to charge-based signal that relies on voltages or currents to represent digital bit values. In addition, techniques are described to use magnetoresistive devices to control the amount of current or voltage that is delivered, where the magnetization state of the magnetoresistive device is set by an optical signal.

TECHNICAL FIELD

The disclosure relates to spin-based computation and logic systems, and more particularly to interconnect in spin-based computation and logic systems.

BACKGROUND

Power consumption and bandwidth limitation in electrical interconnects is a bottleneck to further improve performance and efficiency of charge-based large scale integrated circuits (ICs), such as those based on a complementary metal-oxide-semiconductor (CMOS) platform and architecture. In charge-based systems, electrical charge is used to represent state variables such as digital bits (e.g., high charge represents a digital high, low charge (or no charge) represents a digital low). Transmitting charge-based representations of digital bits at higher data rates requires additional power and bandwidth, which may not be available.

Spin-based systems can potentially address the limitations of charge-based electrical systems based on CMOS ICs at least with respect to power and architectural constraints of transmitting digital bits at higher data rates. In spin-based systems, an electron spin is used represent state variables. For example, through spin-transfer torque (STT), spins of electrons in one direction in a spin-polarized current cause a magnetic moment of a free layer of a magnetoresistive device (e.g. a magnetic tunneling junction (MTJ), a giant magnetoresistive (GMR) device) to align (e.g., magnetic moment to align) in the same direction (e.g., parallel magnetization state), and spins of electrons in the other direction in the spin-polarized current cause the free layer of the magnetoresistive device to align in the opposite directions (e.g., anti-parallel magnetization state).

In the spin-based systems, the magnetization state of the magnetoresistive devices is indicative of the digital bit. For example, a parallel magnetization state represents a digital low, and an anti-parallel magnetization state represents a digital high, or vice-versa. The magnetization state is read out from the measurement of the resistance value of the magnetoresistive devices, low resistance for the parallel magnetization state and high resistance for the anti-parallel magnetization state.

SUMMARY

This disclosure describes schemes for optical interconnects in spin-based computation and logic systems. In examples of spin-optical interconnect systems described herein, a transmitter converts a spin-based signal to an optical signal for transmission in an optical transport (e.g., optical fiber and optical waveguide), and a receiver converts an optical signal from the optical transport to a spin-based signal for reception. The optical signal may include a series of optical pulses, where the polarization of the optical pulses is indicative of the value of the digital bit. To convert the optical signal to the spin-based signal, the receiver may include a magnetoresistive device in which magnetization state of the device is controlled by the polarization of the optical pulses of the optical signal.

Optical transports (e.g., optical waveguides or fiber optic links) allow for optical signals to travel a relatively long distance at a high data rate, and conversion of spin-based signals to optical signals and back may consume a relatively small amount of power. In this manner, the techniques described in this disclosure may provide a scheme to transmit and receive data over medium to long distances in spin-based computation, logic, and communication systems in a high bandwidth medium and power efficient way without needing to convert the spin-based signals to electrical current-based signals.

In some examples, the techniques utilize the magnetoresistive device to control current and voltage. In such examples, the polarization of an optical signal sets the magnetization state of the magnetoresistive device, and the magnetization state defines the electrical properties of the magnetoresistive device. Accordingly, by setting the magnetization stage of the magnetoresistive device with an optical signal, the techniques configure the electrical properties of the magnetoresistive device to control the current and voltage that is delivered to circuit components.

In one example, the disclosure describes a method of data transfer in a spin-based system, the method comprising receiving an optical signal that represents digital bit values. The method also includes converting the optical signal directly into a spin-based signal without converting the optical signal into a charge-based signal. In this example, the spin-based signal represents the digital bit values of the optical signal by magnetization states of a magnetoresistive device.

In one example, the disclosure describes a device comprising a spin-to-optical transmitter configured to convert a spin-based signal into an optical signal for transmission. The spin-based signal represents digital bit values of the optical signal by magnetization states of a magnetoresistive device. The device also includes an optical-to-spin receiver configured to convert the optical signal back to a spin-based signal for reception.

In one example, the disclosure describes an optical-to-spin receiver comprising an input configured to receive an optical signal that represent digital bit values, and a magnetoresistive device configured to directly convert the optical signal into a spin-based signal. The spin-based signal represents the digital bit values of the optical signal by magnetization states of the magnetoresistive device. The conversion of the optical into the spin-based signal occurs without a conversion of the optical signal into a charge-based signal.

In one example, the disclosure describes a device comprising an input circuit, an output circuit, and a magnetoresistive device having a magnetization state. The magnetoresistive device is configured to receive an optical signal and set a magnetization state of the magnetoresistive device based on the optical signal to control voltage or current from the input circuit that is delivered to the output circuit.

In one example, the disclosure describes a magnetoresistive device comprising a first layer comprising ferromagnetic material having an magnetization direction, a second layer having a magnetization direction configurable based on polarization of light received by the second layer, and a third layer sandwiched between the first layer and the second layer. In this example, the magnetoresistive device is configured to output a signal responsive to an alignment of the magnetization direction of the first layer relative to the magnetization direction of the second layer.

DETAILED DESCRIPTION

FIG. 1is a block diagram illustrating a spin-based system10with optical interconnect, in accordance with one or more techniques of this disclosure. System10may be an all spin-based system or a hybrid spin-based system. In the all spin-based system example, all units of system10, including sub-units within a unit, are spin-based. In the hybrid spin-based system, some of the units or sub-units within a unit are spin-based and others are charge-based. As used in this disclosure, “spin-based” refers to examples in which state variables such as digital bit values are represented by electron spins in a spin-polarized current that set the magnetization states of magnetoresistive devices based on the spin direction, and “charge-based” refers to examples in which state variables such as digital bit values are represented by charge levels (e.g., voltage or current levels).

In a spin-based system, the electron spin direction in a spin-polarized current represents a digital bit value (e.g., one spin direction for a digital high and the other spin direction for a digital low). Because the spin direction of the spin-polarized current sets the magnetization state of a magnetoresistive device (e.g., a magnetic tunnel junction (MTJ) or giant magnetoresistance (GMR) device), the magnetization state of the magnetoresistive device can be indicative of the digital bit value in spin-based systems.

For example, electrons spins in one direction in the spin-polarized current set the magnetoresistive device to a first magnetization state indicative of a digital high and electrons spins in the other direction in the spin-polarized current set the magnetoresistive device to a second magnetization state indicative of a digital low, or vice-versa. However, the spin-polarized current can only diffuse over a relatively short length meaning that after a relatively short distance, the spins of the electrons may no longer be in the same direction in the spin-polarized current (i.e., be depolarized), and the digital bit value indicated by the spin of the electrons may be lost. For example, even in the spin channel materials such as graphene the diffusion length is only a few tens of micrometers. Accordingly, using spin-polarized current to transmit data over medium and long distances (e.g., tens of microns to millimeters) between spin-based units may not be feasible in spin-based systems.

