Patent Description:
This invention was made with Government support. The Government has certain rights in this invention.

The present invention relates generally to quantum computing systems, and more particularly to a managing a state of a qubit assembly.

A classical computer operates by processing binary bits of information that change state according to the laws of classical physics. These information bits can be modified by using simple logic gates such as AND and OR gates. The binary bits are physically created by a high or a low energy level occurring at the output of the logic gate to represent either a logical one (e.g., high voltage) or a logical zero (e.g., low voltage). A classical algorithm, such as one that multiplies two integers, can be decomposed into a long string of these simple logic gates. Like a classical computer, a quantum computer also has bits and gates. Instead of using logical ones and zeroes, a quantum bit ("qubit") uses quantum mechanics to occupy both possibilities simultaneously. This ability means that a quantum computer can solve a large class of problems with exponentially greater efficiency than that of a classical computer.

A qubit initialization protocol based on a dissipative environment that can be dynamically adjusted is described in <NPL>.

Described herein is a qubit apparatus according to claim <NUM>. Preferred embodiments are defined in the appended dependent claims.

Solid state quantum bits ("qubits") encode information in quantized excitations of a macroscopic degree of freedom in objects such as semiconducting quantum dots, SQUIDs, or other superconducting devices. In any quantum computer, it is desirable to initialize the quantum bits to a known state with high fidelity. In some architectures, the physical qubits can be recycled throughout the computation, such that the application is sensitive to the speed of the reset operation. A solid state qubit, in accordance with an aspect of the present invention, has the ability to initialize the system in a known state with high fidelity, a process referred to herein as a "reset" of the qubit. In one implementation, the qubit is initialized in the ground state. A related process is the removal of unwanted thermal excitations from the qubit. In this process, referred to herein as "cooling" the qubit, the excited state population of the qubit is reduced to below thermal levels.

The present disclosure relates generally to superconducting circuits, and more particularly to a fast reset or cooling scheme that involves coupling the qubit to a dissipative environment via a tunable coupler, allowing for selective reset or cooling of the qubit. It is often desirable to reuse a qubit in an algorithm, which is expedited by an efficient method for initializing the qubit in the ground state. Since long qubit lifetimes are also desirable, it is intractable to wait out the qubit lifetime to allow the qubit to decay naturally. Therefore, a method is provided for qubit reset that can initialize the qubit quickly, but will not deleteriously affect the qubit lifetime during operation. An external bias tunes the coupling between the qubit and the environment, with the bias idling at a point such that the qubit is isolated from the environment, preserving the qubit lifetime. Fast DC pulses on the external bias are used to activate the coupler when it is desirable to reset the qubit.

<FIG> illustrates a functional block diagram of a qubit assembly <NUM> in accordance with an aspect of the present invention. The qubit assembly <NUM> comprises a qubit <NUM> coupled to a dissipative element <NUM> via a tunable coupler <NUM>. The qubit <NUM> can include any physical assembly having a plurality of energy states that are tunable in response to a control mechanism (not shown). For example, the qubit cell can be implemented as an oscillator that can transfer energy between some combination of an electric field of a capacitor, a magnetic field of an inductor, and one or more superconducting Josephson junctions, such that the qubit cell can include one or more of a charge qubit, a flux qubit, and a phase qubit. Exemplary implementations of a qubit cell can include one or more of a Josephson junction, a quantum dot, a SQUID (superconducting quantum interference device), a Cooper pair box, and an ion trap. It will be further appreciated that the term "coupled" is intended to encompass not only a means of physical coupling, such as a mechanical coupling by means of an electrical conductor, but also any other appropriate coupling means including capacitive, inductive, magnetic, nuclear, and optical coupling, or any combination of the foregoing.