As described in more detail, the disclosure describes using optical interconnect as a way in which to transport spin-based signals optically over medium and long distances, where spin-based signals refer to a series of digital bits that are represented by the magnetization state of a magnetoresistive device or spin-polarized current. In this manner, spin-based signals can be transmitted over relatively longer distances in the form of optical signal without converting the spin-based signals to charge-based signals. For instance, converting spin-based signals to charge-based signals may be inefficient in power consumption and may be limited in bandwidth in a way that using optical interconnect is not (e.g., the bandwidth of electrical wires is far less than the bandwidth of optical transports).

System10is an example of a computing system, such as a spin-based computing system. However, the techniques described in this disclosure are not so limited and may be extended to computation, logic, or communication systems, and generally to spin-based systems in which one unit transmits or receives relatively high data rate signals (e.g., in the 10 giga-bits per second range) over a medium to long distance (e.g., tens of micros to millimeters). It should be understood that although the techniques are described with respect to high data rate signals that travel over a medium to long distance, the techniques described in this disclosure may be useable for data signals with low data rates and/or data signals that need to travel relatively short distances.

As illustrated, system10includes processor12and system memory24. Processor12includes processor core14, local memory16, and memory interface18. Processor12may include more units than illustrated inFIG. 1, and processor core14, local memory16, and memory interface18are illustrated merely to ease with understanding. Other units of processor12may communicate in a manner similar to the description below for processor core14, local memory16, and memory interface18.

In some examples, one or more of processor core14, local memory16, and memory interface18may be spin-based units. For example, processor core14may process data using spin-based logic gates, rather than charge-based logic gates, and local memory16may store digital bits as magnetization states of magnetoresistive devices within local memory16. Accordingly, in some examples, processor12may be a spin-based system. For instance, in such examples, processor12is a spin-based system within the larger spin-based system10.

System memory24is the system memory for system10. For example, local memory16may be memory that only units of processor12can access, whereas other units of system10, including processor12, access system memory24. As illustrated, memory interface18may be the interface by which processor core14stores data to or receives data from system memory24. In some examples, system memory24may store digital bits as magnetization states of magnetoresistive devices within system memory24. In this sense, system memory24may also be a spin-based system that is part of the larger spin-based system10.

In some examples of processor12, there may be multiple processor cores14, each can access its private local memory16, or they can share a common local memory16. Multiple process cores14, local memory16and memory interface18can be on the same chip of integrated circuits.

In general, the techniques described in this disclosure are implemented by a device. One example of a device is the chip that includes processor12(e.g., an integrated circuit (IC) chip). In some examples, system10is configured on a circuit board (e.g., processor12and system memory24are on the same circuit board). The circuit board that includes system10is another example of a device. In some examples, system10is an appliance in which processor12and system memory24are located on different boards. Such an appliance that includes system10is another example of a device. In some examples, system10is configured across different appliances, and a device includes a plurality of different interconnected appliances. There may be different ways to connect components of system10in a spin-based system, and each of these different ways may be additional examples of a device configured to implement the example techniques described in this disclosure.

For example, the spin-based units of the device communicate with one another using an optical interconnect. As illustrated inFIG. 1, processor core14includes spin-optical transceiver20A and20C, local memory16includes spin-optical transceiver20B, memory interface18includes spin-optical transceiver20D and20E, and system memory24includes spin-optical transceiver20F. Spin-optical transceivers20A-20F are collectively referred to as “spin-optical transceivers20.”

Each one of spin-optical transceivers20includes a spin-to-optical transmitter (referred to simply as transmitter) and an optical-to-spin receiver (referred to simply as receiver). As illustrated, each one of spin-optical transceivers20transmits and receives data via respective ones of optical transports22A-22C (collectively referred to as “optical transports22”). Examples of optical transports22include optical waveguides and fiber optic links. For example, for medium distances such as approximately tens of microns to a few millimeters, optical transports22may be optical waveguides. For longer distances such as more than tens of millimeters, optical transports22may be fiber optic links. For instance, optical transport22A is an optical waveguide, and optical transport22C is a fiber optic link. As described in more detail, in some examples, each one of optical transports22may include two optical links, where each optical link is an optical waveguide or a fiber optic link.

As described above, in spin-based system10, the magnetization state of a magnetoresistive device corresponds to a binary value of a digital bit. One example of a magnetoresistive device is a magnetic tunnel junction (MTJ). An MTJ includes two ferromagnetic layers that sandwich an insulator layer. The magnetization direction of one of the two ferromagnetic layers is fixed, and the magnetization direction of the other of the two ferromagnetic layers can be changed. If the magnetization directions of both the ferromagnetic layers are the same, the MTJ is in a parallel magnetization state with low resistance, and if the magnetization directions of both the ferromagnetic layer are different, the MTJ is in an anti-parallel magnetization state with high resistance. Another example of a magnetoresistive device is a giant magnetoresistive (GMR) device that may function similar to the MTJ. A GMR includes two ferromagnetic layers that sandwich a non-magnetic metal spacer layer.

The parallel magnetization state corresponds to one digital value (e.g., a digital low), and the anti-parallel magnetization state corresponds to the other digital value (e.g., a digital high). The parallel magnetization state corresponding to a digital low and the anti-parallel magnetization state corresponding to a digital high is one example assignment between magnetization state and digital bit values. It should be understood that the parallel magnetization state may correspond to a digital high, and the anti-parallel magnetization state may correspond to a digital low in some examples.

In the techniques described in this disclosure, the transmitters of each one of spin-optical transceivers20transmit an optical signal that includes a plurality (e.g., series) of optical pulses via respective optical transports22based on the magnetization states of a magnetoresistive device. One example of how the transmitters transmit the optical signal based on the magnetization states of the magnetoresistive device is described in more detail with respect toFIG. 2. Also, in the techniques described in this disclosure, the receivers of each one of spin-optical transceivers20receive the optical signal from respective optical transports22and convert the optical signal into corresponding magnetization states of a magnetoresistive device. One example of how the receivers convert the optical signal into corresponding magnetization states of a magnetoresistive device is also described in more detail with respect toFIG. 2.

As an illustrative example, when processor core14needs to store data (e.g., a series of digital bits) in local memory16, processor core14sets the magnetization state of an MTJ to the magnetization state that corresponds to the first bit in the series of digital bits via a spin-polarized current with electrons in a first or a second spin direction depending on the digital bit value, as one example. The transmitter of spin-optical transceiver20A outputs an optical pulse based on the magnetization state of the MTJ via optical transport22B. Processor core14then sets the magnetization state of the MTJ to the magnetization state that corresponds to the second bit in the series of digital bits via a spin-polarized current with electrons in the first or second spin direction depending on the digital bit value. The transmitter of spin-optical transceiver20A outputs an optical pulse based on the magnetization state of the MTJ via optical transport22B, and so forth until processor core14transmits all of the digital bits in the series of digital bits.

In some examples, the duration of the optical pulse may be based on the data rate (e.g., for a 100 giga-bits per second signal, the duration of the optical pulse may be 10 picoseconds, which is the temporal length of a bit). Also, as described in more detail, in some examples, the polarization of the optical pulse is based on the digital bit value that is to be transferred.

On the receive end, the receiver of spin-optical transceiver20B receives the optical signal for the first bit in the series of digital bits via optical transport22B and converts the optical pulse into a magnetization state of an MTJ of the receiver that corresponds to the first bit in the series of digital bits based on the polarization of the optical pulse. The receiver of spin-optical transceiver20B receives the optical pulse for the second bit in the series of digital bits via optical transport22B and converts the optical signal into a magnetization state of the MTJ of the receiver that corresponds to the second bit in the series of digital bits based on the polarization of the optical pulse, and so forth under local memory16receives all of the digital bits in the series of digital bits for storage.

By using optical interconnects (e.g., optical transports22) to transmit and receive data (e.g., digital bits), system10may be configured to transmit and receive data at a relatively high data rate (e.g., 10 giga-bits per second and greater) in a spin-based system over medium and long distances. In some charge-based systems, there may be power and bandwidth limitations to transmitting and receiving data at such high data rates. For instance, electrical interconnects such as wires or transmission lines may limit improvements in the performance and efficiency of a charge-based processor or charge-based system.

Optical interconnect addresses the bandwidth limitations of the electrical interconnect. For example, replacing wires or transmission lines with optical waveguides or fiber optic links boosts the bandwidth and reduces energy consumption as compared to charge-based computation and communication systems.

While optical interconnect may address some of the limitations in charge-based systems, optical interconnect may not be able to overcome some of the other limitations in charge-based systems. As one example, even with optical interconnect, charge-based systems, such as CMOS ICs, may consume a relatively large amount of power to operate at relatively high data rates. There may also be architectural constraints to charge-based CMOS ICs. However, spin-based systems may not need to consume as much power as CMOS ICs to operate at high data rates; nor do spin-based systems have the same architectural constraints as charge-based CMOS ICs, and may therefore allow for performance improvements not available in charge-based systems. For example, spin based systems may be more suitable than charge-based CMOS system to implement non-Boolean primitives and non-von Neumann architectures for neuromorphic systems with applications such as visual recognition and machine learning.

As described above, although spin-based systems may provide advantages over charge-based systems, in some cases, spin-based systems are limited in the distance they can transmit data. For instance, a spin-polarized current has a short diffusion length. This short diffusion length of the spin-polarized current means that it is impractical to interconnect spin devices (e.g., processor core14, local memory16, and memory interface18of processor12as one example) with spin-polarized current directly over medium and long distance (tens of microns to millimeters).

To overcome the limitations of transmitting spin-polarized current over medium to long distances, it may be possible to convert the spin-polarized current to a charge-based current, and transmit the charge-based current over medium and long distances. However, converting to the charge-based current is inefficient in energy and would encounter the same bandwidth limitations described above.

Using spin-optical interconnect system for spin based computation and logic systems, as described in this disclosure, provides the advantages of having spin-based units (e.g., processor12and system memory24or processor core14, local memory16, and memory interface18of processor12) without the limitations of transmitting data only over short distances. For example, converting spin-based signals (e.g., digital bits are represented by magnetization state of magnetoresistive devices) to optical signals allows for data transmission via optical transports22along medium or long distances. Also, converting the optical signals back to spin-based signals does not require an intermediate conversion to charge-based current.

FIG. 2is a block diagram illustrating an example of transmitter26of spin-optical transceiver20A, an optical transport22B, and a receiver36of spin-optical transceiver20B, in accordance with one or more techniques of this disclosure.FIG. 2illustrates an example of converting a spin-based signal into an optical signal for transmission, and converting the optical signal back to a spin-based signal for reception. As described in more detail, in the techniques described in this disclosure, transmitter26converts the spin-based signal into an optical signal without an intervening conversion to a charge-based signal. Similarly, receiver36converts the optical signal back to a spin-based signal without an intervening conversion to a charge-based signal.

InFIG. 2, although only transmitter26of spin-optical transceiver20A is illustrated, it should be understood that spin-optical transceiver20A includes a receiver substantially similar to receiver36. Also, although only receiver36of spin-optical transceiver20B is illustrated, it should be understood that spin-optical transceiver20B includes a transmitter similar to transmitter26. Furthermore, althoughFIG. 2illustrates transmitter26of spin-optical transceiver20A and receiver36of spin-optical transceiver20B, spin-optical transceivers20C-20F each include similar transmitters and receivers.

Also, transmitter26and receiver36are one example of ways in which to convert spin-based signals to optical signals and optical signals back to spin-based signals. However, the techniques described in this disclosure are not limited to the specific configuration of transmitter26and receiver36. The techniques described in this disclosure describe using a transmitter to convert spin-based signals to optical signals for transmission in an optical transport, and a receiver to convert the optical signals received from the optical transport to the spin-based signal, and transmitter26and receiver36are merely one example way in which to perform such transmission and reception.

In the example illustrated inFIG. 2, receiver36includes magnetoresistive device40, an example of which is an MTJ or a GMR. Magnetoresistive device40may be a special type of MTJ or GMR whose magnetization state is set by polarization of optical pulses. As one example, magnetoresistive device40may utilize the helicity-dependent all-optical switching (HD-AOS) effect in thin films of rare-earth (RE) and transition metal (TM) alloys.

For example, ultrafast circular polarized (CP) optical pulses have been used in experiments to manipulate spin states in a broad range of magnetic material systems, including ferromagnetic semiconductors and metals, such as those of magnetoresistive device40. The right or left CP light applies an effective magnetic field along or opposite, respectively, the direction of light propagation on the material that light is incident on. The application of an effective magnetic field along or opposite the direction of light propagation is referred to as the inverse Faraday effect (IFE).