The tunable coupler <NUM> can include any appropriate structure for selectively isolating the qubit <NUM> from the dissipative element <NUM>. In one implementation, the tunable coupler is an RF-SQUID with a small inline DC-SQUID, known as a compound Josephson junction (CJJ), which acts as a tunable mutual inductance between two elements <NUM> and <NUM>. The magnetic susceptibility of the coupler can be tuned by applying flux to either the main inductor of the RF-SQUID or to the CJJ. Using this coupler <NUM>, a tunable mutual inductance can be changed in situ. The tunable coupler <NUM> can be controlled via a coupling controller <NUM> that controls the coupling strength of the compound Josephson junction. For example, the coupling controller <NUM> can tune the mutual coupling at the tunable coupler <NUM> between a first value, representing a strong coupling between the qubit <NUM> and the dissipative element <NUM>, for example, a mutual inductance between twenty and four hundred fifty picohenries, and a second value, representing substantial isolation of the qubit from the dissipative element, such as a mutual inductance near zero. The coupling controller <NUM> can utilize single flux quantum (SFQ) logic (e.g., reciprocal quantum logic (RQL) logic), and/or conventional logic. In one implementation, one or more fast DC pulses can be applied via a coupler control line (not shown) to turn on the coupler and reset the qubit. During qubit operation, the coupler will be DC biased and held in a state providing near zero coupling. This allows for a controllable reset of the qubit <NUM> to the ground state quickly without having an undesired, deleterious effect on the qubit lifetime.

The dissipative element <NUM> comprises a circuit element, referred to herein as a load, that receives the energy stored in the qubit <NUM> when the mutual coupling between the qubit and the dissipative element is strong. Essentially the qubit control <NUM> can tune the qubit <NUM> to discharge its associated quantum state along the tunable coupler <NUM> to provide it to the circuit element. In such a case, the dissipative element <NUM> can comprise any element having a non-vanishing real impedance to which it is desirable to provide a single photon input. For example, said circuit element can be an amplifier, a detector, a fiber coupler, an opto-modulator, a beam splitter, or similar component. It will further be appreciated that the qubit assembly <NUM> can be used with resonators, other qubits, or other qubit assemblies having the fast cooling and reset configuration of the illustrated qubit assembly.

<FIG> illustrates a schematic of one example of a circuit <NUM> for reading a state of a qubit. The circuit <NUM> includes a transmon qubit <NUM>, a readout resonator <NUM>, a feedline <NUM>, a transmission line <NUM>, a resistive load <NUM>, and a tunable compound Josephson junction (CJJ) coupler <NUM>, comprising a superconducting loop interrupted by a CJJ <NUM> to form a radio frequency (RF) superconducting SQUID. The tunable compound Josephson junction (CJJ) coupler <NUM> includes a first inductor <NUM> in the superconducting loop that provides a mutual inductance, Mq, to the qubit <NUM> via a galvanic connection, and a second inductor <NUM> that provides a mutual inductance, Mr, to the transmission line <NUM> leading to the resistive load <NUM> via a galvanic connection. In the illustrated implementation, the CJJ <NUM> includes two identical Josephson junctions <NUM> and <NUM>.

A flux, Φα, can be applied to the CJJ <NUM> by applying current to a control line <NUM>, with a bias tee <NUM> on the control line available to allow for both DC and pulsed signals to be applied to the coupler. Since current provided to the control line <NUM> does not generate current in the superconducting loop when the junctions <NUM> and <NUM> in the CJJ <NUM> are identical, the qubit <NUM> is protected from dissipation caused by this line. The coupler represents an effective mutual inductance, Mσff = Mα Mrχ, where χ is the magnetic susceptibility of the coupler. The magnetic susceptibility is a function of the flux applied to the CJJ <NUM>, which can be expressed as: <MAT>
where it is assumed that LQQ is the total geometrical inductance of the coupler, <MAT>, Φ<NUM> is the magnetic flux quantum, approximately equal to <NUM> femtowebers, and Ic is the combined critical current of the two junctions in the CJJ <NUM>.

Using the control line <NUM>, the effective mutual inductance, Meff' between the qubit <NUM> and the resistive load <NUM> can be tuned to allow for selective reset of the qubit state. Since the qubit lifetime is a function of this mutual inductance, we isolate the qubit from the environment by setting Meff = <NUM> and reset it by turning up Meff' The dependence of the qubit lifetime, T<NUM>, with the bias, Φα, applied to the coupler can be seen in Eq. <NUM>: <MAT>
where Cq represents a capacitance of the qubit <NUM>, Lq represents an inductance of the qubit, and Z<NUM> represents an impedance of the dissipative element, such as the resistive load <NUM>.