Using CP light pulses, it is possible to deterministically switch the magnetization state in RE-TM ferrimagnetic film of Gadolinium-Iron-Cobalt (GdFeCo) alloy, as one example, other types of alloys may also be used. For instance, the light of the CP light pulses can be used set the magnetization state of magnetoresistive device40. In other words, if the layer of magnetoresistive device40that can change its magnetization direction is formed from a GdFeCo film, then the magnetization state of magnetoresistive device40can be controlled based on the polarization of the received optical pulse when the magnetoresistive device40receives the light of the polarized optical pulses.

As illustrated inFIG. 2, grating reflector48receives the polarized optical pulses. Magnetoresistive device40couples to grating reflector48, and magnetoresistive device40may be arranged perpendicular to the waveguide that carries the optical pulses. Grating reflector48bends the optical pulses 90° so that the bottom layer of magnetoresistive device40receives the polarized optical pulses. The magnetization state of magnetoresistive device40is then set based on the polarization of the optical pulses, where each of the magnetization states of magnetoresistive device40corresponds to different digital bit values.

For example, right circular polarized (RCP) light pulse may flip a domain with perpendicular magnetization in the opposite direction of light incidence while left circular polarized (LCP) light may have no effect on such a domain. If the domain magnetization is reversed, the effects of RCP and LCP are also reversed. Accordingly, the magnetization state of magnetoresistive device40may only be dependent on the helicity (e.g., LCP or RCP) of the incident light. In this way, the techniques described in this disclosure may utilize the helicity state of the light pulse as a way to transfer digital bits by setting the magnetization state of magnetoresistive device40to the magnetization state that corresponds to the transferred digital bits. Stated another way, from the view of data transfer, the helicity dependent all-optical switching (HD-AOS) effect may directly convert the helicity states of the CP light to the spin states of the magnetic material of magnetoresistive device40.

FIG. 3is a conceptual diagram illustrating an example representation of logical bits by magnetization states and polarization states of circular polarized light, in accordance with one or more techniques of this disclosure. As illustrated inFIG. 3, a right circular polarized (RCP) light pulse causes the GdFeCo layer of magnetoresistive device40to align (e.g., magnetic moment to align) with the top layer of magnetoresistive device40in what is referred to as a parallel magnetization state, and a left circular polarized (LCP) light pulse causes the GdFeCo layer of magnetoresistive device40to be opposite with the top layer of magnetoresistive device40in what is referred to as an anti-parallel magnetization state. In one example, a first helicity state (e.g., RCP) of the light pulse may be associated with a first digital bit value (e.g., a digital low), and a second helicity state (e.g., LCP) of the light pulse may be associated with a second digital bit value (e.g., a digital high).

In some examples, magnetoresistive device40may integrate RE-TM alloy film that can be optically switched to a magnetic tunnel junction (MTJ), which is why an MTJ is one example of magnetoresistive device40. There may be other examples of magnetoresistive device40in addition to an MTJ. Another example of magnetoresistive device40is a current perpendicular to the plane (CPP) giant magnetoresistance (GMR) device.

In some examples, the magnetic layer in an MTJ or an GMR device that could be switched by HS-AOS could be the composite structure that consists of one RE-TM or TM layer ferromagnetically or antiferromagnetically exchanged coupled with another RE-TM or TM layer. For example, the RE-TM layer may be Terbium-Iron-Cobalt (TbFeCo) with a wide range of composition for the optimal compensation temperature and Curie temperature. Examples of the TM layer include Cobalt (Co), Cobalt-Iron (CoFe) alloy, Cobalt-Iron-Boron (CbFeB) alloy, a plurality of sub-layer pairs that include Cobalt/Palladium (e.g., [Co/Pd]n), or a plurality of sub-layer pairs that includes Cobalt/Platinum (e.g., [Co/Pt]n), etc.

Because the RCP light pulse sets magnetoresistive device40in the parallel magnetization state, the parallel magnetization state of magnetoresistive device40may correspond to a digital low. Because the LCP light pulse sets magnetoresistive device40in the anti-parallel magnetization state, the anti-parallel magnetization state of magnetoresistive device40may correspond to a digital high. In this way, if magnetoresistive device40receives a RCP light pulse, magnetoresistive device40may indicate that a digital low was received, and if magnetoresistive device40receives a LCP light pulse, magnetoresistive device40may indicate that a digital high was received. The assignment of RCP light pulse to a digital low or setting the magnetization state of magnetoresistive device40to the parallel magnetization state, and the assignment of LCP light pulse to a digital high or setting the magnetization state of magnetoresistive device40to the anti-parallel magnetization state is provided merely as one example and should not be considered limiting.

Magnetoresistive device40may change magnetization states ultrafast with a time constant of less than a few picoseconds. Accordingly, it may be possible to determine the bits of high data rate signals (e.g., 100 giga-bits per second).

As an illustrative example, assume that it takes 2 picoseconds to change the magnetization state of magnetoresistive device40. Also, assume that magnetoresistive device10receives a LCP pulse for 10 picoseconds, a RCP pulse for 10 picoseconds, a LCP pulse for 10 picoseconds, a LCP pulse for 10 picoseconds, a RCP pulse for 10 picoseconds, and a RCP pulse for 10 picoseconds. In this example, 10 picoseconds is selected because 10 picoseconds is the amount of time of a digital high or digital low that corresponds to 100 giga-bits per seconds.

In this example, for the latter 8 picoseconds of the first 10 picoseconds, the magnetization state of magnetoresistive device40would be anti-parallel (where the magnetization state transitions to the anti-parallel state in the first 2 picoseconds of the 10 picoseconds). For the latter 8 picoseconds of the next 10 picoseconds, the magnetization state of magnetoresistive device40would be parallel (where the magnetization state transitions to the parallel state in the first 2 picoseconds of the 10 picoseconds). For the latter 8 picoseconds of the next 10 picoseconds, the magnetization state of magnetoresistive device40would be anti-parallel (again, 2 picoseconds used to transition to the anti-parallel state). For the next 10 picoseconds, the magnetization state of magnetoresistive device40would remain in the anti-parallel state. For the latter 8 picoseconds of the next 10 picoseconds, the magnetization state of magnetoresistive device40would be in the parallel state, and for the final 10 picoseconds, the magnetization state of magnetoresistive device40would be in the parallel state. In this example, if a 100 giga-Hertz clock, centered in the 10 picoseconds of the light pulses, is used to read the magnetization state of magnetoresistive device40, magnetoresistive device40would indicate that receiver36received digital bits: 101100.

The exact reason why magnetoresistive device40changes states when exposed to polarized light pulses is still under research. For instance, some studies have shown that the ultrafast heating generated by the absorption of ultra-short laser pulses, and not the angular momentum of the laser pulses, causes the switching in the magnetization state. In any event, whatever the cause of the switching in the magnetization state, within a window of pulse energy (e.g., within 10 picosecond laser pulse), deterministic switching can still be achieved using left or right circular polarized (LCP or RCP) laser pulses of a few picoseconds. Also, it should be noted that HD-AOS has also been achieved in other RE-TM materials such as TbFeCo and TbCo and many other magnetic material systems, including ferromagnetic thin films and multilayer structures, synthetic ferromagnetic and ferrimagentic multilayers. Therefore, these example materials can also be used in examples of magnetoresistive device40as receivers in this application.

As described above, magnetoresistive device40may be a magnetoresistive device whose magnetization state is set by the polarization of light incident to magnetoresistive device40. The following describes the structure of such an example of magnetoresistive device40.

FIG. 4is a block diagram illustrating an example magnetoresistive device stack structure with optically switchable layer of ferromagnetic film and integrated on silicon substrate, in accordance with one or more techniques of this disclosure. In the illustrated example, magnetoresistive device40is an MTJ with a ferromagnetic film for GdFeCo. InFIG. 4, magnetoresistive device40is a hybrid MTJ structure with an optically switchable GdFeCo layer, which is exchange-coupled with a perpendicular free layer composed of multilayers of Fe and Pd, a tunneling junction of MgO layer and fixed, multilayers of Fe and Pd. In the example illustrated inFIG. 4, there are 1-5 multilayers of Fe and Pd in the top layer, and 1-20 multilayers of Fe and Pd in the bottom layer. The thickness of the MgO layer may be approximately 0.6 nm to 1.2 nm (as one example).

After post-annealing, the multilayer structure of [Fe/Pd]nlayers or FePd alloy functions as the source of perpendicular anisotropy for the fixed layer and part of the free layer. The multilayer structure may be replaced by other perpendicular material that may have higher magnetoresistance ratio. In some cases, GdFeCo layer or other RE-TM layer may alone be configured with perpendicular anisotropy as well.

In some examples, it may be possible to generate an MTJ structure or GMR with one layer coupled with the HS-AOS. For instance, such example of the magnetoresistive device may include an optical transparent protection layer, HS-AOS sensitive magnetic layer (e.g., layer whose state is set by the helicity), and magnetic layer for anisotropy and magnetoresistance tunnel barrier (for MTJ) or spacer layer (for GMR). The MTJ or GMR of such examples may also include a pinned magnetic layer and a pinning magnetic layer. Such examples of magnetoresistive device40may be directly coupled to the waveguide that transports the optical pulses starting with the HS-AOS sensitive layer or the protector or seed layer.

In the example ofFIG. 4, optical pulses can switch the magnetization of the GdFeCo layer (switchable layer) from parallel to anti-parallel, and the free layer will switch at the same time due to strong exchange-coupled with the switchable GdFeCo layer. In this example, the tunneling magnetoresistance (TMR) of the MTJ may change from low to high so that the optical information is converted to the magnetic states. The example stack structure of magnetoresistive device40inFIG. 4may also improve thermal stability of the switchable layer and increase the TMR ratio of the MTJ. In this manner, the magnetization state of magnetoresistive device40indicates the digital bit value in optical-spin receiver36in spin-optical interconnect system10.

In the example illustrated inFIG. 4, the Fe/Pd multilayer may provide the perpendicular anisotropy and low damping constant for both free and fixed layers. Additional examples of the free and fixed layers materials include Co/Pt, Co/Pd multilayer, L10FePt, L10FePd, L10CoPt, L10CoPd, CoFeB, or their combination. Additional examples of the tunnel barrier include Al2O3, BN, graphene, or others. In some examples, the GdFeCo layer could be replaced by TbFeCo, Co/Pt, Co/Pd, multilayer or their synthetic antiferromagnetic structure coupling through a thin Ru layer. For some applications, longitudinal magnetization or tilted magnetization may be used as well.

As described above, one example of magnetoresistive device40is an MJT structure. In some examples, such an MTJ structure includes a first layer comprising ferromagnetic material, a second layer whose magnetization direction is configurable based on polarization of light received by the second layer, and an insulator layer sandwiched between the first layer and the second layer. The first layer may be pinned by an antiferromagnetic layer or synthetic antiferromagnetic layer. The second layer may be composed of HS-AOS switchable layer (e.g., switchable based on helicity) and another magnetic layer (e.g., CoFeB, CoFe, etc.) that directly contacts with the tunnel barrier.

Another example of magnetoresistive device40is a GMR. Such a GMR may include a first layer comprising ferromagnetic material, a second layer whose magnetization direction is configurable based on polarization of light received by the second layer, and a non-magnetic spacer layer (e.g., Cu, Ag, etc.) sandwiched between the first layer and the second layer. The first layer may be pinned by an antiferromagnetic layer or synthetic antiferromagnetic layer. The second layer may be composed of HS-AOS switchable layer and another magnetic layer (e.g., CoFe, FeNi, etc.).

For example, the first layer of magnetoresistive device40, in examples where magnetoresistive device is an MTJ or GMR, may be a ferromagnetic material having a magnetization direction, and a second layer having a magnetization direction configurable based on polarization of light received by the second layer. In the techniques described in this disclosure, magnetoresistive device40may be configured to output a signal responsive to an alignment of the magnetization direction of the first layer relative to the magnetization direction of the second layer.

For instance, magnetoresistive device40may output a signal (e.g., indicate) a digital low or a digital high based on whether the layers of magnetoresistive device40are aligned parallel or anti-parallel. In such an example, the signal output by magnetoresistive device40may be a spin-based signal in which a respective spin of each electron represents the alignment of the magnetization direction of the first layer relative to the magnetization direction of the second layer. For example, the spin of the spin-polarized current output by magnetoresistive device40may be indicative of the digital value, and the spin may be based on the alignment of the magnetization direction of the layers.

Some charge-based examples may also utilize a magnetoresistive device, as described with respect toFIG. 8. In such charge-based systems, the optical light may set the magnetization state, which in turn sets the resistance of the magnetoresistive device. In such examples, the output of the magnetoresistive device is based on the resistance of the magnetoresistive device, which is set by the polarization of the light. In other words, the signal output by the magnetoresistive device, such as the one illustrated inFIG. 8and described in more detail below, may vary in response to a change in resistance of the magnetoresistive device based on the polarization of the light.