<FIG> is a line chart <NUM> illustrating the variation of the mutual inductance between the qubit <NUM> and the resistive load <NUM>, represented in picohenries on the vertical axis <NUM>, as a function of the control flux provided to the CJJ <NUM>, represented on the horizontal axis <NUM> in units of the magnetic flux quantum. For the purposes of the model illustrated in <FIG>, it is assumed that Mq is one hundred picohenries. From the plotted line <NUM>, it can be seen that that the coupling at Φα = Φ<NUM>/<NUM> is zero, due to the fact that χ goes to <NUM>. For Φ<NUM>/<NUM> < Φα < Φ<NUM>, χ takes on a negative value and eventually grows to be much larger, in absolute terms, than it was at Φα = <NUM>. In one implementation, this large negative coupling can be exploited to give an enhancement in the absolute coupling strength. When gate operations are performed on the qubit <NUM>, the coupler <NUM> is turned off by providing a control flux, Φα = Φ<NUM>/<NUM> so that the qubit is isolated from the resistive load <NUM> and the lifetime of the qubit is not reduced. For reset, a voltage pulse is applied to the Iα line at the bias tee <NUM> such that the control flux, Φα, is raised to Φ<NUM>, thus turning the coupler <NUM> on. In order to reduce the effects of capacitors on the bias line, the readout can be followed with a negative pulse of equal magnitude on the Iα line.

<FIG> is a line chart <NUM> illustrating a projected relaxation time of the qubit <NUM>, represented logarithmically in nanoseconds on the vertical axis <NUM>, as a function of the control flux provided to the CJJ <NUM>, represented on the horizontal axis <NUM> in units of the magnetic flux quantum. For the purposes of the model illustrated in <FIG>, it is assumed that Mq is one hundred picohenries, an energy of the ground to excited transition of the qubit is <NUM>, an impedance of the resistive load is <NUM>Ω, and a capacitance of the qubit is <NUM> fF. From the plotted line <NUM>, it can be seen that that the relaxation time is maximized at Φα = Φ<NUM>/<NUM> and Φα = 3Φ<NUM> /<NUM> , where the coupling between the qubit <NUM> and the dissipative element is minimized. The relaxation time is minimized at Φα =Φ<NUM>, where the coupling is maximized.

<FIG> is line chart <NUM> illustrating the measured lifetime, T<NUM>, of the qubit <NUM>, represented logarithmically in microseconds on the vertical axis <NUM>, as a function of the control flux provided to the CJJ <NUM>, represented on the horizontal axis <NUM> in units of the magnetic flux quantum. From the plotted line <NUM>, it can be seen that a maximum lifetime of <NUM> was achieved when the qubit <NUM> was isolated from the dissipative element and a minimum lifetime of <NUM> ns was achieved during reset. Accordingly, the qubit lifetime, in this specific implementation, is reduced by a factor of approximately one thousand, although depending on the implementation, the lifetime can be shortened by a factor of between five-hundred and five-thousand. The method appears to be capable of resetting the qubit <NUM> with at least <NUM>% fidelity in less than <NUM> ns.

The proposed device has been tested via a four-frame measurement intended to reduce the effect of low frequency drift in the readout signal. One frame was a simple T<NUM> decay in which the qubit <NUM> was π-pulsed and some time was allowed to lapse before measurement. This is referred to as the "no-reset" case in <FIG> below. The second was a measurement of the qubit when no π pulse was applied, with some time allowed to elapse before measurement. In a third frame, the qubit <NUM> was π-pulsed, and a pulse was applied to the reset coupler that set Φα = Φ<NUM> for some time before measuring. This is referred to as the "reset" case in <FIG> below. In a fourth case, the qubit was not excited, but a pulse was still applied to the reset coupler. The magnitude of the integrated transmitted signal through the readout resonator feedline was measured for <NUM> million samples of the four frames when the reset/wait time was set to <NUM> ns. The reset signal had a <NUM> ns turn on and turn off with an error function profile, so the shortest delay for all measurements was <NUM> ns and this <NUM> ns is included in the reset/wait time.