Referring back toFIG. 2, there may be many ways in which receiver36receives an optical pulse in a first polarization indicative of a first digital bit value and receives an optical pulse in a second polarization indicative of a second digital bit value andFIG. 2illustrates one example way. For instance, receiver36includes polarization converter38. Polarization converter38includes two input ports coupled to optical links42and44of optical transport22B and one output port coupled to grating reflector48that couples to magnetoresistive device40. The input ports coupled to optical links42and44are one example of the input of receiver36.

Grating reflector48may not be needed in every example, such as in examples where it is possible for magnetoresistive device to receive optical pulses directly from polarization converter38. In examples where magnetoresistive device40is arranged perpendicular to polarization converter38(e.g., polarization converter38is laid out horizontally on a board and magnetoresistive device40is coupled vertically to the board), grating reflector48may reflect the optical signal 90° so that magnetoresistive device40receives the optical pulses.

FIG. 5is a schematic diagram illustrating an example of a 2D grating coupling emitting left circular polarized and right circular polarized optical pulses, respectively, in accordance with one or more techniques of this disclosure. In particular,FIG. 5illustrates one example of grating reflector48. Grating reflector48is a 2D grating coupler design that can emit circularly polarized optical pulsed in perpendicular direction. Magnetoresistive device40can be directly integrated on top of grating reflector48to receive the optical pulses.

Returning toFIG. 2, polarization converter38receives a linearly polarized light pulse via either optical link42or optical link44. For example, if receiver36is to receive a digital high, receiver36receives an optical pulse via optical link42, and if receiver36is to receive a digital low, receiver36receives an optical pulse via optical link44. In other words, receiver36receives optical pulses of the optical signal via optical link42for the digital bit values equal to a digital low, and receives optical pulses of the optical signal via optical link44for the digital bit values equal to digital high.

When receiver36receives an optical pulse via optical link42, polarization converter38left circular polarizes the optical pulse and outputs the left circular polarized optical pulse to magnetoresistive device40which sets magnetoresistive device40into an anti-parallel magnetization state. When receiver36receives an optical pulse via optical link44, polarization converter38right circular polarizes the optical pulse and outputs the right circular polarized optical pulse to magnetoresistive device40which sets magnetoresistive device40into a parallel magnetization state.

As illustrated, receiver36converts the optical pulse to a right circular polarized optical pulse or a left circular polarized optical pulse based on whether the optical pulse is received from optical link42or optical link44. However, the techniques described in this disclosure are not so limited.

In some examples, it may be possible to circular polarize the optical pulse at the transmitter26end. In these examples, rather than having two optical links42and44, optical transport22B may include only one optical link. For instance, transmitter26may include left or right circular polarizers like polarization converter38. If transmitter26is to transmit a digital high, transmitter26may output an optical pulse via the left circular polarizer of transmitter26that travels via the single optical link of optical transport22B, and if transmitter26is to transmit a digital low, transmitter26may output an optical pulse via the right circular polarizer of transmitter26that travels via the single optical link of optical transport22B.

However, there may be certain drawbacks in polarizing the optical pulse in at transmitter26, rather than at receiver36. For example, propagation of circular polarized optical signals in integrated waveguides may not be feasible because of strong material and modal birefringence of the waveguides. For instance, for transmitting and receiving optical signals within processor12, processor12may include optical waveguides that are integrated within integrated circuit of processor12(e.g., optical transports22A and22B may be optical waveguides). Such optical waveguides may degrade the optical pulse (e.g., via chromatic dispersion) so that the polarization of the optical pulses is lost.

For instance, circular polarized light is composed of two orthogonal linear polarized modes (with a phase difference of +π/2) which propagate with different phase velocity in the waveguides because of its birefringence. Therefore, the final polarization state of the optical pulse may be undetermined (e.g., neither left circular polarized nor right circular polarized, but somewhere in the middle) over a long propagation distance if the birefringence is not controlled.

Controlling the birefringence of the optical waveguide may be difficult. Accordingly, in one example, rather than setting the polarization of the optical pulse at the transmitter26end, the techniques set the polarization of the optical pulse at the receiver36end. For example, transmitter26may transmit linearly polarized optical pulses that are converted to polarized optical pulses by polarization converter38to circumvent the degradation caused by the birefringence of the optical waveguide. However, such techniques may utilize two optical links42and44within optical transport22B, rather than a single optical link within optical transport22B.

Accordingly, polarization converter38converts optical pulses received at the upper or lower input ports to right circular polarized light or left circular polarized light, respectively, at the output port. In the example techniques described inFIG. 2, transmitter26only propagates linearly polarized optical pulses over medium to long distances via two input waveguides (e.g., optical links42and44of optical transport22B). In this manner, transmitter26may not need to encode information in the helicity states of the light pulses. Instead, transmitter26may encode the digital bit values of the spin-based signal into the two optical links42and44that carry the digital high or a digital low, respectively, to polarization converter38.

Also, although magnetoresistive device40is illustrated as being external to polarization converter38, the techniques described in this disclosure are not so limited. In some examples, to further minimize the effects of birefringence, magnetoresistive device40may be integrated with polarization converter38. In this manner, an additional optical waveguide may not be needed from the output of polarization converter38to magnetoresistive device40.

FIG. 2also illustrates transmitter26, which includes voltage source28, resistor30, magnetoresistive device32, and photonic switch34. Magnetoresistive device32may be different than magnetoresistive device40. For example, magnetoresistive device40may a type of magnetoresistive device whose magnetization states are configurable based on the polarization of the optical pulse that magnetoresistive device40receives. The magnetization state of magnetoresistive device32may not need to be configurable based on the polarization of light.

Magnetoresistive device32may be an MTJ or a GMR. One of the characteristics of magnetoresistive device32may be that the resistance of magnetoresistive device32is a function of the magnetization state. For example, in the parallel magnetization state, the electrical resistance of magnetoresistive device32may be lower than when magnetoresistive device32is in the anti-parallel magnetization state.

In this manner, the voltage at node33may be a function of the magnetization state of magnetoresistive device32. For instance, resistor30and magnetoresistive device32form a voltage divider that divide the output voltage of voltage source28, and the voltage at node33is the divided voltage. Accordingly, if magnetoresistive device32is in the parallel magnetization state, the voltage at node33may be less than if magnetoresistive device32is in the anti-parallel magnetization state. In other words, if magnetoresistive device32is in the parallel magnetization state, the voltage at node33is at a first voltage level, and if magnetoresistive device32is in the anti-parallel magnetization state, the voltage at node33is a second voltage level. In this example, the first voltage level is less than the second voltage level.