The measurements were performed using high-power readout, so the readings separated into two distributions due to the bright state behavior of the readout resonator, with the "bright state counts" in one distribution representing a measure of |<NUM>〉 state population. As expected, the bright state counts were highest when an excitation pulse was applied to the qubit, although it will be noted that even when no excitation pulse is applied to the qubit there are some counts in the "bright state" distribution, and the number obtained in the no excitation and no reset frame provides a baseline for the differencing measurement used below. The reset provides a significant drop in the bright state counts when the qubit <NUM> has been excited, and even a slight reduction in the peak of the distribution for the non-excited case.

<FIG> is a line chart <NUM> comparing the evolution of the population of the excited state for a qubit after an excitation pulse. A vertical axis <NUM> represents the fractional population of the excited state, on a logarithmic scale, and the horizontal axis <NUM> represents time, in nanoseconds, on a logarithmic scale. A first plotted line <NUM> represents the no-reset case and the second plotted line <NUM> represents the reset case. The data for each case is normalized to the difference between the no reset measurement and the no excitation with reset measurement taken with a <NUM> ns reset time. It will be appreciated that the reset curve decays to the noise floor of the measurement by <NUM> ns, as would be expected for a T<NUM> equal to <NUM> ns. Unfortunately, the noise on the measurement is such that although some measurements at reset times of ><NUM> ns fall below <NUM> %, the scatter in the data gives an error bar of about <NUM>%.

In view of the foregoing structural and functional features described above, an example method will be better appreciated with reference to <FIG>. While, for purposes of simplicity of explanation, the example method of <FIG> is shown and described as executing serially, it is to be understood and appreciated that the present examples are not limited by the illustrated order, as some actions could in other examples occur in different orders, multiple times and/or concurrently from that shown and described herein. Moreover, it is not necessary that all described actions be performed to implement a method.

<FIG> illustrates one example of a method <NUM> for operating a qubit. At <NUM>, a first value of a control flux is provided to a tunable coupler linking the qubit and a dissipative element such that the qubit is substantially isolated from the dissipative element. In one example, the tunable coupler includes a superconducting loop interrupted by a compound Josephson junction, and the first value of the control flux is provided to the compound Josephson junction. At <NUM>, a quantum operation is performed at the qubit. At <NUM>, a second value of the control flux is provided to the tunable coupler such that the qubit is coupled to the dissipative element. In one implementation, the first value of the control flux is substantially equal to one-half of a flux quantum, and the second value of the control flux is greater than one-half of a flux quantum and less than or equal to the flux quantum. The second value of the control flux can be selected to provide a strong, negative coupling between the qubit and the dissipative element. At <NUM>, a reset time is allowed to elapse while the qubit relaxes to a ground state during coupling to the dissipative element. The reset time can be between <NUM> ns and <NUM> ns, and in one implementation, a reset time substantially equal to <NUM> ns can be used.

What have been described above are examples. It is, of course, not possible to describe every conceivable combination of components or methodologies.

Claim 1:
A qubit apparatus (<NUM>, <NUM>) comprising:
a resistive load (<NUM>);
a qubit (<NUM>, <NUM>);
a tunable compound Josephson junction coupler (<NUM>, <NUM>) coupling the qubit to the resistive load,
the tunable compound Josephson junction coupler comprising a superconducting loop interrupted by a compound Josephson junction (<NUM>), the compound Josephson junction being implemented as an inline direct current superconducting quantum interference device, DC SQUID, which acts as a tunable mutual inductance, to form a radio frequency superconducting quantum interference device, RF SQUID, the tunable compound Josephson junction coupler further comprising a first inductor (<NUM>) that provides a mutual inductance to the qubit via a first inductor galvanic connection, and a second inductor (<NUM>) that provides a mutual inductance to a transmission line (<NUM>) leading to the resistive load via a second inductor galvanic connection; and
a coupling controller (<NUM>) that applies flux to either the superconducting loop of the RF SQUID or to the compound Josephson junction to tune a magnetic susceptibility of the coupler to control the coupling strength of the tunable compound Josephson junction coupler such that a coupling between the qubit and the resistive load is a
first value, at which the qubit is coupled to the resistive load, when a reset of the qubit is desired and a second value, at which the qubit is substantially isolated from the resistive load, during operation of the qubit.