As illustrated, the voltage at node33drives photonic switch34, and photonics switch34receives as input a linearly polarized laser. One example of photonic switch34is a silicon micro-ring resonator based optical switch. If magnetoresistive device32is in the parallel magnetization state, which corresponds to a digital low, the voltage at node33is at the first voltage level. In this example, the voltage at node33being at the first voltage level causes photonic switch34to output the linearly polarized laser via optical link44. If magnetoresistive device32is in the anti-parallel magnetization state, which corresponds to a digital high, the voltage at node33is at the second voltage level. In this example, the voltage at node33being at the second voltage level causes photonic switch34to output the linearly polarized laser via optical link42.

There may be various ways in which to control the magnetization state of magnetoresistive device32. As one example, a spin-based polarized current may set the magnetization state of magnetoresistive device32. For instance, the spin-based polarized current may set the magnetization state of magnetoresistive device32every 100 picoseconds, which means that photonic switch34transmits an optical pulse of the linearly polarized laser for 100 picoseconds. In this manner, transmitter26may convert spin-based signals to optical signals for transmission at a relatively high data rate (e.g., 10 giga-bits per second in this example). As described above, receiver36receives the optical pulses representative of digital bit values and converts the optical pulses into spin-based digital bit values to complete the optical interconnect and transfer of data from transmitter26to receiver36.

In some examples, photonics switch34may be capable of transmitting data at more than 10 giga-bits per second with energy consumption of less than 50 fJ/bit. The performance of such photonics switches is improving for data transmission at even greater data rates. Therefore, such photonic switches may be well suited for spin-to-optical conversion for high bandwidth, low power data communication in spin-based systems.

However, it should be understood that receiver36may receive optical pulses in ways other than the way in which transmitter26transmits the optical pulses. For example, a transmitter that is configured in a way other than transmitter26may transmit spin-to-optical converted data signals to receiver36. Similarly, a receiver that is configured in a way other than receiver36may receive optical pulses from transmitter26.

FIG. 6is a schematic diagram of the example illustrated inFIG. 2. As illustrated inFIG. 6, the micro-ring switch, which is one example of photonics switch34, receives the laser input. Based on magnetization state of magnetoresistive device32, the micro-ring switch outputs the optical pulse from the laser via optical link42or optical link44of optical transport22B. Polarization converter38receives the optical pulse via optical link42or optical link44and left or right circular polarizes the optical pulse based on whether polarization converter38receives the optical pulse via optical link42or optical link44. The GdFeCo layer of magnetoresistive device40receives the polarized optical pulse via grating reflector48(not shown inFIG. 6) and sets its magnetization state based on the polarization.

In this way, this disclosure describes examples of spin-optical interconnect systems that includes a spin-to-optical transmitter and an optical-to-spin receiver that are interconnected with integrated optical waveguides or fibers. The transmitter converts spin information (e.g., a digital bit value represented by the electron spins of a spin-polarized current that sets a magnetization state of a magnetoresistive device) to an optical signal. In the illustrated examples, the optical signal indicates whether a digital high or low is transmitted based on the polarization or the pathway of the optical signals. In some examples, the transmitter (e.g., transmitter26) may indicate whether the optical signal represents a digital high or a low based on the amplitude or phase modulation.

The optical signal may transmit through optical waveguides integrated on the chip (e.g., optical waveguides in optical transports22A and22B that are integrated on the chip of processor14) over a medium distance of tens of microns to a few millimeters. In some examples, the optical signal may transmit through fiber optics (e.g., fiber optics in optical transport22C) over a long distance such as over tens of millimeters.

The optical-to-spin receiver converts the optical signal back to spin information by all-optical switching of magnetization of a magnetic device (e.g., a digital bit value indicated by the magnetization state of magnetoresistive device40). As described above, the magnetization state of magnetoresistive device40can be deterministically controlled by either the direct angular momentum transform from the optical pulses to magnetic domains or by ultrafast heating in the material induced by the optical pulses.

FIG. 7is a flowchart illustrating one example technique in accordance with the disclosure. As illustrated, transmitter26converts spin-based signal to an optical signal for transmission (50). As described above, the spin-based signal includes digital bit values represented by magnetization states of a magnetoresistive device.

Transmitter26transmits optical pulses of the optical signal for digital bit values equal to a first digital bit value via a first optical link (52), and transmits optical pulses of the optical signal for digital bit values equal to a second digital bit value via a second optical link (54). For example, the voltage at node33causes photonics switch34to output the laser via optical link42for digital bit values equal to a digital high and to output the laser via optical link44for digital bit values equal to a digital low.

Receiver36receives optical pulses of the optical signal for digital bit values equal to a first digital bit value via the first optical link (56), and receives optical pulses of the optical signal for digital bit values equal to a second digital bit value via the second optical link (58). Polarization converter38of receiver36polarizes optical pulses received via the first optical link to a first polarization (60), and polarizes optical pulses received via the second optical link to a second polarization (62).

To convert the optical signal back to a spin-based signal, magnetoresistive device40sets its magnetization state to a first magnetization state representative of a first digital value for the optical pulses with the first polarization (64), and sets its magnetization state to a second magnetization state representative of a second digital value for the optical pulses with the second polarization (66). For example, the light of the left circular polarized optical pulses sets magnetoresistive device40into the anti-parallel magnetization state, which is representative of a digital high. The light of the right circular polarized optical pulses sets magnetoresistive device40into the parallel magnetization state, which is representative of a digital low.

FIG. 8is a block diagram illustrating one example device100for controlling functional characteristics with magnetoresistive device104, in accordance with one or more techniques of this disclosure. Similar toFIG. 1, examples of device100include a chip comprising one or more integrate circuits, a board that includes the components of device100illustrated inFIG. 8, and one or more appliances.

As illustrated, device100includes input circuit102, magnetoresistive device104, output circuit106, and controller108. In one example, input circuit102and output circuit106may be charge-based circuits. For instance, input circuit102may be configured to deliver voltage or current output circuit106. Input circuit102may be a voltage source or a current source. In some examples where input circuit102and output circuit106are charge-based circuits, input circuit102may be driver. Output circuit106includes circuitry that receives voltage or current. For instance, output circuit106may be a load driven by input circuit102. Output circuit106may include one or more components that receive power from input circuit102.

As described above, input circuit102and output circuit106may be charge-based circuits (e.g., operate with voltages and currents) or hybrid circuits (hybrid of spin-based and charge-based). However, the techniques described in this disclosure are not so limited. In some examples, input circuit102and output circuit106may be spin-based components. In general, input circuit102and output circuit106may be spin-based, hybrid, or charge-based circuits, where input circuit102outputs to output circuit106. For illustration, input circuit102and output circuit106are described as charge-based circuits.

Magnetoresistive device104may be similar to magnetoresistive device40, in some examples. For example, like magnetoresistive device40, the polarization of an optical pulse sets the magnetization state of magnetoresistive device104. However, whereas magnetoresistive device40facilitated data transfer, magnetoresistive device104may function to control the interconnection between input circuit102and output circuit106.

For example, the resistance of magnetoresistive device104is a function of the magnetization state of magnetoresistive device104(low resistance for parallel magnetization state and high resistance for anti-parallel magnetization state). Controller108may output an optical signal, via an optical waveguide or fiber optic link, with a specific polarization based on whether magnetoresistive device104is to be low resistance or high resistance. For example, similar to above, right circular polarized optical pulses may set magnetoresistive device104to the parallel magnetization state and left circular polarized optical pulses may set magnetoresistive device104to the anti-parallel magnetization state (or vice-versa).

Controller108may determine the amount of voltage or current that output circuit106is to receive and set the magnetization state of magnetoresistive device104with the appropriate polarized optical pulse based on the determination. For example, if input circuit102is a voltage source, controller108may determine the amount of voltage or current that output circuit106should receive. If the voltage or current at output circuit106is to be relatively high, controller108may set the magnetization state of magnetoresistive device104to the parallel magnetization state (low resistance state). If the voltage or current at output circuit106is to be relatively low, controller108may set the magnetization state of magnetoresistive device104to the anti-parallel magnetization state (high resistance state).

In the above example, magnetization device104is describes as functioning as a resistive element. However, the techniques described in this disclosure are not so limited. In some examples, magnetization device104may function as form of a switch. For example, magnetization device104may be a transistor with a MTJ or GMR built on the transistor, where the magnetization state of the MTJ or GMR is controllable by the polarization of the optical pulse outputted by controller108.

As one example, assume that the MTJ or GMR is built on the gate of the transistor, and the drain of the transistor is connected to input circuit102and the source of the transistor is connected to output circuit106, or a collector of the transistor is connected to input circuit102and the emitter of the transistor is connected to output circuit106. In this example, assume that if the MTJ or GMR is in the anti-parallel magnetization state (e.g., high resistance), the transistor is in the cutoff mode. Also, assume that if the MTJ or GMR is in the parallel magnetization state (e.g., low resistance), the transistor is the active mode.

In such an example, if controller108determines that output circuit106should not receive any current, controller108may output an optical pulse that sets the magnetization state of the MTJ or GMR to the anti-parallel magnetization state so that the transistor is turned off and no current from input circuit102can flow to output circuit106. If controller108determines that output circuit106should receive current, controller108may output an optical pulse that sets the magnetization state of the MTJ or GMR to the parallel magnetization state so that the transistor is turned on and current from input circuit102flows to output circuit106. In this way, magnetoresistive device104may function as a switch that allows or blocks current from flowing from input circuit102to output circuit106.

Accordingly, in some examples, magnetoresistive device104is configured to receive an optical signal from controller108and set a magnetization state based on the optical signal. In some examples, controller108may set the magnetization state of magnetoresistive device104to control an amount of voltage or current from input circuit102that is delivered to output circuit106.

In some examples, such as those of spin-based systems, the magnetization state of magnetoresistive device104may control the flow of spin-polarize current between input circuit102and output circuit106(e.g., in examples where input circuit102and output circuit106are spin-based circuits). Accordingly, even in examples where input circuit102and output circuit106are spin-based, magnetoresistive device104may be configured to receive an optical signal and set a magnetization state based on the optical signal to control current (e.g., spin-polarize current) from input circuit102that is delivered to output circuit106. In charge-based examples of input circuit102and output circuit106, magnetoresistive device104may be configured to receive an optical signal and set a magnetization state based on the optical signal to control current or voltage (e.g., an amount of current or voltage) from input circuit102that is delivered to output circuit106.

A method of data transfer in a spin-based system, the method comprising converting a spin-based signal into an optical signal for transmission, wherein the spin-based signal comprises digital bit values represented by magnetization states of a magnetoresistive device, and converting the optical signal back to a spin-based signal for reception.

The method of example 1, wherein converting the spin-based signal into the optical signal comprises converting the spin-based signal directly into the optical signal without an intervening conversion to a charge-based signal, wherein the charge-based signal represents digital bit values using voltage or current.

The method of any of examples 1 and 2, wherein converting the optical signal back to the spin-based signal comprises converting the optical signal directly into the spin-based signal without an intervening conversion to a charge-based signal, wherein the charge-based signal represents digital bit values using voltage or current.

The method of any of examples 1-3, further comprising transmitting optical pulses of the optical signal for digital bit values equal to a first digital bit value via a first optical link, and transmitting optical pulses of the optical signal for digital bit values equal to a second digital bit value via a second optical link.

The method of any of examples 1-4, further comprising receiving optical pulses of the optical signal for digital bit values equal to a first digital bit value via a first optical link, and receiving optical pulses of the optical signal for digital bit values equal to a second digital bit value via a second optical link.

The method of example 5, wherein the magnetoresistive device comprises a first magnetoresistive device, the method further comprising polarizing the optical pulses of the optical signals received via the first optical link to a first polarization, and polarizing the optical pulses of the optical signals received via the second optical link to a second polarization, wherein converting the optical signal back to the spin-based signal comprises receiving with a second magnetoresistive device the light of the optical pulses with the first polarization and the optical pulses with the second polarization, setting a magnetization state of the second magnetoresistive device to a first magnetization state representative of a first digital value for the optical pulses with the first polarization, and setting a magnetization state of the second magnetoresistive device to a second magnetization state representative of a second digital value for the optical pulses with the second polarization.

A method comprising receiving light of an optical signal with a magnetoresistive device, and setting a magnetization state of the magnetoresistive device based on the light of the optical signal.

A method of data transfer in a spin-based system, the method comprising converting a spin-based signal into an optical signal, wherein the spin-based signal comprises digital bit values represented by magnetization states of a magnetoresistive device, and transmitting the optical signal.

The method of example 8, wherein transmitting the optical signal comprises transmitting optical pulses of the optical signal via a first optical link for the digital bit values equal to a first digital bit value, and transmitting optical pulses of the optical signal via a second optical link for the digital bit values equal to a second digital bit value.

An optical-to-spin receiver comprising a magnetoresistive device configured to receive light an optical signal, wherein a magnetization state of the magnetoresistive device is set by the light of the optical signal.

A system comprising a spin-to-optical transmitter configured to convert a spin-based signal into an optical signal, wherein the spin-based signal comprises digital bit values represented by magnetization states of a magnetoresistive device, and transmit the optical signal.